Mitosis, Meiosis and Chromosomal Basis of Disease

Question 1

Cell Cycle Phases

Answer 1

G1- growth
S- synthesis (DNA replicates –> 2 sister chromatids attached at centromeres)
G2- growth/prep for mitosis
M- mitosis (meiosis in germ cells); cytokinesis

**Transitions regulated by cell cycle checkpoints

Question 2

Stages of Mitosis

Answer 2

Prophase- chromosomes condense
Prometaphase- nuclear envelope breaks down and individual sister chromatids attach to spindle fibers
Metaphase- sister align in center of cell on metaphase plate
Anaphase- release of sister chromatid attachment and go to poles
Telophase- reform nuclear envelope and cytokinesis

Question 3

Male v Female Meiosis

Answer 3

Male- prod 4 sperm AND germ cells dormant until puberty

Female- prod 1 egg and other 3 products are polar bodies AND female born w/ complete set of oocytes

Question 4

How is variation in offspring ensured? (2 ways)

Answer 4

1- Independent assortment of chromosomes in Meiosis I

2- Crossing over in meiosis I (recombination)

Question 5

When does crossover recombination occur?

Answer 5

Prophase of meiosis 1

Question 6

Mechanism of crossover

Answer 6

• strands held together by synaptonemal complex

- Programmed dbl strand breaks THEN repaired by DNA damage repair pathway (use homolog as template)

Question 7

Why is crossover so important to meiosis?

Answer 7

-Crossover is required to hold homologs together on meiosis I spindle so w/o crossover the homologs would separate independently (not split evenly into 2 daughters)

Question 8

Nondisjunction (meiosis I and II)

Answer 8

• Generally… it is if chromosomes don’t segregate
• In Meiosis I… homologs do not separate –> 2 diploid gametes and 2 empty gametes
• In Meiosis II…sister chromatids do no separate –> 2 normal gametes, 1 diploid gamete and 1 empty gamete

Question 9

What does 2 sister chromatids in same gamete indicate?

Answer 9

-Nondisjunction in Meiosis II

Question 10

What does 2 homologs in same gamete indicate?

Answer 10

-Nondisjunction in Meiosis I

Question 11

Balanced Carrier

Answer 11

-Have complete chromosomal complement (all the info) but 2 chromosomes are not homologous
(reciprocal exchange of material b/n 2 non-homologous chromosomes)

-Not harmful to individual but will lead to problems when they go to have children

Question 12

3 Types of Translocations

Answer 12

1-Alternate –> 2 good gametes (1 normal 1 balanced carrier)
2-Adjacent –> 2 bad gametes
**Dep how the quadrivalent segregates
** So 2/3 chance infertility in balanced carriers

3- Robertsonian - fusion of small acrocentric chromosomes

Question 13

How do chromosomal insertions and deletions arise?

Answer 13

• Usually unequal crossing over in areas of REPEATS
• Also if balanced carrier of inversions and translocations has kids (adjacent translocation)

**If deletion…only the portion of chromosome w/ centromere will be stably inherited

Question 14

How does triploidy occur?

Answer 14

-2 sperms fertilize 1 egg

Question 15

2 Types Inversions

Answer 15

1- pericentric - include centromere (so easier to see b/c change in relative position of centromere)

2- paracentric - only on one arm (ID by changes in chromosomal landmarks)

**Paracentric more deleterious b/c lead to chromosomes breaking (broken chromosomes can then fuses to other parts of genome which wreaks havoc)

Question 16

Inversion Loops

Answer 16

• Formed when the unbalanced gametes of a inversion heterozygote carrier go to make chromosome pairs
• Either the normal DNA or the inverted DNA can end up w/in the loop (if inverted part in loop it does not participate in recombination BUT if normal gene in loop and inverted DNA outside loop…inverted DNA undergoes recombination –> deleterious effects)

Question 17

Isochromosomes

Answer 17

• mirror image of one chromosome arm w/ concomitant loss of other arm
• result in unbalanced karyotype w/ partial monosomy and partial trisomy

Question 18

Marker Chromosomes

Answer 18

• small unidentified chromosomal fragments seen in chromosome preparations
• seen in additional to normal complement of chromosomes (“supernumerary chromosomes”)
• not sure what their phenotypic consequence is
• Usually unstable b/c no centromeres but some mitotically stable

Question 19

Ring Chromosomes

Answer 19

• Type of marker chromosome where extra DNA has fused into circle
• Result of 2 dbl strand breaks in chromosome OR telomere dysfunction that is healed by nonhomologous end joining
• Usually only in some cells b/c not mitotically stable