Mol Lecture #21

Question 1

Meiosis (Overview process)

Answer 1

• Take a 2n cell and make 4 1n cells.
• Each of these cells will have 1 of each chromosome type.
  Note: Meiosis only occurs in germ cells (Gametogenesis)

Question 2

4 stages of meiosis:

Answer 2

→ Pre-meiotic interphase (DNA replication and growing cells) (G1, S phase, G2)
→ 1st meiotic division: paternal + maternal chromosomes (1 chromosome from each parent- chromosome versions separate) (Meiosis I)
→ Interkinesis- interphase w/o S phase
→ Second meiotic division (Meiosis II)- sister chromatids made in the first interphase will separate.

Question 3

Meiosis I (Important notes)

Answer 3

Essential: (Synapsis) Homologous chromosomes line up together
→ linked together via synaptonemal complex (tetrad)
- Genetic remcombination through homologous chromosomes→ results in additional genetic variation.
- Homologous connected in tetrad with kinetochore and spindles (different than what we saw in mitosis)

Question 4

Recombination

Answer 4

• By then, we have two new variants of this homologous pair
• Useful for genetic variation in sexual reproduction
• The crossover (chiasma) is another term for the genetic recombination event. (indicative of where it takes place)

Question 5

Separation of Chromosomes

Answer 5

• Anaphase is conducted similarly (to mitosis), but we move homologous pairs from one parent into different cells. (No longer diploid)

Question 6

The end of meiosis I:

Answer 6

• 2 cells (haploid cells- because they only have 1 version of each chromosome type). But have the same number of total chromosomes (DNA content is the same as when we began).

Question 7

Sex Chromosomes

Answer 7

• X and Y
• act like homologous chromosomes despite having different genes in different orders.

Question 8

2X v.s. XY

Answer 8

• 2X= homologous chromosomes
• XY= at like homologous chromosomes

Question 9

Meiosis II (Overview)

Answer 9

• Interkinesis: no new S phase
  (Alter what’s happening during cell division)
• The same number of chromosome types, but half the chromosome number.

Question 10

Similarity between meiosis II and mitosis

Answer 10

• Similar checkpoints in meiosis II

Question 11

Genetic variability introduced in Meiosis

Answer 11

• Independent assortment
  →Gives us the potential for 2^n combinations of gametes possible
  → for humans that’s 2^23 (chromosome types)= 8 million gamete types
• Crossing over
  →In humans, it is estimated that there are 2-3 crossover events per chromosome pair

Question 12

Independent Assortment

Answer 12

• random distribution of chromosomes from each parent

Question 13

Crossing Over

Answer 13

• genetic recombination between homologous chromosomes (can apply to many genes or 1 gene- not specified)

Question 14

Random fertilization

Answer 14

Individual ovum is fertilized by an individual sperm cell to give the zygote.
→ (about) 64 million possible combinations (without even thinking about crossing over)

Question 15

Meiosis and Non-disjunction (What is it, and what it produces)

Answer 15

• When homologous chromosomes in Meiosis I or sister chromatids in Meiosis II do not separate properly
  → daughter cells with non-normal numbers of chromosomes.
  → medical consequences
• This could happen in mitosis (there are mechanisms to handle it, though)
• The outcomes differ depending on meiosis I or meiosis II (sometimes there are normal number gametes- n)

Question 16

Gametogenesis

Answer 16

• Cellular mechanism in which gametes are formed
• Involved meiosis as the process of cell division
• Occurs in specialized cells called germ cells

Question 17

Gametogenesis in animals

Answer 17

• occurs in specialized reproductive organs called gonads
• Female gonads are called ovaries, and these produce female gametes called ova or eggs
• Male gonads are called testes, and these produce male gametes called spermatozoa or sperm.

Question 18

Spermatogenesis (Process Overview)

Answer 18

• Spermatogonium is the first cell to undergo meiosis to give us a secondary spermatocyte, then go through Meiosis II to create sperm.
• Loss of cytoplasm and construction of specialized cellular structures, such as the acrosome and flagellum
• We acquire 4 1n cells

Question 19

Oogenesis (Part I)

Answer 19

• We acquire the secondary oocyte in meiosis I and the first polar body
  → Cytoplasm and organelles are kept in the secondary oocyte.
  →First polar body does not undergo meiosis II
• Secondary oocyte undergoes meiosis II and acquires the second polar body
  →Both 1st and second polar bodies are degraded
  →And then we have 1 1n cell made

Question 20

Oogenesis and Mammalian Eggs

Answer 20

• Mammalian eggs usually complete prophase I (in meiosis I), and then wait until fertilization is done to complete meiosis (and make the egg) (alterations with specialized features)