9Chapter 9

Introduction to Genetics:

One Cell Becomes Two:

Mitosis and Cytokinesis

Why do cells divide?

• Growth

• Repair/regeneration

• Reproduction– asexual

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.

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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 ChromatidsEach strand is an identical copy of the other one

CentromereWhere the two chromatidsAre 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 protein5.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 = 38

2n = 78

2n = 94

2n = 42

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

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.

Prophase

Mother cell

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.

Occurs after nucleus has been duplicatedOccurs after nucleus has been duplicatedBegins in anaphase in animal cells Begins in anaphase in animal cells

by the formation of a by the formation of a cleavage furrowcleavage furrow

Begins in telophase in plant cells Begins in telophase in plant cells

by the formation of a by the formation of a

cell plate.cell plate.

Cytokinesis

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

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IISister chromatids are separated

Crossing over can occur

Homologous pairs are separated

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Gametogenesis

• OogenesisProduction of an

egg

One mother cell produces one egg cell and three polar bodies that die

• SpermatogenesisProduction 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 9.13, 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 brakes3. Cyclin-dependent kinases, act in linked chain of

protein activity

Skin cancer

The End