Lecture 2 - Intro to cytogenetics

Question 1

What are the 2 types of cell division? What are their resultant cell types?

Answer 1

Mitosis results in 2N cells (aka somatic cell division) and meiosis results in N cells (aka germ cell division).

Question 2

How does the chromosome number and DNA content change during mitosis? When do these changes happen?

Answer 2

During mitosis, the chromosome number always stays the same (2N), but the DNA content doubles (2C to 4C) during interphase. The DNA content returns to normal at metaphase when the cells divide and each gets a copy of DNA.

Question 3

How does the chromosome number and DNA content change during meiosis? When do these changes happen?

Answer 3

At interphase I, the cell doubles its DNA content from (2C to 4C). During anaphase I (splitting), the DNA content divides and is returned to 2C but the chromosome number is reduced from 2N to N. During the second meiotic division, both are reduced to their single part (N and C) resulting in germ cells.

Question 4

What is reduction division?

Answer 4

The first mitotic division (anaphase I) where the number of chromosomes is halved resulting in 2 N cells.

Question 5

What is recombination? When can it occur?

Answer 5

Recombination results from crossing over, when homologous chromosomes overlap and exchange portions of DNA resulting in reassortment. It can occur in prophase I.

Question 6

What is non-disjunction? What is the result?

Answer 6

Non-disjunction is the failure of chromosomes or chromatids to separate during anaphase. The result is cells that have an abnormal number of chromosomes.

Question 7

What is the difference between meiotic non-disjunction I and II?

Answer 7

Non-disjunction I happens during anaphase I and II happens in anaphase II. NDJ I will result in 2 heterodisomic cells and 2 nullosomic cells. NDJ II results in 2 normal cells, 1 heterodisomic cell, and 1 nullosomic cell.

Question 8

By what process do eggs develop? What is the result?

Answer 8

Eggs develop by oogenesis and it results in 1 egg cell and 2-3 polar bodies.

Question 9

Describe the timeline of oogenesis.

Answer 9

In the fetus the the egg cells go through prophase I and remain suspended in that phase until ovulation as primary oocytes. Upon ovulation, the oocytes complete meiosis I and are now known as secondary oocytes (the first polar body is formed here as well). The secondary oocyte gets fertilized and completes meiosis II, forming the zygote and another polar body.

Question 10

By what process do sperm develop? What is the result?

Answer 10

Sperm develop by spermatogenesis and it results in 4 equal gametes.

Question 11

What are the three main differences between oogenesis and spermatogenesis?

Answer 11

1. sperm can be made throughout life and eggs are all present at birth.
2. sperm can be made continually and eggs are matured only during ovulation.
3. the products are 4 equal sperm vs. 1 mature egg cell.

Question 12

How many chromosomes do humans have? How many pairs?

Answer 12

46 chromosomes, 23 pairs (22 autosomal and 1 sex pair).

Question 13

What two features does the Y chromosome have?

Answer 13

TDF - testis determining factor and SRY - sex determining region of Y.

Question 14

What region of sex chromosomes can participate in recombination?

Answer 14

Pseudoautosomal region. This region also helps hold the chromosomes together during meiosis.

Question 15

Describe female development.

Answer 15

Because females lack the TDF region (and a Y chromosome…) they will develop ovaries from differentiation of the Muellarian ducts. Also Wilffian ducts regress because they are not needed for male internal reproductive organs.

Question 16

Describe male development.

Answer 16

Because males have a Y chromosome, they have the TDF gene. The TDF causes degeneration (inhibition) of the Muellerian ducts via testosterone production by the developing testis. The testes develop by function of the Wolffian duct system.

Question 17

Is TDF the only sex determining factor needed?

Answer 17

No, genes transcribed on autosomal chromosomes helps determine sex characteristics too.

Question 18

How do females have the same enzyme production from X chromosomes? What is the result?

Answer 18

Females and males end up having the same gene transcription because females undergo X inactivation (this process is known as the Lyon hypothesis). It results in one functioning X and a Barr body.

Question 19

When does X inactivation happen? In what proportions does it happen?

Answer 19

X inactivation happens 3-7 days after fertilization. It happens randomly in equal proportions between mother and father Xs normally, but if an X is damaged, it may be preferentially inactivated.

Question 20

If the female is heterozygous, how does somatic mosaicism work?

Answer 20

Some cells will express the maternal X genes and some will express paternal X genes. An example is calico cats that express black fur with some cells and yellow fur with other cells.

Question 21

How are X chromosomes inactivated? What gene segments can escape inactivation?

Answer 21

Epigenetic modification via methylation especially at the XIST region (primary inactivation center). The pseudoautosomal region can escape inactivation to be present during meiosis so both Xs will be active.

Question 22

What are two types of chromosomal abnormalities?

Answer 22

Numerical - a change in the number of chromosomes and structural - a change in shape or size in one of the chromosomes.

Question 23

What specimens can be used to identify chromosomal abnormalities?

Answer 23

Bone marrow, tissue, blood (white cells only), amniotic fluid, chorionic villi.

Question 24

What three pieces of information can be gathered about a chromosome from a karyogram?

Answer 24

Chromosome size, centromere, and banding pattern.

Question 25

What is the unreplicated region in the middle of a chromosome called? What are its functions?

Answer 25

Centromere - it is the point of attachment for micotubules and divides the chromosome into arms.

Question 26

What are the names of the short and long arms of the chromosomes called?

Answer 26

The short arm is the p arm and the longer is the q arm.

Question 27

What is the chromosome called when the centromere is equidistant from both ends? When off-center? With modified short arms?

Answer 27

Metacentric, submetacentric, and acrocentric.

Question 28

What is a telomere?

Answer 28

Tandemly repeated sequences that stabilize the chromosome during replication. They shorter as we age due to loss during replication.

Question 29

What is chromosome polymorphism?

Answer 29

There are more than 1 functional structural forms of a chromosome. They are usually benign occurrences.

Question 30

How are banding patterns visualized? What stain do you use?

Answer 30

They are represented in an ideogram and the chromosomes are stained using Giemsa or Wright’s stain.