Week 2 - Meiosis and Genetic Variation

Question 1

What are genes?

Answer 1

Specific locations on a chromosome/strand of DNA that code for a particular trait or traits

Question 2

What is a locus?

Answer 2

A specific location where a gene is located on a certain chromosome

Question 3

What are gametes?

Answer 3

Reproductive cells that pass on genetic information to their offspring

Question 4

What are sister chromatids?

Answer 4

Two genetically identical chromosomes formed after DNA replication bound together by a centromere

Question 5

How many chromosomes do humans have?

Answer 5

46

Question 6

How many pairs of chromosomes do humans have?

Answer 6

23

Question 7

How many chromosomes do we inherit from each parent?

Answer 7

23

Question 8

When are chromosomes most condensed?

Answer 8

Metaphase

Question 9

What is a karyotype?

Answer 9

The number, sizes, and shapes of the entire set of condensed chromosomes

Question 10

What are some examples of staining methods for karyotyping?

Answer 10

Q, R, and G banding

Question 11

What is G Banding?

Answer 11

A type of dying used in karyotyping. Most common in human cytogenetics.
Heterochromatin stains are dark compared to light staining euchromatin.

Question 12

What is ploidy?

Answer 12

Refers to the number of chromosomes in a set. I.e. diploid = 2n, haploid = 1n, triploid = 3n

Question 13

How many sets of chromosomes do human somatic cells contain?

Answer 13

2 (2n)

Question 14

How many sets of chromosomes do human gamete cells contain?

Answer 14

1 (1n)

Question 15

What’s the difference between homologous chromosomes and sister chromosomes?

Answer 15

Sister chromosomes are identical copies of a chromosome that is made after DNA replication
Homologus chromosomes are similar chromosomes that are genetically different, each inherited from a different parent.

Question 16

What are three events that are unique to meiosis over mitosis?

Answer 16

1
Synapsis and crossing over in prophase 1
In metaphase 1, chromosomes are paired in

2
homologous pairs (tetrads), instead of individual replicated chromosomes

3
At anaphase 1, homologous chromosomes instead of sister chromosomes separate

Question 17

Outline the steps in crossing over

Answer 17

After interphases, sister chromatids are held together by proteins called cohesion.

Each pair of homologs associated along their lengths

DNA molecules of two non-sister chromatids are broken at precisely corresponding points

The chromatin of the chromosomes begins to condense

A protein called synaptonemal completed forms and attached one homolog to the other

Each homolog swaps over and DNA from each homolog is swapped over

Chromosomes remain slightly attached and move towards the metaphase plate.

Question 18

What three events during meiosis and fertilisation contribute to the genetic variety of offspring?

Answer 18

Crossing over
Independent assortment of maternal and paternal chromosomes
Random union of gametes

Question 19

What is independent assortment? Why is it important?

Answer 19

The arrangement of homologus pairs during metaphase 1.
Occur randomly and have many possible arrangements, therefore there are many different possible gametes that can be produced, leading to great genetic variation.