Mol Lecture #21 Flashcards
Meiosis (Overview process)
- 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)
4 stages of meiosis:
→ 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.
Meiosis I (Important notes)
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)
Recombination
- 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)
Separation of Chromosomes
- Anaphase is conducted similarly (to mitosis), but we move homologous pairs from one parent into different cells. (No longer diploid)
The end of meiosis I:
- 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).
Sex Chromosomes
- X and Y
- act like homologous chromosomes despite having different genes in different orders.
2X v.s. XY
- 2X= homologous chromosomes
- XY= at like homologous chromosomes
Meiosis II (Overview)
- Interkinesis: no new S phase
(Alter what’s happening during cell division) - The same number of chromosome types, but half the chromosome number.
Similarity between meiosis II and mitosis
- Similar checkpoints in meiosis II
Genetic variability introduced in Meiosis
- 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
Independent Assortment
- random distribution of chromosomes from each parent
Crossing Over
- genetic recombination between homologous chromosomes (can apply to many genes or 1 gene- not specified)
Random fertilization
Individual ovum is fertilized by an individual sperm cell to give the zygote.
→ (about) 64 million possible combinations (without even thinking about crossing over)
Meiosis and Non-disjunction (What is it, and what it produces)
- 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)
Gametogenesis
- Cellular mechanism in which gametes are formed
- Involved meiosis as the process of cell division
- Occurs in specialized cells called germ cells
Gametogenesis in animals
- 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.
Spermatogenesis (Process Overview)
- 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
Oogenesis (Part I)
- 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
Oogenesis and Mammalian Eggs
- 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)
