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Unit 1: DNA and the Genome Sub-Topic (1.3) Gene Expression

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Unit 1: DNA and the Genome Sub-Topic (1.3) Gene Expression Unit 1: DNA and the Genome Sub-Topic (1.3) Gene Expression Higher Biology Pupil Course Notes Unit 1: DNA and the Genome Sub-Topic (1.3) Gene Expression On completion of this subtopic I will be able to State the meanings of the terms genotype, phenotype and allele. State that the phenotype of an organism is determined by the proteins produced as a result of gene expression. This is affected by environmental factors acting inside and outside the cell. Know that only a fraction of the genes inside a cell are expressed. State that gene expression is controlled by the regulation of transcription and translation. State the differences between DNA and RNA. Describe the process of transcription. State that RNA polymerase is the enzyme responsible for transcription. It unwinds and opens up (unzips) the DNA strand to bring about the synthesis of an mRNA molecule. State that introns are non-coding regions within a gene and play no part in coding for a polypeptide. State exons are coding regions within a gene that have a part to play in coding for a polypeptide. Know that during RNA splicing introns (non-coding regions) within the primary mRNA transcript are removed and exons (coding regions) are joined together to form the mature transcript. Know that translation of mRNA into a protein takes place at the ribosome. Describe the process of translation. Describe the structure and function of mRNA, tRNA and rRNA. Different mRNA molecules are produced from the same primary transcript depending on which exons are included in the mature mRNA pranscript. State that many proteins can come from one gene as a result of alternative mRNA splicing. Know that proteins can be modified after the translation process by cutting and combining polypeptide chains or by adding phosphate or carbohydrate groups. Know that proteins have a three dimensional shape; peptide bonds link amino acids together in the polypeptide chain which then folds as a result of hydrogen bonds forming between amino acids. Cross connections may also form between amino acids in one or more chains to form the protein’s final structure. Know that the final three dimensional structure of the proteins allows it to carry out its function. Duncanrig Secondary LS 2016 page 1 of 17 Higher Biology Pupil Course Notes Prior Learning Unit 1.4 DNA and the production of proteins Each section of DNA which codes for a protein is known as a gene. Proteins are built from subunits called amino acids. A polypeptide is a chain of amino acids linked together. The sequence of bases on the DNA molecule encodes information for the sequence of amino acids in proteins. The sequence of the bases on the DNA molecule therefore determines the function of the proteins they code for. Messenger RNA (mRNA) is a single stranded molecule which carries a complementary copy of the code from the DNA, in the nucleus, to a ribosome. The ribosome is the site of protein synthesis. Proteins are assembled from amino acids at the ribosome. 1. Gene Expression Genes are DNA sequences that code for particular proteins. The instructions contained within the DNA molecule must be interpreted and translated into proteins that carry out actions in the cell. Although every individual will have genes for the same proteins, there may be different forms of the gene, these are called alleles. In humans, since you have two copies of each chromosome you will have two copies of each gene. The copies may not be in the same form. The phenotype of an organism is determined by the proteins produced as a result of the genes expressed. This is influenced by environmental factors acting inside and outside the cell. Only a fraction of the genes in a cell are expressed. State the meaning of the word phenotype: ___________________________________________________________________ State the meaning of the word genotype: ____________________________________________________________________ Duncanrig Secondary LS 2016 page 2 of 17 Higher Biology Pupil Course Notes Gene Expression through protein synthesis A protein’s exact molecular structure, shape and ability to carry out its function depends on the sequence of its amino acids. The order of the amino acids is determined by the sequence of bases on the DNA molecule. Therefore, the information for the manufacture of a protein has to be carried from the nucleus to the ribosome where proteins are synthesised. This is carried out by a nucleic acid called ribonucleic acid (RNA). RNA is a single strand of RNA nucleotides. They are similar to DNA nucleotides but the sugar is a ribose sugar rather than a deoxyribose sugar and one of the bases is different- Uracil replaces thymine. Complete the diagram of the RNA nucleotide below: - Bases: Complete the table below to summarise the differences between a DNA and an RNA molecule: DNA RNA Found in Number of strands Sugar in nucleotide Adenine paired to Duncanrig Secondary LS 2016 page 3 of 17 Higher Biology Pupil Course Notes 2. The Genetic Code As we have learned, the order of the amino acids in a protein is determined by the sequence of bases on the DNA molecule. The sequence of the bases along the strand represents a sequence of ‘codewords’. This is called the genetic code. Each group of three bases on the DNA molecule, or triplet, is called a codon. These three letter codes are designated by the code letters of the bases e.g. CGC, AAT depending on the order of the nucleotides. Each amino acid is coded for by one or more triplets of bases. The triplet code allows for 64 combinations which is more than enough to code for 20 amino acids. Gene expression is controlled by the regulation of transcription and translation. 3. Transcription Transcription is the first step in gene expression. In this process a molecule of mRNA is formed using the DNA as a template. This occurs in the nucleus. An enzyme called RNA polymerase attaches to a sequence of DNA called the promoter. It moves along the DNA unwinding the double helix and opens up (unzips) the DNA molecule breaking the hydrogen bonds holding the base pairs together. Free RNA nucleotides align with the complementary base pairs on the DNA moving from 3’ to 5’. RNA nucleotides are held in place by hydrogen bonds while strong covalent bonds form between the phosphate of one nucleotide and the 3’ carbon of the adjacent nucleotide. The molecule lengthens until a terminator sequence of nucleotides is reached on the DNA strand and the RNA polymerase enzyme is released. As a result of the base pairing rule, the mRNA strand gets a nucleotide sequence complementary to one of the two DNA strands. The mRNA strand becomes separated from the DNA template. This is known as the primary transcript of mRNA. Duncanrig Secondary LS 2016 page 4 of 17 Higher Biology Pupil Course Notes Modification of the primary transcript Eukaryotic organisms have long stretches of DNA that exist within a gene that do not play a part in coding for a polypeptide chain. These non-coding regions are called introns. These are located between coding regions called exons. This means that the region in the primary transcript responsible for coding for the polypeptide is fragmented. RNA splicing The primary transcript needs to be modified in a process called RNA splicing. The introns are cut out of the primary transcript and the exons are spliced together to form a mature mRNA strand with a continuous sequence of nucleotides. The mature mRNA passes out of the nucleus and enters the cytoplasm. Once in the cytoplasm ribosomes and another RNA molecule called transfer RNA (tRNA) work together to translate mRNA into a polypeptide chain. Duncanrig Secondary LS 2016 page 5 of 17 Higher Biology Pupil Course Notes RNA splicing Mature mRNA transcript Complete the diagram below by: 1. labelling the template strand of DNA. 2. adding the missing complementary base-pairs on the DNA strand. 3. labelling the mRNA strand and adding the complementary base pairs. 4. labelling the sugar phosphate bonds between adjacent nucleotides on the mRNA molecule. G T A T C C A T C A C C G G T A T C T A A C Duncanrig Secondary LS 2016 page 6 of 17 Higher Biology Pupil Course Notes Answer the questions that follow: 1. How many bases in the genetic code are needed to code for one amino acid? 2. Complete the diagram to show the mRNA strand that would be transcribed from the section of DNA in the diagram below. ATA TCG CGA CCT TGA DNA strand mRNA stand 3. Name the enzyme that would direct this process. 4. State the difference between an exon and an intron. 5. Which of these is removed during modification of the primary transcript of mRNA? 6. State the name of the process in which this occurs. Ribosomes Translation of mRNA into a protein takes place at the ribosome Duncanrig Secondary LS 2016 page 7 of 17 Higher Biology Pupil Course Notes Ribosomes are small spherical structures found in all cells. They can be found free in the cytoplasm or attached to endoplasmic reticulum. They are formed from a type of RNA called ribosomal RNA (rRNA) and protein. They contain enzymes required for protein synthesis. Ribosomes allow mRNA and transfer RNA (tRNA) to come together during a process called translation. 4. Translation Translation is the process that translates the mRNA into a polypeptide. The ribosome reads the information carried by the mRNA molecule as three letter ‘codewords’. Each group of three bases on the mRNA (or triplet) is called a codon. Remember! The information contained in the triplet of bases on the mRNA molecule is complementary to a triplet of bases on the original DNA molecule.
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