Session 1: Meiosis and Inheritance

Question 1

What are homologous chromosomes?

Answer 1

Chromosomes that share: The same structural features (e.g. same size, same banding patterns, same centromere positions). The same genes at the same loci positions (while the genes are the same, alleles may be different). Homologous chromosomes must be separated in gametes (via meiosis) prior to reproduction, in order to prevent chromosome numbers continually doubling with each
generation.

Question 2

What is meiosis?

Answer 2

Meiosis is the cell division process of making haploid sex cells (gametes) via three key events:
 Interphase, which copies chromosomal DNA (sister chromatids)
 Meiosis I (reductional division) separates the homologous chromosomes (2n - n) and halves the chromosome number
 Meiosis II (multiplication division) separates the sister chromatids (genetically distinct)

Question 3

Describe the interphase of meiosis.

Answer 3

DNA is replicated during the S phase of interphase. Replicated chromosomes will consist of genetically identical sister chromatids that remain attached to each other at the centromere

Question 4

Describe what happens during prophase I (meiosis).

Answer 4

During Prophase I, homologous chromosomes become connected via synapsis. This allows connected chromosomes to be arranged for a reduction division. The connected homologous chromosomes are known as bivalents (or tetrads)

Question 5

Describe crossing over (synapsis).

Answer 5

Non-sister chromatids may recombine with their homologous partner and exchange sections of DNA leaving physical connections called chiasmata. The genetic exchange results in the formation of recombinant chromosomes that possess unique combinations of alleles.

Question 6

Describe metaphase I in meiosis.

Answer 6

Homologous chromosomes align at the metaphase plate/line up in a random orientation, referred to as
independent assortment. There is an equal chance of the resulting gamete containing either of the pair.

Question 7

Describe anaphase I in meiosis.

Answer 7

The chromosomes with two sister chromatids are separated, and they begin to migrate to the opposite poles. This separation is achieved because of the contraction of the spindle fibres attached to each chromosome’s centromere.

Question 8

Describe telophase I in meiosis.

Answer 8

Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair. However, there are still 2 copies of that single chromosome.

Question 9

Describe meiosis II (prophase II, metaphase II, etc.)

Answer 9

Prophase II: DNA does not replicate, new spindles form in both daughter cells.
Metaphase II: Chromosomes (sister chromatids) align at the metaphase plate).
Anaphase II: Centromeres divide and sister chromatids migrate separately to each pole.
Telophase II: Chromosomes decondense, nuclear membrane reforms, cells divide.
Cytokinesis: Cell division is completed. Four haploid daughter cells are obtained, these cells are likely to be genetically distinct due to the crossing over that occurs during Prophase I (recombination of sister chromatids).

Question 10

What are karyotypes?

Answer 10

A karyotype is a picture of all of the chromosomes found in an individual. It can be used to diagnose certain genetic problems, either before birth (using CVS or amniocentesis) or after birth. Chromosomes are stained, photographed, and grouped by size and banding patterns. Chromosomes seen are in the form of duplicated sister chromatids, which is why you may see X shapes

Question 11

What is non-disjunction?

Answer 11

Sometimes during meiosis, the division of chromosomes does not occur normally, and a gamete ends up with the incorrect number of chromosomes. This can occur during Anaphase I or Anaphase II. If a gamete formed following a nondisjunction event is used in fertilisation, this leads to aneuploidy in the zygote and an organism with an abnormal number of chromosomes. The higher the maternal age, the higher the risk of non-disjunction.

Question 12

What are monosomy and trisomy?

Answer 12

Monosomy refers to lack of one chromosome of the normal complement (one less than normal). Trisomy refers to the presence of three copies, instead of the normal two, of a particular chromosome (one more than normal - usually when there’s 24 chromosomes in a gamete instead of 23).

Question 13

Overall, what is the concept of genes and Inheritance?

Answer 13

Genes contain the information for the production of proteins. Proteins are responsible for the physical expression of genes as phenotypic characteristics or traits, such as eye colour. Since genes are inherited, traits are also inherited.

Question 14

What are alleles?

Answer 14

Genes occupying the same position (locus) on
homologous chromosomes. Alleles are versions of the same gene that code for a variant of the same protein.

Question 15

What is genotype and phenotype?

Answer 15

The genotype of an organism refers to its genetic make-up. The phenotype of an organism refers to its observable features or traits.

Question 16

Overview of meiosis I and II and its products.

Answer 16

A cell with 46 chromosomes (92 chromatids) divides into 2 cells with 23 chromosomes each (46 chromatids) (meiosis I). These cells then copy into 2 more cells each - four in total. All 4 cells have 23 chromosomes (23 chromatids). (meiosis II)

Question 17

What does 2n - n mean?

Answer 17

2n is a diploid and has the full number of chromosomes (46). n is a haploid and has half the number of chromosomes (23). This is used in meiosis I and refers to one cell splitting into 2 cells.

Question 18

Describe the example: translocation.

Answer 18

Translocation is when a fragment of a chromosome is moved (“trans-located”) from one chromosome to another - joins a non-homologous chromosome. The balance of genes is still normal (nothing has been gained or lost) but can alter phenotype as it places genes in a new environment. Can also cause difficulties in egg or sperm development and normal development of a zygote.