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Cell Cycle

The cell cycle involves the growth, replication, and division of a eukaryotic cell. Interphase, Mitosis, & the Cell Cycle The two main phases of the cell cycle are interphase and M phase. During interphase, a cell's chromosomes are duplicated, but no cell division is occurring. Most of a cell's life is spent in interphase.

M phase includes mitosis and cytokinesis. During mitosis, the nucleus of a cell divides into two daughter nuclei that each contain the same number of chromosomes as the parent nucleus. The two nuclei that are formed during mitosis are separated into two identical daughter cells during cytokinesis.The end result of M phase are two cells that genetically identical to the parent cell. Mitosis is directly involved in the division of a cell's nucleus during the cell cycle. Meiosis, on the other hand, is not directly involved in the cell cycle.Meiosis is a process in which a cell undergoes two successive nuclear divisions. Meiosis produces haploid daughter cells with half of the species' usual number of chromosomes. These resulting daughter cells are called gametes and aid the organism in sexual reproduction.

Cell Growth & Reproduction

All cells come from pre-existing cells. Cell division is a key process involved in growth, repair, and reproduction of organisms. Cell Division Most of the cells found in living things are able to reproduce by dividing to form new cells that are identical or genetically similar to themselves. Cell division is triggered as cells become too large to efficiently import nutrients and export wastes across their cell membrane. This occurs because as a cell grows larger, its volume grows more rapidly than its surface area. As cells continue to divide, they will proliferate to fill whatever medium they are in. Once the cells have spread so much that two cells contact each other, they can signal to each other to stop dividing through a process called inhibition. Cellular reproduction in multicellular organisms occurs primarily through the process of mitosis. The purpose of mitosis is to form new somatic cells.Somatic cells are those cells that form the body of an organism. Mitosis & Cytokinesis Mitosis refers specifically to the division of the cellular nucleus. Therefore, it only occurs in eukaryotes. After the chromosomes are replicated during interphase, the cell enters the first stage of mitosis— prophase. Following the completion of mitosis, the entire cell divides through a process called cytokinesis. The result of which are two identical daughter cells. The major events that occur during mitotic cellular division are described below.

Interphase Interphase occurs before mitosis. During interphase, the chromosomes containing the genetic information of the cell are copied.

Mitosis Begins

Prophase Genetic material (chromatin) condenses into rod-like structures called chromosomes . Metaphase Chromosomes line-up along the equator of the cell.

Anaphase Chromatids separate and move to opposite sides of the cell

Telophase A nuclear membrane forms around each set of chromosomes and mitosis is complete.

Mitosis Ends Meiosis

Meiosis is a kind of eukaryotic cell division that reduces the number of chromosomes in a cell by half. Overview of Meiosis Only eukaryotic cells can undergo meiosis. Meiosis is a form of cell division that produces haploid (N) daughter cells that contain only half of the species' usual number of chromosomes. These resulting daughter cells are calledgametes and aid the organism in sexual reproduction. The following video describes the process involved to create egg and sperm cells.

Clip provided by Education Clip Library with permission from ITN Source Stages of Meiosis Chromosomes are copied during interphase prior to the start of meiosis. This short period of interphase is known as S phase for synthesis. The following stages of meiosis are summarized in order.

Prophase I During prophase I, homologous chromosomes pair and become tetrads (two chromosomes or four chromatids). Crossing over between homologous chromosomes occurs at this stage.

Metaphase I After crossing over occurs, homologous chromosomes line-up along the equator. Anaphase I Whole chromosomes separate from the tetrad formation and move to opposite sides of the cell. Each chromosome still has two sister chromatids.

Telophase I During telophase I, a nuclear membrane forms around each set of chromosomes. Each cell now has one set of chromosomes and is haploid (n).

Prophase II Sister chromatids become short and thick at the beginning of prophase II.

Metaphase II The chromosomes migrate to the center of the nucleus and line-up along the equator by the end of metaphase II. Anaphase II During anaphase II, sister chromatids are pulled apart by microtubules to opposite poles.

Telophase II A nuclear envelope forms around each set of chromosomes and meiosis II is complete.

Cytokinesis The cells divide to create four haploid cells. Genetic Variation During meiosis, crossing over—a process unique to meiosis—can occur.Crossing over occurs during prophase I when two chromosomes pair up andexchange parts of their DNA. Crossing over provides genetic diversity between the parents and their offspring. Genetic variation can also occur when alleles are randomly sorted during meiosis. Since each offspring receives a different combination of alleles from the parent organisms, phenotypic diversity results.

Genes

The set of instructions, or code, that is responsible for all the inherited traits of an organism is held in genetic material calledDNA.

A gene can be defined as a basic unit of hereditary information.It refers to a specific segment of DNA that influences a particular trait or group of traits. Inherited Traits & Genes All of your inherited traits are the result of your genes. You received half of your genes from your mother and half from your father. In many other species, genes are passed to an offspring from one parent only. The reason that half of the genetic information contained within a human comes from each parent is because of chromosome recombination during meiosis and fertilization. When a sperm cell fertilizes an egg cell, their nuclei fuse, and the 23 sets of chromosomes pair up. The resulting zygote has a new, full DNA sequence that is half from the mother and half from the father. So, what is a gene? In general, a gene refers to a specific segment of an organism's DNA. The unique DNA code in that segment influences one or more traits of the organism. Sometimes, a single gene can control a single trait. Sometimes, multiple genes work together to control a single trait. Sometimes, a single gene can influence many traits. Organization of Genetic Material gene → DNA molecule → chromosome → genome It is important to know how genetic material is organized. A gene is one "piece" of a DNA molecule. A molecule of DNA is "packaged" and carried by a larger structure called a chromosome. The genome of an organism refers to its complete genetic makeup and includes the organism's entire set of chromosomes. A human has a total of 46 chromosomes: 23 chromosomes come from the mother and 23 come from the father. Homologous Pairs In the cells of a sexually-reproducing organism, a pair of similar chromosomes with the same genes in the same locations is known as a homologous pair.This means that every normal human body cell contains a 22 homologous pairs of autosomal chromosomes and 1 pair of sex chromosomes. Homologous pairs are found in diploid cells. Each member of the pair was received from one of the organism's parents. The genes on a pair of homologous chromosomes often have alternate forms, or alleles, which influence the organism's traits. Sex Determination An individual's sex is determined by its combination of sex chromosomes.Females have two X chromosomes (XX), and males have an X chromosome and a Y chromosome (XY).

DNA

DNA, or deoxyribonucleic acid, is a type of nucleic acid that contains genetic information. This information provides instructions for an organism's development and growth, and it is passed from generation to generation. DNA Structure DNA is composed of two nucleotide chains wound together into a double helix.

Each nucleotide consists of:  a five carbon sugar (deoxyribose)  a nitrogenous base (adenine, cytosine, guanine, or thymine)  a phosphate group The two DNA strands are held together by hydrogen bonds between specific pairs of nucleic acids. Adenine (A) only bonds with thymine (T), and cytosine (C) only bonds with guanine (G). These pairings are known as complementary bases. DNA molecules are different from one another because they contain a unique sequence of nucleotides. Even though there are only four nitrogenous bases, these bases can be ordered in innumerable ways. In fact, no two organisms possess the exact same DNA sequences in their cells. DNA can be divided into small segments known as genes. Genes can influence a single trait or multiple traits. DNA, along with its associated proteins, can also be organized into larger molecules known aschromosomes. An organism's complete genetic makeup, including its entire set of chromosomes, is known as its genome.

DNA Replication

DNA replication begins when enzymes unwind and separate the two strands of the molecule. Each strand serves as a template for polymerases to add complementary nucleotides (A-T and C-G). The process results in two identical DNA molecules.

During DNA replication...  The two original strands of DNA are separated with the help of enzymes known as DNA helicases. Helicases work by breaking the hydrogen bonds holding the nucleotide bases together.

 Enzymes known as DNA polymerases add complementary nucleotides to each strand. Adenine bonds with thymine, and cytosine bonds with guanine.

 Two DNA molecules, which are identical to the original DNA molecule, form. Each newly formed DNA molecule consists of two strands of DNA, one from the parent molecule and one built from scratch using the parent molecule as a template.

DNA Replication is said to be semi-conservative. Each copy contains one newly-replicated strand and one strand from the original molecule. The process of DNA replication is biologically significant because it allows the cells of living organisms to copy their DNA before cell division.

Gene Expression

The genetic information that is passed from a parent to its offspring is found in DNA molecules. Segments of DNA known as genes code for the production of proteins. These proteins cause specific traits to be expressed.

Two main processes are involved in gene expression - transcription and translation.  During transcription, DNA in the nucleus of a cell is copied into messenger RNA, or mRNA, molecules.

 The mRNA then moves into the cell's cytoplasm and attaches to a ribosome, where it is translated into proteins. Central Dogma

Once DNA is transcribed into mRNA and translated into a protein, the process cannot be reversed. That is, information cannot be transferred from the protein back to the nucleic acid. This is known as the central dogma of molecular biology.

Transcription

The sequence of the nucleotides within a strand of DNA provides the genetic instructions needed to construct proteins. In order to express these proteins, a segment of DNA must first be transcribed, or copied, to a complementary strand of messenger RNA (mRNA).

Three processes occur during transcription:  Initiation - Enzymes bind to a DNA sequence and unzip the molecule.  Elongation - As the molecule unzips, RNA nucleotides pair to complementary DNA nucleotides on one of the DNA strands. For example, if the DNA strand reads AGT, the new RNA strand would read UCA.

 Termination - Once base pairing is complete, the new RNA molecule (mRNA) breaks away from the DNA strands and the DNA strands re-attach.

The process of transcription occurs in the nucleus of a cell, but the mRNA that is created travels into the cytoplasm once it is made. Translation

During transcription, segments of DNA are copied to a complementary strand of messenger RNA, or mRNA.Translation is the process through which amino acids that correspond to codons, or triplet nucleotide sequences, in the mRNA molecules are joined together to form functional proteins.

During translation...  A codon on the mRNA molecule attaches to a ribosome.

 Then, transfer RNA, or tRNA, molecules, carrying specific amino acids, approach the ribosome.

 The tRNA molecule that corresponds to the codon (called the tRNA anticodon) attaches to the mRNA codon.

 The ribosome slides to the next codon on the mRNA molecule and repeats the process.

 As amino acids are added next to each other, peptide bonds link the amino acids together.  The chain of amino acids continues to grow until the ribosome reaches a stop codon on the mRNA strand. This signals that no more amino acids should be added, and the protein is complete.

Protein Synthesis - Organelles

Protein synthesis starts with the transcription of mRNA from DNA in the nucleus. Organelle Interactions During Protein Synthesis Beginning with the transcription of messenger RNA in the nucleus, protein synthesis involves many cellular structures, including: ribosomes, the endoplasmic reticulum, and the Golgi apparatus.

Many proteins are synthesized by ribosomes located on the rough ER. Some of these proteins continue to the Golgi apparatus where they are packaged and sorted. Some proteins also involve other organelles. For example, the heme portion of hemoglobin is manufactured in mitochondria. Ribosomes and Protein Synthesis The translation of mRNA to a polypeptide chain takes place on ribosomes. The nucleus is the location where the portion of DNA that codes for a specific protein is transcribed into messenger RNA. After the mRNA leaves the nucleus, it is delivered to a ribosome in the cytoplasm. Translation of the protein takes place on the ribosome. At the end of translation, the protein has completed the primary structure, which is simply a polypeptide chain that will still undergo folding and other possible modifications. Endoplasmic Reticulum and Protein Synthesis The endoplasmic reticulum aids in transporting proteins out of the cell or to other organelles within the cell. Some proteins are synthesized by ribosomes found on the rough portion of the endoplasmic reticulum. The term rough refers to its appearance due to the attachment of ribosomes. The proteins made here are destined for other organelles or to be secreted by the cell. The endoplasmic reticulum also plays a part in the folding and modification of some proteins. The ER aids in the creation of disulfide bonds, which provide stability to the structure of the protein, and glycosolation, which attaches carbohydrates to proteins. Golgi Apparatus and Protein Synthesis The Golgi apparatus is responsible for sorting, modifying, and packaging proteins. The Golgi apparatus works closely with the rough ER. It receives proteins synthesized by ribosomes on the endoplasmic reticulum. These proteins sometimes undergo further modifications. Phosphorylation, or the addition of phosphates, to some proteins occurs here. Glycosolation also continues in the Golgi. The Golgi plays an important role in sorting and tagging proteins which aid in their transportation to the correct location.

Heredity & Genetic Material

Reproduction is a characteristic of all living organisms. During reproduction, the instructions for inherited traits are passed from parents to offspring through their genetic material. The passage of these genetic instructions from one generation to the next generation is heredity. Chromosomes, Genes & DNA Genetic information is encoded in DNA. DNA is located in the chromosomesof cells. Chromosomes appear very dark (when viewed under a microscope) and are located in the cell nucleus, if the cell has one. A gene is a section of DNA located on a chromosome.

Adapted from image courtesy of NIH. A section of a chromosome that controls a particular trait is a gene. Genes are a kind of blueprint for an organism. They contain all the information necessary to build, repair, and keep the organism running, including how to make all the different proteins and other materials the body needs.

Mendel's Laws of Genetics

Gregor Mendel's studies with pea plants formed the basis of three laws governing inheritance. Mendel's Experiments During the 1800s, Gregor Mendel studied genetic traits in pea plants. He selected seven different traits to study, including plant height, pea pod color, and flower color. Each of the traits that Mendel studied had two possible alleles, such as tall or dwarf for height and purple or white for flower color. The seven traits of pea plants studied by Gregor Mendel during his genetic experiments.

Adapted from image courtesy of Wikipedia. Mendel began his experiments by obtaining pure lines of pea plants. These pure lines were groups that produced offspring with the same traits. For example, Mendel's pure line plants with purple flowers produced offspring that all had purple flowers. Mendel knew his lines were pure because each generation kept passing on the same traits to its offspring, but he did not yet know how these traits were passed. He would later understand that these pure lines were homozygous for several traits.

Test Cross After obtaining the pure lines, Mendel crossed a pure line that had purple flowers with a pure line that had white flowers. He called this the P generationfor parent generation. Once the pollinated flowers produced seeds, Mendel planted them. The cross of purple and white flowers resulted in F1 generationoffspring that all had purple flowers (100%).

Next, Mendel crossed two of the offspring from the F1 generation. Both parents had purple flowers, but about

¾ of the resulting F2 generationoffspring had purple flowers, and about ¼ had white flowers. Based on his data analysis, Gregor Mendel formulated three laws governing inheritance: 1. The Law of Dominance 2. The Law of Segregation 3. The Law of Independent Assortment Law of Dominance The law of dominance states that when an organism has two or more alleles for a trait, the allele that is expressed over the other alleles is considereddominant. The other alleles are considered recessive. In Mendel's pea experiments discussed above, the allele for white flowers was recessive to the allele for violet flowers because the allele for violet flowers masks the allele that codes for white flowers. Example 1: A man and his wife have four children, all with freckles. The man does not have any freckles, but the wife does have freckles. Why do all of the children have freckles? All of the children have freckles because they inherited a dominant allele for freckles from their mother. From the Law of Dominance, it can be assumed that if the parents have different alleles for the same trait, but all of the offspring have the same allele for that trait, the inherited allele is dominant. Law of Segregation The law of segregation states that different alleles for the same trait separate when gametes are formed. Thus, a mother that is heterozygous for brown eyes (Bb) could pass either a dominant brown allele (B) or a recessive blue allele (b) for eye color to her offspring. Example 2: A plant with a red flower and a plant with a white flower are crossed, resulting in all red flowers. The offspring are heterozygous for flower color. What allele(s) for flower color can the offspring plant pass on to its own offspring? The plant can pass on the allele for red or white flower color to its offspring. A heterozygous organism has both a dominant and a recessive allele. Since the alleles separate into different gametes, an offspring could receive a gamete with one allele or the other. Law of Independent Assortment The law of independent assortment states that when pairs of alleles separate, they do so independently of each other. Thus, the alleles for hair color and the alleles for eye color in humans are not inherited together. Example 3: Margie's father is heterozygous for brown hair, a dominant trait, and homozygous for blue eyes, a recessive trait. Margie's mother is homozygous for blonde hair, a recessive trait, and heterozygous for brown eyes, a dominant trait. Is it possible for Margie to inherit brown hair and brown eyes? Yes. Since the alleles for each trait separate into gametes independent of each other, it is possible to inherit some traits from one parent and some from the other. Margie could inherit any one of several combinations of traits from her parents (blonde hair and blue eyes, brown hair and blue eyes, or blonde hair and brown eyes). Demonstration of Mendel's Laws The chart below shows all of the possible variations of offspring that could be produced from a cross between two pea plants that are both heterozygous for two traits: pod form and color. During meiosis, alleles are separated and assorted independently. This results in greater variation among offspring.

Heredity

The genetic makeup of an organism (set of genes present) is referred to as its genotype, and the visible traits that we can observe as a result of these genes are its phenotype. Allele Dominance An allele is a variation, or one possible form, of a gene that codes for a particular trait. Different alleles can be dominant, recessive or intermediate. Dominant alleles mask recessive genes, meaning that when a dominant and a recessive allele are both present, the dominant allele's phenotypic trait is observed. For example, if an individual pea plant has a dominant allele for tallness (T) and a recessive allele for shortness (t), the observed phenotype of the pea plant is tall. This plant is heterozygous for the height gene and has genotype (Tt). Traditionally, a capital letter represents the dominant allele and a lowercase version of the same letter represents the recessive allele. A homozygous plant is one that has two copies of the same allele. Both allelic copies may be dominant (e.g. TT - tall) or recessive (e.g. tt - short). Incomplete Dominance Some alleles produce intermediate traits. That is, if one allele is incompletely dominant over the other, then a phenotype that is intermediate between the two variations can be observed. For example, some flowering plants have an allele for red (R) color and an allele for white (r) color. However, when the plant is a heterozygote (Rr), the plant produces pink flowers rather than red or white. Pink flowers are a result of incomplete dominance. Whenever expressed traits show a blending of two alleles, it is usually due to incomplete dominance. Codominance & Multiple Alleles Codominance occurs when two alleles are equally dominant. In these circumstances, the alleles are expressed simultaneously, resulting in organisms that have some kind of mixed pattern. For example, a flowering plant with codominant color genes might exhibit white and red speckled flowers in the heterozygote. In addition, some gene loci may have multiple traits, or more than two different traits present in varying amounts in a given population. When there are multiple traits, some genes may be dominant, others recessive, and still others may be incompletely dominant to one another. The ABO blood group is a good example of a gene locus with codominance and multiple alleles. The alleles for A blood proteins and B blood proteins are codominant to each other.Unlike incomplete dominance, there is no blending. Someone with AB blood type will produce both A proteins and B proteins.

Image is courtesy of NIH and Wikipedia. Polygenic Inheritance Sometimes a particular phenotype may be determined by more than one gene. This is referred to as polygenic inheritance, where more than one gene locus has a similar and additive effect on the same trait. Traits that are coded for by many genes tend to have large variations. Skin color, for example, is determined by three separate gene loci in humans, each of which has alternate traits with an additive effect on how light or dark skin color is. Sex-Linked Traits Sex-linked traits are those carried on the X chromosome. Recall that in humans, a normal female has two X chromosomes. A normal male inherits an X chromosome from the mother and a Y chromosome from the father.Therefore, normal males inherit sex-linked traits only from their mothers.

Recessive, Sex-linked Traits Recessive, sex-linked traits are more common in boys than girls. Some examples include:  hemophilia  colorblindness  Duchenne muscular dystrophy

Males inherit recessive, sex-linked traits from their mothers.

Image is courtesy of U.S. National Library of Medicine. The daughters of affected males will all become carriers. The sons will be unaffected because they do not inherit X chromosomes from their father. Image is courtesy of U.S. National Library of Medicine.

Punnett Squares

A Punnett square is a chart which predicts all of the possible gene combinations from two parents for a particular trait.

Punnett squares are named for the English geneticist Reginald Punnett, who discovered some of the basic principles of genetics, including sex-linkage and sex-determination. In a Punnett square, one parent's alleles are written across the top of a grid and the other parent's alleles are written down the left side of the grid. Then, the predicted genotypes of the offspring are determined inside the grid, like a multiplication table. In the example below, two parents that are both heterozygous (Bb) for a coat color trait are crossed.

Since black coat color (B) is dominant over brown (b), the resulting progeny will approximately be:  3:4 (75%) black coated o 1:4 (25%) homozygous dominant (BB) o 1:2 (50%) heterozygous (Bb)  1:4 (25%) homozygous recessive (bb) and brown coated. This is an example of a monohybrid cross because there is only one characteristic present (coat color). However, Punnett squares can also be used to predict the gene combinations of multiple linked traits.

Genetic Variation

Genetic variation is important for the survival of a species.Greater variation within a species increases the chances of survival for the species because the variant organisms within the species are able to respond differently to the environmental changes. Overview Sexual reproduction results in a great variety of possible gene combinations that can be produced in the offspring of any two parents. This variety is due to the sorting and recombination of genes that occurs during meiosis. Since each offspring receives a different combination of alleles from the parent organisms, both genotypic and phenotypic diversity results. Although asexual reproduction is faster and requires less energy than sexual reproduction, offspring are almost always genetically identical to their parents; there is little to no genetic variation. Meiosis & Genetic Diversity Meiosis is the form of cell division by which unique gametes (sex cells) are produced. Each diploid parent cell divides twice during meiosis and produces four haploid daughter cells. During meiosis, the process of crossing over results in new combinations of alleles due to the fact that genes are located on separate chromosomes.When crossing over occurs, different parts of chromosomes are exchanged, meaning that genes (and their alleles) are transferred to new chromosomes.When meiosis separates these chromosomes, the new combination of alleles is transferred to the offspring, resulting in a new combination of traits.

Errors made during crossing over can also result in genetic variation. For example, genes, or even entire chromosomes, can be deleted or duplicated, resulting in even more genotypic possibilities. Transposons Transposons, or jumping genes, can independently replicate and insert new copies of themselves within an organism's genome. Since transposons can replicate and insert themselves several times during the formation of gametes, these genes can cause a large amount of genetic variation. Fertilization & Genetic Variation Since parents are genetically different, when their gametes and chromosomes are united during fertilization, genetic variation results. No two organisms produced by sexual reproduction will share an identical genome unless they come from the same fertilized egg. Chromosomal Mutations and Genetic Variability Though all types of mutations contribute to genetic variability, only chromosomal mutations are covered in this section.

Deletion Many diseases can be caused by chromosomal abnormalities. For example, the disease Cri du chat, which causes children to have a cat-like cry, is caused by the deletion of part of chromosome 5.

Translocation Chromosome translocation occurs when material is exchanged between two chromosomes, or part of one chromosome becomes fused onto another chromosome. Some human disorders are caused by chromosome translocation, such as cancer, infertility, and translocation Down syndrome. Translocation Down syndrome, for example, occurs because a section of chromosome 21 becomes fused onto another chromosome. It accounts for less than 5% of the total cases of Down syndrome reported.

Nondisjunction Nondisjunction occurs when chromosomes do not separate correctly during cell division. The resulting daughter cells will either be missing or have extra copies of chromosomes. The picture below compares normal cell division to cell division in which nondisjunction has occurred.

Notice the abnormal number of chromosomes in the daughter cells on the right. One daughter cell has 2 copies of the chromosome whereas the second cell is missing a copy. Some chromosomal abnormalities occur in the sex chromosomes. Trisomy X, for example, is a type of chromosomal abnormality in which a female has three X chromosomes (XXX). Females with trisomy X often have learning disabilities and may be taller than normal, but they do typically undergo normal sexual development and are able to conceive children. Turner syndrome, on the other hand, is caused by a female only receiving one complete X chromosome and can result in infertility and other health problems. Both Trisomy X and Turner syndrome are examples of genetic diseases caused by nondisjunction.

Inversion Inversion occurs when part of a chromosome breaks off and reattaches in the reverse direction. Mutations

A change in the sequence of nucleotides in an organism's genetic material is known as a mutation. Background Mutations can occur randomly during DNA replication when base pairs are added, deleted, or substituted, or they can be caused by environmental factors, such as overexposure to radiation or toxic chemicals. Mutations can occur in any cell in an organism's body, but they will be passed on to an organism's offspring only if the mutation occurs in the sex cells (gametes) of the organism. If a mutation occurs in the body cells of an organism, such as skin cells, bone cells, muscle cells, and nerve cells, it cannot be passed on to potential offspring. These mutations can only be passed on to the mutant body cells' daughter cells (cells that are produced when the mutant cell divides). Sometimes mutations create changes in an organism's appearance or behavior. Some of these changes may be beneficial; other changes may be detrimental. And sometimes mutations have no effect on an organism at all. Point Mutations A point mutation is a mutation in a single base pair in a strand of DNA. Some genetic diseases, like cystic fibrosis, color blindness, hemophilia, and sickle cell anemia, can occur as a result of a point mutation. The image below demonstrates an example of a point mutation. Notice how the substitution of a single base pair results in a different amino acid. The remainder of the protein sequence remains unchanged.

Silent Mutation A silent mutation is a specific type of point mutation. Because many amino acids have more than one codon, it is possible for a mutation in a single base pair to have no effect on the polypeptide sequence. The example below shows how some mutations are silent, meaning they have no effect on the translated protein. Though the second codon, or triplet code, has changed, the amino acid sequence of the resulting protein is the same as that coded for by the normal mRNA strand. Nonsense Mutation A nonsense mutation changes an amino acid codon into a stop codon. This causes the normal polypeptide sequence to be shorter.

The image above shows how proteins are truncated by nonsense mutations. Frameshift Mutations A frameshift mutation is a mutation that causes the reading frame of a codon sequence to be shifted. Since a codon is a sequence of three nucleotides that code for a specific amino acid, any insertion or deletion of nucleotide base pairs that are not in multiples of three will cause a frame shift mutation. Insertions or deletions in multiples of three will cause a protein to be shorter or longer than normal, but the entire sequence of the amino acids will not be shifted. The image below shows how insertions and deletions affect the polypeptide sequence.

Chromosome Translocation Chromosome translocation is caused when material is exchanged between two chromosomes or part of one chromosome becomes fused onto another chromosome. Some human disorders, such as cancer, infertility, and translocation Down syndrome, are caused by chromosome translocation.

Heredity & Technology

Biotechnology is a branch of science that uses living organisms to manufacture food, drugs, or other products. History of Genetic Engineering Rudimentary biotechnology has been around for thousands of years.Whenever yeast is used to bake bread or whenever grapes, malt, or milk is fermented to make wine, beer, or cheese, someone is using biotechnology.

Another popular biotechnological technique that is frequently used by society is selective breeding. Using this technique, farmers or ranchers select certain crops or animals to breed based off of desirable traits (e.g. the strongest bull, the fastest horse, the most virus-resistant corn, the largest tomato, etc.).Although this technique has been around for centuries, Charles Darwin's studies on natural selection and Gregor Mendel's studies in genetics helped to explain the scientific mechanism behind this popular technique. Current Trends in Biotechnology Recent biotechnology methods, such as genetic engineering, gene therapy, and genetic counseling, require more advanced technology and a more advanced prior knowledge of natural systems. Current biotechnology methods include genetic modification (also known as genetic engineering, gene splicing, gene therapy, transgenics, or recombinant DNA technology), cloning, and DNA fingerprinting. All genetic modification techniques involve inserting, deleting, or substituting DNA segments into an organism's natural genomic material. Typically these modifications are made in order to somehow enhance the organism. For example, crops, such as tomatoes, corn, and cotton, or animals, such as cattle and sheep, can be genetically modified to be more resistant to diseases that often infect them. Genetic modification techniques require extensive knowledge of the organism's genome, however, because genes cannot be safely manipulated if the traits that they express are unknown. Even complete understanding of gene functionality cannot prevent problems from occurring. There are still risks involved with the introduction of genetically modified organisms. Engineered crops pose the danger of spreading and reproducing with wild types. In addition, the modifications may not necessarily work. Just as insects can become resistant to chemical pesticides, there is the same risk that they could become resistant to chemicals produced due to the insertion of genes from other organisms. In 1980, scientists began mapping the human genome. This project was completed in 2003. With the completion of this project comes the hope that genetic manipulation in humans might be able to cure certain genetic disorders. This process of replacing absent or faulty genes is known as gene therapy. The following video discusses the results of the Human Genome Project, their potential applications, and the possible implications of this knowledge on society. Impact of Genomics

The Human Genome Project was a 13-year-long research effort which included scientists from several countries around the world. The scientists working on the Human Genome Project were attempting to identify all of the approximately 20,000-25,000 genes in human DNA and find the sequences of the 3 billion chemical base pairs that make up human DNA. The scientists completed the project in 2003.

The directors of the Human Genome Project are hoping that the information gained from the project will help other scientists develop new forms of biotechnology. Knowing more about the human genome may help improve technology in the following fields:  Molecular medicine - involves the diagnosis and treatment of different genetic diseases.

 Microbial genomics - involves the use of genetically engineered bacteria for bioremediation, pollutant detection, the production of biofuels, and more.

 Bioarchaeology and anthropology - involves the study of human evolution and migration.

 DNA forensics - involves the use of DNA to help solve crimes, including the use of a DNA fingerprint (the unique pattern of dark bands on photographic film that is made when an individual’s DNA restriction fragments are separated by gel electrophoresis, probed, and then exposed to an X-ray film).

 Agricultural genetic engineering - involves the genetic engineering of pest- or drought-resistant crops, healthier farm animals, biopesticides, crops containing life-saving vaccines, and more. As you can see from the list above, many of the applications of the Human Genome Project involve genetic engineering. Genetic engineering is the process of manipulating genes for practical purposes. Genetic engineering often involves the use of recombinant DNA, which is DNA made from two or more different organisms. DNA Fingerprinting

One of the ways scientists judge whether two species or two individuals are related is through the study of the organisms' genetic sequence, or DNA profile.

While each species or individual has its own unique DNA sequence, sequences of related organisms will be more similar than sequences of unrelated organisms.

In this example of a DNA profile, species 2 and species 4 are more closely related to each other than to the other species. This can be determined by comparing the location of the bands in the different samples. The more band locations the organisms have in common, the more likely it is that the organisms are related.

Genetic Engineering

Through the process of genetic engineering, the genetic code of plants, animals, or microorganisms is manipulated to obtain a desired product. Genetic engineering has many practical applications in medicine, agriculture, and biology. Potential Applications Examples of the potential applications of genetic engineering include:  using bacteria with recombinant DNA to produce medicines, such as insulin;

 inserting functioning genes into the cells of individuals with mutated genes to cure genetic disorders;  modifying the genetic makeup of certain crops to make them more nutritious or more resistant to bacteria;

 genetically altering fruit to contain vaccines so that they can be administered more cheaply and easily; and,

 using bacteria that have been modified to consume toxins to clean up spilled pollutants in the environment. The Process Genetic engineering can be done using recombinant DNA technology.Using this technology, different enzymes can be used to cut, copy, and move segments of DNA. Characteristics produced by the segments of DNA can then be expressed when these segments are inserted into new organisms, such as bacteria. The diagram below shows the process of recombinant DNA technology. Gene Therapy

The genetic techniques developed have opened the door to a variety of applications. One such application is the ability to alter an individual’s genes through gene therapy.

During gene therapy, specific gene sequences are inserted into an individual's cells to replace a defective or mutant allele. Scientists have found that the most efficient and effective way to accomplish this is by using viruses to insert the gene sequences into cells. This is possible because all viruses naturally insert their genetic material into their host cells as part of the viruses' replication cycle.

Image courtesy of NIH and the National Human Genome Research Institute.

Gene therapy is currently being used to try to cure genetic disorders, such as sickle cell anemia and cystic fibrosis. During this process, specific DNA sequences are inserted into an individual to try to replace faulty or absent genes so that normal gene expression can occur rather than abnormal gene expression.

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