Chromosomal Abnormalities (Megee L1)

Question 0

Two key differences between mitosis and meiosis

Answer 0

1. meiosis: pat. and mat. derived homologs pair at prophase I
   mitosis: pat. and mat. derived homologs segregate independently
2. meiosis: reciprocal recombination between mat. and pat. sister chromatids = chiasmata between homologs
   mitosis: recombination rare

(also, mitosis 2n –> 4n –> 2n
meiosis 2n-> 4n -> 2n -> n)

Question 1

Cell cycle - Mitosis

Answer 1

Interphase [G1 –> S –> G2] –> Mitotic [M] phase

Within M-phase: Prophase -> Prometaphase -> Metaphase -> Anaphase -> Telophase -> cytokinesis

Question 2

What is a chiasma (pl. chiasmata)? How many are there per homolog pair and how do they form?

Answer 2

a physical linkage between homologs that occurs during the onset of meiosis (prophase I) allowing for reciprocal recombination between homologs.
2-3 per homolog pair
each link requires two double-strand breaks in order to form

Question 3

At what points during cell division do chromosomal abnormalities arise and what are they?

Answer 3

Meiosis: Germ line mutations; genetic reassortment during meiotic crossovers

Mitosis: Somatic and Germ line mutations; mosaicism

Question 4

What are pseudoauosomal regions?

Answer 4

Regions of sex chromosomes that can cross over during male meiosis. That is, part of the Y chrom. can cross over with part of the X chrom.
Allows for proper alignment and segregation of these two chromosomes
Often appears as an autosomal pattern in a pedigree.

Question 5

What is a bivalent structure in and what is the structure that holds a bivalent together at the beginning of prophase I?

Answer 5

When mat. and pat. homologs become paired (synapsed) along their entire length.
Held together by synaptonemal complex (proteinaceous structure)

Question 6

At the end of prophase I what holds the bivalent together?

Answer 6

just the chiasmata since the synaptonemal complex has disassembled.

Question 7

When does recombination occur?

What is the total estimated variability this can produce?

Answer 7

prophase I of meiosis

80^23 woah

Question 8

What is the difference between sister chromatids and homologous chromosomes?

Answer 8

Sister chromatids are generated when a single chromosome replicates. Now there are two copies

Homologous chromosomes are two different copies of a chromosome (one from mom, one from dad)

Question 9

In meiosis, when do sister chromatids and homologous chromosomes separate?

Answer 9

Homologous chromosomes: Meiosis I; Anaphase I

Sister chromatids: Meiosis II; Anaphase II

Question 10

What is nondisjunction?

Answer 10

Abnormal segregation of homologues or sister chromatids.

Question 11

Metacentric, Submetacentric, Acrocentric?

Answer 11

Meta: centromere in the center
Submeta: centromere slightly off of center
Acro: centromere near one end

Question 12

Acrocentric is unique. Describe. Which human chromosomes are acrocentrei?

Answer 12

On the small end, they have a small bit of chromatin called a satellite attached to the centromere by a stalk.

13, 14, 15, 21, 22

Question 13

What is contained in the satellite and the stalk of an acrocentric chromosome?

Answer 13

Satellite region is many bp repeats.

Stalk contains DNA that encodes for rRNA (which, as we know, makes up a large portion of the ribosomal structre) as well as repeats.

Question 14

What is Giemsa?

Answer 14

Stain used to identify individual chromosomes. Produces an unique banding pattern on a chromosome.

Question 15

The short and long arms of a chromosome are labeled as what?

Answer 15

p = short arm (petite)
q = long arm

Question 16

What does “der” stand for?

Answer 16

derivative. when there is translocation we use “der” to designate where a piece of translocated DNA and centromere came from

Question 17

Aneuploidy and Polyploidy?

Answer 17

Aneuploidy = loss or gain of a particular chromosome (e.g. trisomy 21 or monosomy X)

Polyploidy = extra copies of all chromosomes (e.g. 3n, 4n)

Question 18

What are the three viable (survive to birth) trisomies and why are they (likely) to be such?

Answer 18

13, 18 and 21. These chromosomes contain the fewest gene-encoding regions of all the chromosomes. Only the Y chromosome has fewer genes located on it.

Question 19

Discuss the mechanisms that can produce polyploidy.

Answer 19

1. Triploidy: di-spermy or diploid egg or diploid sperm

2. Tetraploidy: DNA duplication but no cell division (endomitosis)

Question 20

List some factors leading to Aneuploidy.

Answer 20

Meiotic chromosome nondisjunction
* location of recombination too close to centromere or telomere
* reduced frequency of crossover (recombination) events
Maternal age > 35 years

Question 21

Mosaicism

Answer 21

presence of 2 or more genotypes in cells in a tissue derived from a single zygote.

Question 22

How many mitosis divisions do germ cells undergo before entering the final meiotic division?

Answer 22

about 30 - 50

Question 23

Two examples of somatic chromosomal errors that occur during development and lead to mosaicism.

Answer 23

1. 47, XX +21 / 46, XX mosaic trisomy 21 (Down’s syndrome)

2. 46, XX / 46, XY true hermaphroditism

Question 24

Somatic chromosomal mosaicism can lead to…

Answer 24

overexpression of oncogenes or loss of tumor suppressor genes

Question 25

How do the children of mosaic parents fare?

Answer 25

children will be fully affected.

Question 26

Common cause of mosaicism

Answer 26

nondisjunction in an early post-zygotic mitotic division.

Question 27

germ line mosaicism can result from…

Answer 27

mitotic nondisjunction in germ cell precursor

Question 28

Nondisjunction event in meiosis I will lead to…

Answer 28

100% abnormal gametes

2 are N+1 and the other 2 are N-1

Question 29

Nondisjunction event in Meiosis II can lead to…

Answer 29

50% abnormal gametes

2 are N (normal), the other 2 are N-1 (abnormal)