Bridle Genetics

Question 1

Why does evolution happen by natural selection?

Answer 1

1. Overproduction (too many offspring and finite resources)
2. Variation between individuals in ability to use these resources
3. Some of this variation can be passed between generations

Question 2

When did Eukaryotes evolve?

Answer 2

1.5-2 billion years ago

Question 3

What are the four levels of packing of DNA so it fits in a nucleus?

Answer 3

1. Nucleosomes (subunit of chromatin)
2. 30nm looped fibre
3. Looped domains (basic structural unit of eukaryotic chromatin
4. Metaphase chromosome

Question 4

What percentage of DNA is in functional genes?

Answer 4

5%, the rest is ‘junk’ DNA

Question 5

How does the p53 protein regulate the cell cycle?

Answer 5

p53 levels are increased in damaged cells
It blocks the cell cycle if DNA is damaged, giving time to repair the DNA
If the DNA damage is severe the p53 protein can cause apoptosis
This prevents damaged cells skipping the resting stage and replicating without control
A p53 mutation is the most frequent mutation leading to cancer

Question 6

What are the 6 steps of mitosis?

Answer 6

Prophase
Prometaphase
Metaphase
Anaphase
Telophase 
Interphase

Question 7

What is prophase?

Answer 7

Condensation and spindle formation. Chromatic begins to condense and becomes visible as chromosomes. The nucleolus disappears and centrioles begin moving to opposite poles of the cell. Some fibres cross the cell to form the mitotic spindle

Question 8

What happens in prometaphase?

Answer 8

Migration to centre
The nuclear membrane dissolves, and proteins attach to the centromeres creating kinetochores. Microtubules (spindle fibres) attach at the kinetochores and the chromosomes begin moving

Question 9

What happens in metaphase?

Answer 9

End of migration
Spindle fibres align the chromosomes along the middle of the cell. This line is the metaphase plate and it helps to ensure that when the chromosomes are separated each new nucleus will receive one copy of each diploid chromosome

Question 10

What happens in anaphase?

Answer 10

Separation and migration to the poles of the cell

The pie chromosomes separate at the centromeres and move to opposite sides of the cell

Question 11

What happens in telophase?

Answer 11

Splitting of cells
Chromatids arrive at opposite sides of the cell and new membranes form around the daughter nuclei. The chromosomes and spindle fibres disperse and are no longer visible under a light microscope. Cytokinesis may begin

Question 12

What happens in interphase?

Answer 12

The G1, S and G2 of the cell cycle. Chromosomes not visible

Not part of mitosis

Question 13

Who discovered meiosis and when?

Answer 13

Oskar Hertwig

1876

Question 14

Cost of meiosis

Answer 14

Sexual reproduction affects short term fitness
Risk of increased predation
Risk of time lost
Risk of energy lost
Each allele only has 50% chance of being passed on

Question 15

What happens in prophase I of meiosis?

Answer 15

DNA replication has already occurred, the homologous chromosomes pair and synapse to form bivalents/tetrads (4 chromatids). Chiasma form and recombination occurs at the equatorial plate. Pairs of chromosomes and genes are exchanged

Question 16

What happens in late prophase I of meiosis?

Answer 16

The nuclear membrane disappears. One kinetochore forms per chromosome, rather than per chromatid in mitosis. The chromosomes begin to move towards the metaphase plate

Question 17

What happens in metaphase I of meiosis?

Answer 17

Bivalents align at the metaphase plate. Orientation is random so either parental chromosome of each homologous pair can be on either side of the metaphase plate. The pairs separate and migrate to opposite ends

Question 18

What happens in anaphase I of meiosis?

Answer 18

The chiasma separate. Chromosomes move to separate poles of the cell. Daughter cells will now be haploid

Question 19

What happens in telophase I of meiosis?

Answer 19

Nuclear envelope reforms and cell goes on to cytokinesis, or the cell me go straight onto meiosis II. The cell splits into two cells with two haploid dyads (double chromosomes)

Question 20

What happens in meiosis II?

Answer 20

Each dyad splits in two within the two daughter cells, generating four haploid cells. Meiosis II is just like mitosis, except there is no DNA synthesis and each haploid cell is unique because of recombination

Question 21

How many chromosome combinations do humans have? (ignoring recombination)

Answer 21

2^n = 2^23 = 8.4 million

Question 22

Problems with meiosis

Answer 22

1. Nondisjunction - homologues don’t separate properly in meiosis I, can be lethal
2. Aneuploidy - gain or loss of chromosomes e.g. Downs sydrome
3. Translocation or deletion - transfer a piece of one chromosome to another, or loss of a fragment of a chromosome
4. Non-reciprocal crosses - chromosomes exchange unequal-sized segments of DNA, so one partner loses more genes than it receives

Question 23

What is the benefit of gene duplication?

Answer 23

Sub-functionalisation can follow where the new gene is free to take on a new function