Introduction to Genetics: One Cell Becomes Two: 9 Chapter 9 Introduction to Genetics: One Cell Becomes Two: Mitosis and Cytokinesis

Why do cells divide? Growth Repair/regeneration Reproduction asexual

This means the cell has twice as much DNA in it after replication Chromatin Invisible most of the time - Only visible during cell division (mitosis or meiosis) During S-phase of the cell cycle – the DNA replicates (makes an exact copy of itself) This means the cell has twice as much DNA in it after replication Once a chromosome has replicated, it shortens and thickens and can now be seen in our microscopes. See animation

Each descendant of a cell, in addition to requiring nutrients, cell membrane, and organelles, must have this information of DNA to survive. 1. Simple division would mean the new cells had half of what the old cell had. 2. Duplication of both cytoplasmic and nuclear contents preceeds division so that new cells have a complete set of everything. 3. Replication—duplication of DNA: Figure 9.5

When cells divide by mitosis, each daughter cell receives the same number of chromosomes as its mother cell has – 2n. In order to do this, the chromosomes must be copied first, then one of each copy is placed in the new cells. 46 46 46 46 46 92 46

II. DNA Is Packaged in Chromosomes (Section 9.3) A. Organization of these long pieces of DNA 1. DNA is divided into long strands wrapped around protein (chromatin). 2. Each strand is packaged and condensed into a single chromosome: Figure 9.6a. Why? Analogy of moving and packing boxes. You only need your stuff out when you are using it. When you move, you won’t be using it and you want it to take up less space: Figure 9.8

One Chromosome Sister Chromatids Centromere DNA Each strand is an identical copy of the other one Centromere Where the two chromatids Are attached to each other – This is different for each chromosome DNA

B. DNA is the key to reproduction, development, and maintenance. 1. Genome = complete collection of an organism’s genetic information as linked genes in a long strand of DNA.

3. Information is found in letters A, C, G, and T in the double helix: Figure 9.2 4. Path from DNA to protein 5. Humans have about 100,000 genes that have all the information to make all the proteins (especially enzymes) a cell needs.

3. Replication takes one chromosome and makes two identical copies, called sister chromatids: Figure 9.6b (Interactive Activity 3)

Chromosome Number Each species has the same number of chromosomes in all their cells that are made by mitosis. This is the diploid number (2n). In humans this number is 46. So cells of your skin and muscle and liver each have 46 chromosomes in them. Look how many chromosomes are in the cells of these creatures: 2n = 42 2n = 78 2n = 38 2n = 94

Sex Chromosomes Homologous in females: XX Not homologous in males: XY

A male karyotype: 22 pairs of homologous chromosomes; one pair of sex chromosomes

The cell cycle keeps record of progress of a cell over time, like a clock: See animation

1. The cell cycle is made up of a repeating pattern of growth, genetic duplication, and division. 2. Typical animal cell cycle lasts about 24 hours. 3. Two main phases: interphase and mitotic phase (about 30 minutes). 4. Interphase = G1 (gap 1 for growth, 12 hours) + S phase (synthesis, for replication of DNA, 6 hours) + G2 (gap 2, 6 hours): -animation of figure 9.9 available under the resources for this chapter

Mitosis Nuclear division resulting in nuclei identical to parent cell – asexual reproduction for some organisms. Begins after interphase Ends before cytokinesis Four phases: Prophase Metaphase Anaphase Telophase P M A T

1. Prophase (P for “plain to see”)—Chromosomes condense, nuclear envelope breaks down, formation of spindle fibers (microtubules) from the centrosomes. 2. Metaphase (M for “middle”)—Chromosomes are aligned on the equator by pushing along spindle with each sister chromatid facing opposite poles. 3. Anaphase (A for “apart”)—Sister chromatids separate; each new chromosome moves to the opposite pole. 4. Telophase (T for “two nuclei”)—Chromosomes de-condense, spindle breaks down, nuclear envelope forms around the two separate complements of chromosomes.

Mother cell Prophase

Prophase Chromosomes become visible Spindle forms from protein microtubules Nuclear envelope disintegrates Nucleolus disintegrates In animal cells, centrioles migrate to opposite ends of the cell (poles) and spindle fibers attach to them

Metaphase Chromosomes line up single file at the equator of the cell

Anaphase Sister chromatids are pulled apart toward opposite poles In animal cell, cleavage furrow begins to form

Telophase Nuclear membrane forms around each group of chromosomes Nucleolus reappears in each nucleus Spindle fibers disappear Chromosomes become invisible again as chromatin Cytokinesis begins in plant cell by formation of cell plate; cleavage furrow in animal cell completely separates the two nuclei into two different cells.

Cytokinesis Begins in telophase in plant cells by the formation of a Occurs after nucleus has been duplicated Begins in anaphase in animal cells by the formation of a cleavage furrow Begins in telophase in plant cells by the formation of a cell plate.

V. Variations in Cell Division (Section 9.5) A. Plant cells—Everything is similar except for cytokinesis because plant cells have to break down and reform the cell wall: Figure 9.12, animation available under the resources for this chapter 1. Vesicles fuse near the metaphase plate for form a cell plate that grows outward to form a cell wall.

Find the different stages of mitosis in these onion cells:

Meiosis Cell division producing cells that have half the number of chromosomes of the mother cell Produces gametes – eggs and sperm Occurs so that fertilization doesn’t increase the number of chromosomes in each generation.

Gametes = sex cells Eggs or sperm Have half the normal number of chromosomes = haploid (n) = 23 in humans Sexual reproduction needs these to combine DNA from two different parents, producing offspring that is different from each parent

Meiosis = Reduction Division Two complete cell divisions First cell division – separates homologous chromosomes (reduction of chromosome number) Second cell division – separates sister chromatids (like mitosis) - Division Produces 4 haploid cells

II M M E E I I O O S S Crossing over can occur Homologous pairs are separated Sister chromatids are separated

Gametogenesis Oogenesis Production of an egg One mother cell produces one egg cell and three polar bodies that die Spermatogenesis Production of sperm One mother cell produces 4 equally sized sperm cells

Fertilization The sperm unites with the egg forming a zygote (fertilized egg). The zygote then divides by mitosis to produce the trillions of cells that make up a multicellular body like yours.

B. Prokaryotes (no nucleus) binary fission: Figure 9 B. Prokaryotes (no nucleus) binary fission: Figure 9.13, animation available under the resources for this chapter

III. When Cell Division Runs Amok: Cancer (Sidebar) A. Unrestrained cell division—cancer 1. Mechanisms that induce cell division can become hyperactive. (carcinogens) 2. Mechanisms that suppress cell division can fail. (carcinogens) B. Genes 1. Oncogenes: stuck accelerator 2. Tumor suppressor genes: failed brakes 3. Cyclin-dependent kinases, act in linked chain of protein activity

Skin cancer

The End