UNIT 3.3

Question 1

Meiosis 1

Answer 1

The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated

Question 2

Meiosis 1 - Phophase 1

Answer 2

DNA supercoils: chromatin condenses and becomes sister chromatids, which are visible under light microscope
Nuclear membrane dissolves
Homologous chromosomes form bivalents
Crossing over occurs
Spindle fibers begin to form

Question 3

Meiosis 1 - Metaphase 1

Answer 3

Chromatids line up at the equator randomly , this causes random orientation
Spindle fibers from opposing centrosomes connect to bivalents (at centromeres)
Different combinations is 223

Question 4

Meiosis 1 - Anaphase 1

Answer 4

Spindle fibers contract and split the bivalent
Homologous chromosomes move to opposite poles of the cell (no centromere is separating
These are already haploid cells

Question 5

Meiosis 1 - Telophase 1

Answer 5

Chromosomes decondense/ uncoiled to become chromatin
Nuclear membrane may reform at opposite poles
Cell divides (cytokinesis) to form two haploid daughter cells
However, each chromatid still has the replicated sister chromatid still attached (not homologous pairs anymore)

Question 6

Meiosis 2

Answer 6

The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)

Question 7

Meiosis 2 - Prophase 2

Answer 7

DNA supercoils: chromatin condensed again
Nuclear membrane dissolves
Centrosomes move to opposite poles (perpendicular to before)
Spindle fibers begin to form

Question 8

Meiosis 2 - Metaphase 2

Answer 8

Spindle fibers from opposing centromeres attach to chromosomes/sister chromatids (at centromere)
Align them along the cell equator

Question 9

Meiosis 2 - Anaphase 2

Answer 9

Spindle fibers contract and separate the sister chromatids,
Chromatids (now called chromosomes) move to opposite poles

Question 10

Meiosis 2 - Telophase 2

Answer 10

Chromosomes decondense and uncoils
Nuclear membrane reforms
Cells divide (cytokinesis) to form four haploid daughter cells

Question 11

Final Outcome after Meiosis 1 and Meiosis 2

*Check Good Notes for Diagram (Chart)

Answer 11

The final outcome of meiosis is the production of four haploid daughter cells
In males, these four cells are all sperm cells
In females, one of the cells is an egg cell while the other three are polar bodies (small cells that do not develop into eggs

Question 12

Mitosis Vs. Meiosis

Number of Divisions:
Tyes of Cells Produced:
Daughter Cells:
Crossing Over:
Chiasmata Formation:
Homologous Chromosomes:
Sister Chromatids:
Extra:

Answer 12

Mitosis
Number of Divisions: 1
Tyes of Cells Produced: 2 Diploid (somatic cells) → growth and repair
Daughter Cells: Are identical to parent cells
Crossing Over: No
Chiasmata Formation: No
Homologous Chromosomes: Yes
Sister Chromatids: Separate in Anaphase
Extra:

Meiosis
Number of Divisions: 2
Tyes of Cells Produced: 4 Haploid → gametes for reproduction
Daughter Cells: Differ from parent cell producing genetic variation
Crossing Over: Yes in prophase I (homologous chromosomes associate as bivalents)
Chiasmata Formation: Yes
Homologous Chromosomes: Yes
Sister Chromatids: Homologous pairs separate in anaphase I
Sister chromatids separate in anaphase II
Extra: Homologous chromosomes pair up whereas in mitosis they do not

Question 13

Sexual Life Cycle

Answer 13

Fertilization of two haploid gametes (egg+sperm) → zygote (diploid) → can grow via mitosis

If chromosome number was not halved in gametes, total chromosome numbers would double each generation (polyploidy)

Question 14

Genetic Variation

Answer 14

Genetic variation is due to:
1) Crossing over (in prophase I)
2) Random assortment of chromosomes (in metaphase I)
3)Random fusion of gametes from different parents

Question 15

Genetic Variation: Crossing Over
*Check Good Notes for Diagram

Answer 15

Involves the exchange of segments of DNA between homologous chromosomes during prophase I

This happens between non-sister chromatids at points called chiasmata

As a consequence, all 4 chromatids that comprise the bivalent will be genetically different

Chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinants

Question 16

Genetic Variation: Random Orientation/Assortment
*Check Good Notes for Diagram

Answer 16

During metaphase I, the homologous chromosomes line up randomly

Question 17

Genetic Variation: Random fertilisation

Answer 17

A random sperm fuses with the egg

Identical twins are formed after fertilization, by the complete fission of the zygote into two separate cell masses

Question 18

Non Disjunction

Answer 18

Non-disjunction: when chromosomes fail to separate correctly during anaphase I, resulting in gametes with one extra or one missing

Failure in Anaphase I → 4 affected daughter cells
Failure in Anaphase II → 2 affected daughter cells

Question 19

Non Disjunction: Down Syndrome
*Check Good Notes for Diagram Chart

Answer 19

Have 3 copies of chromosome 21 (trisomy 21)

*One of the parental gametes has 2 copies of chromosome 21 as a result of nondisjunction
*The other parental gamete was normal and had a single copy of chromosome
*When the 2 fused during fertilization it resulted in three copies

Strong positive correlation between maternal age and Down syndrome

Question 20

Karyotyping

Answer 20

Karyotyping: the process by which chromosomes are organized and visualized for inspection

• Used to determine the gender of an unborn child and test for chromosomal abnormalities
• Cells are harvested from the fetus before being chemically induced to undertake cell division (so chromosomes are visible)
• The stage during which mitosis is arrested will determine whether chromosomes appear with sister chromatids
• Chromosomes are stained and photographed, before being organized according to structure
  The visual profile generated is called a karyogram

Question 21

Chronic Vili Sampling

Answer 21

Chorionic Villi Sampling: involves removing a sample of the chorionic villus (placental tissue) via a tube inserted through the cervix
It can be done at ~11 weeks of pregnancy with a slight risk of inducing miscarriage (~1%)

SUCTION TUBE

Question 22

Amniocentesis

Answer 22

Amniocentesis: involves the extraction of a small amount of amniotic fluid (contains fetal cells) with a needle
It is usually conducted later than CVS (~16 weeks of pregnancy) with a slightly lower risk of miscarriage (~0.5%)

NEEDLE