Lecture 6 - Meiosis (Chromosomes and Cell Division)

What is Cytogenetics?

Study of chromosomes w/ relation to cell behaviour

What is Chromatin?

DNA + proteins (Histones)

What is the relationship between DNA and Histones?

DNA wraps around histones to form nucleosomes

What is Euchromatin?

Loosely packed chromatin

What is Heterochromatin?

TIghtly packed chromatin

How is Chromatin organised?

Organised into fibres

How does compaction affect Chromatin?

Affects whether genes in a region of DNA are in/active

What is the Telomere?

Protective caps to stop DNA degrading in Replication

What is a Chromatid?

1 copy of replicated DNA

What happens before Cell Division?

Chromosomes replicate

What is the purpose of Chromatids during Cell Division?

2 identical copies (chromatids) remain attached at centromere

What happens after Cell Division?

Each daughter cell has 1 chromatid

What are Diploid Organisms?

2 complete sets of each chromosome, 1 from each parent

What is a Haploid?

Half a Diploid

What must Gametes be?

Haploid

What is Meiosis?

Gamete formation

What is Mitosis?

Growth and repair

What is the issue with Mitosis in Sperm Generation?

Zygote of generation 1 would have 4 copies, gen 2 would have 8, haploid needed

Describe the 3 stages of Meiosis?

• Replication of chromosomes

• Separation of Homologous pairs

• Separation of Chromatids

Describe the Replication of Chromosomes in Meiosis?

• Diploid Parent

• Replication

• Result

• Homologous pair of chromosomes in diploid parent cell

• Chromosomes replicate

• Homologous pair of replicated chromosomes

Describe the Separation of Homologous Pairs in Meiosis

• Chromosomes

• Haploid

• Homologous chromosomes separate

• Haploid cells w/ replicated chromosomes

Describe the Separation of Chromatids in Meiosis

• Sisters

• What do they form?

• Sister chromatids separate

• Haploids w/ unreplicated chromosomes

What happens before Meiosis 1?

Interphase

Describe Interphase

Chromosomes duplicate, DNA replication occurs here

Describe Prophase 1

• Condensation

• Chromosomes

• Condensation of chromosomes

• Homologous chromosomes pair and exchange

Describe Metaphase 1

Tetrads line up

Describe Anaphase 1

• Splitting

• Poles

• Pairs of homologous chromosomes split up

• Pulled apart to opposite poles

Describe Telophase 1 and Cytokenesis

• Haploids

• Chromosomes

• 2 haploids form

• Chromosomes are still double

What does Meiosis 2 involve?

Prophase 2, Metaphase 2, Anaphase 2, Telophase 2 & Cytokenesis

What happens in Meiosis 2?

2nd round of division, sister chromatids separate

What is the result of Meiosis 2?

4 Haploid Daughters, containing single Chromosomes

What are the possible types of haploid daughters?

100% paternal, 100% maternal, cross of both

How does Meiosis increase Genetic Diversity?

Produces new combos of genes in gametes

How does Independent Assortment produce new combos of Genes in Gametes?

Independent assortment of maternal/paternal chromosomes into gametes

How many pairs of Chromosomes are formed in Meiosis?

23, 2²³ Combos

How does Crossing Over produce new combos of Genes in Gametes?

Exchange of maternal and paternal DNA during Prophase 1

What is Aneuploidy?

Loss/gain of a Chromosome

Are most cases of Aneuploid survivable?

No

What is Non-Disjunction?

Chromosomes don’t separate properly in Meiosis 1 or Sister Chromatids don’t in M2

What would be the products if Non-Disjunction happened in Meiosis 1?

n+1, n+1, n-1, n-1

What would be the products if Non-Disjunction happened in Meiosis 2?

n+1, n-1, n, n

What is the role of Trypsin Enzymes?

Partially digest chromosomal proteins

What is the role of a Giemsa Stain?

Produces dark and light bands

What is the name of a Dark Giemsa Stain?

AT rich

What is the name of a Light Giemsa Stain?

GC rich

What is a Karotype?

Collection of chromosomes in a cell

How do Karotypes work?

Arrange chromosomes by size, morphology (shape) & bonding to make a karyogram

How many Chromosomes are there?

46, 23 Pairs

What are the 2 types of Chromosomes?

22 pairs of autosomes, 1 pair of sex chromosomes

How do Cytogenetics use Karotypes?

To identify extra or missing chromosomes

What is the 1 case where Aneuploidies are survivable?

Chromosome 21

How many copies of Chromosome 21 are there?

3

How does the mother’s age affect aneuploidy in live births in Trisomy 21?

• < 1 in 2000 in u30

• > 1 in 100 in 30+

What is Trisomy 21 (Down Syndrome)?

Aneuploidy in Chromosome 21

What are the effects of Down Syndrome?

• Growth

• Disabilities

• Immunity

Physical growth delays, intellectual disabilities, poor immune function

What causes Down Syndrome?

Excess gene products from Chromosome 21 relative to genes they interact with

How do Sex Chromosome Aneuploidies relate to other types?

More survivable

What are the different Aneuplodies in the XY Karotype?

• Loss of XY - Turner Syndrome

• Gain of Y - XYY

• Loss of X - Fatal

• Gain of X - K Line Felter Syndrome

What are the different Aneuplodies in the XX Karotype?

• Loss of X - Turner Syndrome

• Gain of X

What is Polyploidy?

Extra set of all Chromosomes

What are some different types of Polyploidy?

Diploid - 2n, Triploid - 3n, Tetraploid - 4n, Hexaploid - 6n

What type of Polyploidy is very common in Plants but rare in Animals?

Hexaploid

What animal is tetraploid?

Red Viscacha Rat - 102 chromosomes