4.2 Meiosis

Question 1

Define gamete

Answer 1

Haploid cells which fuse together during fertilisation to form a zygote (also called sex cells)

Question 2

Describe a diploid cell

Answer 2

Contains 2 complete sets of chromosomes; chromosomes are in bivalents/tetrads. Body cells (somatic cells) are diploid.

Question 3

Describe a haploid cell

Answer 3

Contains 1 complete set of chromosomes. Gametes (sex cells) are haploid.

Question 4

Define tetrad

Answer 4

A pair of homologous chromosomes joined by a centromere (also called a bivalent)

Question 5

Define meiosis

Answer 5

A reduction division of a diploid nucleus to form four haploid nuclei

Question 6

Define homologous chromosomes

Answer 6

Homologous chromosomes are chromosomes that share:
• The same structural features
• The same genes at the same loci positions (while genes are the same, alleles may be different)

Question 7

Outline Prophase I of Meiosis I

Answer 7

1. Nuclear membrane breaks down, DNA in form of chromatin supercoils, condensing into chromosomes.
   2. Homologous chromosomes undergo synapsis whereby they form Tetrads
   
   3. ‘Crossing over’ occurs
      
      4. The centrosomes move to the poles of the cell and spindle fibres begin to form and attach to the centromeres of the chromosomes.

Question 8

Outline Metaphase I of Meiosis I

Answer 8

The homologous pairs are lined up horizontally along the equator by contraction of the spindle fibres.

Question 9

Outline Anaphase I of Meiosis I

Answer 9

Continued contraction causes the homologs to separate and pulls the chromosomes to the opposite poles of the cell.

Question 10

Outline Telophase I of Meiosis I

Answer 10

Nuclear membrane reforms, spindle fibres break down, chromosomes decondense back into chromatin. Cell divides into two haploid daughter cells (cytokinesis)

Question 11

State the result of meiosis

Answer 11

4 haploid cells produced from 1 diploid cell

Question 12

Define non-disjuncture

Answer 12

When a set of homologs in a tetrad do not separate during Anaphase I

Question 13

Explain the cause of down-syndrome due to non-disjunction

Answer 13

• Caused by extra chromosome 21.
• Chromosomes or chromatids do not separate and go to the same pole.
Either
• non-separation of homologous chromosomes during anaphase I due to incorrect spindle attachment.
• non-separation of chromatids during anaphase II due to centromeres not dividing.
• Occurs during oogenesis or spermatogenesis, but more common in egg formation.
• Egg or sperm contain an extra copy of chromosome 21, therefore zygote has 3 copies of chromosome 21.
• Thus Down syndrome is also known as trisomy 21.

Question 14

Define karyotyping

Answer 14

When chromosomes are arranged in pairs according to their structure

Question 15

Outline the process of karyotyping

Answer 15

• Pictures can be taken of the human chromosomes during the metaphase
  
  • They can then be arranged into pairs on the basis of size and structure
  • Chromosome 1 is the shortest, chromosome 22 is the longest
  • The chromosomes appear as pairs of sister chromatids

Question 16

Outline one use for karyotyping

Answer 16

• Amniotic fluid or chorionic tissue can be sampled as they contain foetal cells in mitosis.
  * Can be used for diagnosis of chromosomal abnormalities, such as Down syndrome, identifying extra chromosome 21.

Question 17

Define synapsis

Answer 17

The process by which tetrads are formed

Question 18

Outline ‘Crossing Over’ of chromosomes in Prophase I

Answer 18

The homologous chromosomes in a tetrad cross over at a points called chiasmata. ‘Crossing over’ of genetic material between non-sister chromatids can occur at these points, resulting in new gene combinations (genetic variation).

Question 19

Define chiasma

Answer 19

The point at which homologous chromosomes in a tetrad cross over