Larger Cells Have More Demand Put on Their DNA

Cell Reproduction C-10-1

Living things get larger because there are more cells produced - not because the cells get bigger!

Reasons: ~ larger cells have more demand put on their DNA ~ larger cells have more trouble moving materials across the cell membrane

DNA overload - if a cell gets too big, there may not be enough info on the DNA to meet the cells needs! ie: library in a small town - everyone can check out a book with no problem - if the town gets bigger, but the library stays the same, not enough books for everyone - people have to wait!

Exchanging materials - the rate at which food and waste is brought across the membrane depends on the surface area of the cell - the rate at which the food and waste is produced depends on the volume of the cell ex: if a town gets bigger, but the roads stay the same, it's harder for people to get around town - same applies to cell materials in a large cell!

So before a cell gets too big... it divides!

Chromosomes! Ch. 14

Chromosome - rod shaped structure that forms when a single DNA molecule and its associated proteins coil tightly before cell division *Made up of genes which make up DNA - segment of DNA that carries info from the parent to the offspring

*Usually the DNA strand is uncoiled when in use. *When the cell prepares to divide, the DNA coils up and compacts for easier division

DNA looks like this:

Chromatid - one of a pair of strands of DNA that make up a chromosome *DNA replicates itself before cell division so each new cell will have the same genes The chromatids are joined in the center by a centromere made of protein.

They look like this:

We have 23 chromosomes! *body cells have 2 of each type of chromosome (diploid) *the 2 copies of each type are homologous chromosomes *one copy from Dad, one from Mom Ex:

*sex cells (gametes) only have 1 of each (haploid)

When a sperm fertilizes and egg, a zygote is formed! The new cell is now diploid!

Chromosomes affect development: *Most babies with less than 46 die before they become a fetus. * A person can live with more…like 47: ~ Down Syndrome = has extra chrom. #21 ~ trisomy – extra chromosome in a 2n cell ~ can determine this with a karyotype – look at a map of the chromosomes… Features of Down Syndrome:  short stature  round face  upper eyelids that cover the inner corner of the eyes  some degree of mental retardation

Can happen to any race!

Usually seen born to mothers older than 35 years but can happen to anyone. ` Due to the eggs’ DNA deteriorating over time and chrom. not separating properly

Chrom. # 21 does not separate at time of cell replication = nondisjunction ie: one cell gets both chrom’s and the other gets none.

Can test through amniocentesis or chorionic villi sampling:  are very invasive tests  can sometimes cause premature labor  use needles to obtain amniotic fluid or a sample of the villi from the placenta and make a karyotype to look for genetic abnormalities

Alterations in chromosome structure: Mutations – changes in an organism’s genetic material

Types of mutations: 1.) Deletion – piece of chrom. breaks off and is lost when the cell divides

2.) Duplication – piece could attach to its homologous chrom. and then the new cell will have 2 copies

3.) Inversion – piece reattaches itself to the original chrom. in the reverse order

4.) Translocation – piece may attach itself to a nonhomologous chrom.

Chromosomes determine your sex Autosomes – the 22 pairs that are NOT sex chromosomes

Sex chromosomes are the only pair the determine whether you’re female or male! 2 forms = X and Y The presence or absnce of the Y chromosome is what determines the sex…this is because the hormone that makes a male is located on the Y

 Without it, the organism is female!

With us… Females = XX Males = XY

Since females only have an X to donate to their offspring, sex is always determined by the males.

Some oddities may arise like these: XXY = Klinefelter’s syndrome = male will be strerile  affects genital development  may have slight breasts  little body hair  long limbs

XYY = male is fertile (no syndrome name) XXX = Triplo-X syndrome = female is fertile

Mitosis and cell division! C-10-2

The simplest form of cell division is done by bacteria – binary fission *This is an asexual process that produces identical offspring

Bacteria: have no nucleus and no membrane bound organelles - DNA is circular and attached to the inner plasma memebrane - cell division has 2 steps: o copy DNA o divide the cell

It looks like this:

In eukaryoteic cells, the process is more complicated.  undergo the Cell Cycle – sequence of growth and division

In 5 Phases: G1 – S – G2 – M – C

1.) G1 phase – growth phase, takes place between divisions 2.) S phase – DNA is copied , at end you have a chromosome with 2 chromatids 3.) G2 phase – growth, mitochondria and other organelles replicate, microtubules reassemble ( form spindle apparatus) 4.) M phase – Mitosis occurs! a. nucleus divides into 2 nuclei: each with same # chrom. 5.) C phase – cytokinesis = cytoplasm divides

* The first 3 phases are collectively called Interphase ~ this is what the cell spends most of its life in

* When interphase ends, Mitosis now begins (M phase) Here’s what takes place now…

Prophase: * chrom. begin to condense * enzymes break down nuclear envelope * cylindrical centrioles start to separate and move to opposite ends of the cell – called “polar bodies” * protein fibers from centriole form spindle fibers – made of microtubules **NOTE! Plants do NOT have centrioles!**

Metaphase: * chrom. keep condensing * chrom. line up in the center of the cell – “equator” * more microtubules extend out of the centromere at the kinetochore – hold the chroms in place * one chromatid is attached to one centriole, one to the other

Anaphase: * centromere divides and the 2 chromatids separate * the new chroms move to opposite sides as the spindle fibers shorten

Telophase: * chroms uncoil at either end of cell *new nuclear envelope forms * spindle fibers break down and disappear

Cytokinesis begins – cell is cleaved in half * cell membrane grows to close both * done by a belt of protein threads

In plants… golgi bodies form a cell plate at the center of the cell to build a new cell wall - once the wall is complete, the cell separates!

Regulating the Cell Cycle C-10-3

Some cells divide often while others stop dividing once they are grown. Controls on Cell Division: - place cells in a nutrient broth in a petri dish - cells will grow until they cover the bottom - when they touch, they stop growing - if remove cells from center, cells will grow until the gap is closed! - shows that cell division can be turned on and off

* same happens in the body when a cut or injury happens! ~ cells divide to until they close the gap - then slows down and returns to normal

Checkpoints: Cell inspects itself prior to cell division to make sure everything is good to go!

Several checkpoints... 1.) G1 checkpoint - checks surroundings before copying DNA if good - go to S phase if bad - rests and waits

2.) G2 checkpoint - checks for mistakes in DNA copying and makes sure cell is big enough to divide

3.) Mitosis checkpoint - checks at metaphase to see if chrom. are lined up correctly and attached to spindle fibers ~ ensures DNA is evenly split between daughter cells

Cancer ~ uncontrolled division of cells ~ can cause tumors which invade and damage healthy tissues

* Caused by loss of control of DNA ~ if division isn't regulated, damaged DNA could be copied and therefore cause more defective cells to be produced - causes tumors * Tumors - 2 kinds ~ benign - does not spread ~ malignant - can spread - called metastasis - invades and destroys healthy nearby tissues

* Treatment ~ drugs kill cancerous cells - chemotherapy ~ radiation therapy - high energy rays focus on the cells and kill them

* Prevention ~ avoid cancer causing agents! - UV rays - cigarettes / smoke