Ch 7: Meiosis

Lecture Notes and Concepts

Both mitosis and meiosis…

1. begin with a diploid (2n) cell

2. undergo the PPMAT stages and cytokinesis

Differences between miosis and mitosis…

Mitosis

1. only undergoes one round of division

2. produces 2 genetically identical diploid (2n) cells

Meiosis

1. undergoes two rounds of division

2. produces 4 genetically unique haploid (1n) gametes

Chromosome definition

long strand of DNA

Replicated chromosome defintion

after S phase, chromosomes are now composed of two identical copies called sister chromatids

What are homologous chromosomes?

a pair of matching chromosomes, one inherited from each parent (contain the same genes, though may have different versions)

Humans are diploid, meaning…

we have two sets of chromosomes per somatic cell (body tissue cell)

• 23 homologous pairs = 46 total chromosomes

Gametes are haploid, meaning…

each gamete (eggs and sperm) only contains one copy of each chromosome

Gametes vs. somatic cells

• Gametes: created during meiosis in animals

• Somatic cells: created during mitosis in animal cells

Prophase vs. Prophase I

1. homologous chromosomes pair to form tetrads

2. crossing over occurs between homologs at chiasmata

   1. sections of DNA are exchanges (increases genetic variation)

   2. crossing over can occur at any point along the length of the homologous chromosomes —> this exchange is very important in producing genetic diversity

What is genetic recombination?

rearrangement of DNA sequences by breaking and rejoining chromosome segments

Metaphase vs. Metaphase I

homologous pairs are attached at chiasmata, so they line up together

Anaphase vs. Anaphase I

Homologous chromosome pairs are pulled apart —> centrosomes do NOT separate, so each chromosome still contains two sister chromatids

Telophase vs. Telophase I

1. same process for both

2. but, meiosis I results in cells containing half the number of chromosomes as the parent cell (one copy of each homologous pair) and those chromosomes are replicated

Prophase II

1. follows cytokinesis

2. meiosis II is almost identical to mitosis

   1. however, the starting cells are haploid (not diploid)

   2. the resulting daughter cells from meiosis will only get one copy of each chromosome

Metaphase II

replicated chromosomes are lined up individually along the equatorial plate by the spindle

Anaphase II

the chromosomes are pulled in half; daughter chromosomes are segregated into separate cells during Telophase II and cytokinesis

Product of meiosis

four genetically distinct haploid (n) gametes

• number cells increases from 1 to 4

• number of chromosomes per cell decreases 6 to 3 (for example)

• every cell has a unique combination of genetic material

What is aneuploidy?

an abnormal number of chromosomes in a cell

What is nondisjunction?

homologous chromosomes or sister chromatids fail to separate during anaphase

What is the result of nondisjunction?

• one daughter nucleus has an extra chromosome

• one is missing a chromosome

What causes aneuploidy?

nondisjunction

Mitotic aneuploidy common occurs in…

cancer cells

Example of nondisjunction in mitosis

sister chromatids fail to separate —> one cell lacks a red daughter chromosome, and the other has an extra

Example of nondisjunction in meiosis

homologous pair fails to separate in Anaphase I —> results in 4 gametes with 2 missing chromosome (n-1) and two with extra copy (n+1)

• this can also occur in Anaphase II, with sister chromatids not separating

Nondisjunction in meiosis in sperm

if those sperm fertilize a normal egg..

• two monosomic zygotes (2n-1) —> missing chromosome

• two trisomic zygotes (2n+1) —> extra chromosome

Most human embryos from aneuploid zygotes…

do not survive

What is Trisomy 21?

down syndrome; one of the few viable aneuploidies

Down syndrome karyotype?

only chromosome 21 is trisomic (extra copy), while all others are diploid; people with down syndrome have 47 chromosomes instead of 46

What is polyploidy?

organisms with triploid (3n), tetraploid (4n), and even higher numbers

How does polyploidy occur?

1. failure of the spindle to form

2. failure of cytokinesis

3. polyspermy = more than one sperm fertilizes the egg

• common in plants and fungi but rarely beneficial in animals

Polyploidy is important in…

agriculture!

What would happen if cell division was unregulated?

1. Unicellular: population growth and crash (overuse of resources)

2. Multicellular: cells might not benefit the organism as a whole (could lead to cancer or non-functional tissues)

How is cell division regulated in eukaryotes?

highly regulated by internal signals; 4 checkpoints ensure that the DNA is replicated and passed on properly

1. G1 checkpoint: requires sufficient resources within the cell, lack of DNA damage, and external signals

2. S checkpoint: must be no DNA damage and completed replication

3. G2 checkpoint: must be no DNA damage

4. M checkpoint

How are checkpoints for regulation regulated?

by cyclin-dependent kinases (CDKs)

• CDKs are activated by binding to cyclin

• cyclins are synthesized when the cell received signals telling it that is can pass through a checkpoint

• CDKs phosphorylate checkpoint proteins that regulate these checkpoints

Regulation via CDKs

1. specific cyclin produced in response to cell signals

2. cyclin binds to CDK

3. specific proteins attach are are phosphorylated

4. phosphorylated proteins then regulate the cell cycle

retinoblastoma protein (RB) blocks cell cycle progression until phosphorylated

Ways cells can die…

1. necrosis

2. apoptosis

Necrosis

cell is damaged or starved of oxygen/nutrients

1. cell swells and bursts, causing inflammation

2. when an organism dies, all of its cells die via necrosis

Apoptosis

genetically programmed cell death —> beneficial cell death for the organism if the cell is:

1. malfunctioning

2. infected

3. no longer needed

4. reached its Hayflick limit

   1. most human somatic cells can only divide 40-60x

Cells that lack a Hayflick limit…

1. stem cells: cells that can develop into many different specialized cells; important for growth and repair of tissues

2. cancer cells: malignant cells that divide without restraint; do not require external signals or respond to cell cycle checkpoints

What cell cycle regulation mutations can lead to cancer?

all can result in unregulated cell division, which can lead to cancer

1. cells produce mutated CDKs that do not need cyclins

2. cells produce cyclins w/o cell division signals

3. cells lack functional checkpoint proteins

4. cells lack functional apoptosis machinery