Topic 4.2 Meiosis

Question 1

4.2.1 State that meiosis is a _______ division of a diploid nucleus to form haploid nuclei.

Answer 1

Meiosis is a reduction division of a diploid nucleus to form haploid nuclei.

Question 2

4.2.2 Define homologous chromosomes.

Answer 2

Homologous chromosomes: 2 chromosomes that code for the same genes; genes are in same location on a chromosome.

Question 3

4.2.3 Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells.

Answer 3

Interphase

1. DNA is in uncoiled form
2. Cell is carrying out interphase growth and processes
3. Before meiosis begins, DNA is replicated and chromosomes now consist of 2 sister chromatid

Prophase I

1. Centrioles replicate and separate to poles of cell
2. Spindle fibres produced by centrioles
3. Chromatin condenses into chromosomes
4. Homologous chromosomes pair up
5. Sections of sister chromatid cross over and are exchanged
   
   • area where crossing over occurs is called the chiasma
6. Nuclear membrane breaks down

Metaphase I

1. Spindle fibres connect to centromere
2. Chromosomes migrate to equator of cell
3. Homologous chromosomes line up across from each other (line up in twos)

Anaphase I

1. Homologous chromosomes are separated and move to opposite poles of cell

Telophase I

1. New nuclear membranes form
2. Cell splits into 2 haploid daughter cells

Prophase II

1. Centrioles replicate & separate to poles of cell
2. Spindle fibres produced by centrioles
3. Chromatin condenses into chromosomes
4. Nuclear membrane breaks down

​Metaphase II

1. Spindle fibres attach to centromeres
2. Chromosomes migrate to equator of cell
3. Centromeres divide

Anaphase II

1. Chromatid are separated & move to opposite poles of cell

Telophase II

1. New nuclear membranes form
2. 2 cells split into 4 daughter cells
3. Chromosomes uncoil
4. Nucleoli appear

Question 4

4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).

Answer 4

Non-disjunction: process by which 2 homologous chromosomes stick together, instead of separating, during meiosis.

Because an extra chromosome moves to 1 pole of cell during meiosis, results in gametes with 1 chromosome too many/too few.

Down syndrome (aka trisomy 21) is an example of non-disjunction; when gamete is fertilized, the zygote produced has 3 chromosomes of type 21 in its karyotype instead of 2 chromosomes.

Question 5

4.2.5 State that, in karyotyping, chromosomes are arranged in pairs according to their ____ and ________.

Answer 5

In karyotyping, chromosomes are arranged in pairs according to their size and structure.

Question 6

4.2.6 State that karyotyping is performed using cells collected by _________ ______ sampling or ____________, for pre-natal diagnosis of chromosome abnormalities.

Answer 6

Karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

Bonus:

Chorionic Villus sampling: cells in placenta that are part of the fetus; needle used to remove fetal cells from chorionic villi.

Amniocentesis: sample of fluid (contains cells from fetus - e.g. sloughed off skin cells) is removed by long needle from amniotic sack.

Question 7

4.2.7 Analyse a human karyotype to determine gender and whether nondisjunction has occurred.

Answer 7

Gender

Males: XY chromosomes

Females: XX chromosomes

Non-disjunction

look for 1 extra/missing chromosome on numbered types on karyotype