DNA - Mitosis and Meiosis

Question 1

DNA - why is it so special?

Answer 1

A combination of just four bases can encode all amino acids.
DNA is 100x more stable than RNA
Fidelity of DNA is easier to maintain than RNA:
* Cytosine rapidly deaminates to uracil, so any uracil in DNA is incorrect.
* Double helix of DNA provides a template for repair.
* Double helix shows were there are mismatches for repair.
Double stranded DNA is less vulnerable to chemical damage.
Helix enables translesion & proofreading during replication.

Question 2

DNA Structure

Answer 2

The DNA consist of a sugar-phosphate backbone with a base attached to it. The bases can be divided into purines (adenine and guanine) and pyrimidines (thymine and cytosine).
Adenine always pairs with thymine (joined by 2 hydrogen bonds).
Cytosine always pairs with guanine (joined by 3 hydrogen bonds) and is more stable due to more hydrogen bonds.

Question 3

Semi-conservative DNA Replication

Answer 3

Ensures one ‘old’ strand and one ‘new’ strand.
* Leading and lagging strands
* Okazaki fragments (RNA primers) required for lagging strand
* DNA polymerase (alpha occurs in a 5’ to 3’ direction
* Proofreading exonuclease activity 3’ to 5’ direction.

Question 4

Types of DNA

Answer 4

A-DNA (protein complexes) - Right handed; 11 bases per turn
B-DNA (canonical): Right handed; bases stack; 10.5 bases per turn
Z-DNA (Gene regulation, DNA instability): Left handed; alternating purine and pyrimidine tracts; 12 bases per turn.
H-DNA (triplex DNA) involved in gene regulation.

Question 5

Organising DNA into Chromosomes

Answer 5

Polynucleotide chain —> The DNA double helix —–> supercoiling of DNA ——> The metaphase chromosome

Chromosome consist of a waist in the middle called a centromere and ends called telomeres.

Question 6

Mitotic Cell Cycle

Answer 6

G1-S-G2 - mitosis (approx. 24 hours)
G1-2-G2 - interphase
G0 = quiescence (reversible)
Checkpoints - cycle arrest in response to DNA damage (involves retinoblastoma and p53 gene which are tumour suppressor genes)
Apoptosis - programmed cell death
Senescence - stable cycle arrest.

Question 7

Prophase

Answer 7

Prophase: The cell starts to generate spindle fibres in the microtubules and the chromosome condense in prophase.

Question 8

Metaphase

Answer 8

Metaphase: Chromosomes align on the microtubules and move to the metaphase plate and align at the equator of the cell. A checkpoint is here to make sure the chromosomes are aligned properly (to ensure daughter cells have equal number of chromosomes)

Question 9

Anaphase

Answer 9

Anaphase: Spindle fibres contract and split the centromere causing the chromosomes to move to the pole of the cells.

Question 10

Telophase

Answer 10

Telophase: We start to get the invagination of the cell.

Question 11

Cytokinesis

Answer 11

Cytokinesis: the formation of the nuclear envelope and this total cell division and the production of two new adult cells.

Question 12

Key Stages of Meiosis

Answer 12

Before meiosis DNA replication occurs (each chromosome now consist of two sister chromatids but the cell is still 2n - diploid)
Pairing (synapsis) and crossing over of homologous chromosomes
Synaptonemal complex disassembly and chiasma visible.
First meiotic division - reductive division each cell is haploid but each chromosomes still comprises two sister chromatids.
Second meiotic division - separation of sister chromatids into sister chromosomes and equational division resulting in 4 haploid cells.

Question 13

Meiosis I - Law of Segregation

Answer 13

Following DNA replication - each chromosome of two sister chromatids (2n)
Unique to gametogenesis
Pairing (synapsis) of homologous chromosomes
Crossing over (chiasmata formation) and exchange of DNA (recombination)
Reductive division - separation of homologous chromosomes
Daughter cells contain 1 set of chromosomes 1n
Each chromosome comprised of sister chromatids.

Question 14

Prophase I

Answer 14

Prophase I: Chromosomes that have been replicated condense and pair with homologous chromosomes.

Question 15

Metaphase I

Answer 15

Metaphase I: Homologous chromosome held together by chiasmata. Chromosomes arrange themselves on the equator of the spindle.

Question 16

Anaphase I

Answer 16

Anaphase I: Homologous chromosomes separate and migrate to opposite poles of the cell

Question 17

Telophase I

Answer 17

Telophase I: The chromosome have formed two groups. The cell begin to constrict across the middle. Separates into two daughter cells.

Question 18

Meiosis II - Law of Independent Assortment

Answer 18

Similar to mitotic cell division
Start with haploid (1n) containing one set of chromosomes
Each chromosome comprised of two sister chromatids
Random separation of sister chromatids into daughter cells
Results in 4 daughter cells.

Question 19

Prophase II

Answer 19

Prophase II: The chromosome of the two daughter cells condense again in preparation for second meiotic cell division.

Question 20

Metaphase II

Answer 20

Metaphase II: The chromosomes then migrate to the equator.

Question 21

Anaphase II

Answer 21

Anaphase II: The newly separated chromosomes of the two daughter cells move to opposite poles of their spindle.

Question 22

Telophase II

Answer 22

Telophase II: The cells constrict across the nuclear membrane. Four haploid nuclei are formed, each with one member of each pair of chromosomes from the original nucleus.

Question 23

Human Spermatogenesis

Answer 23

Still have the primordial sperm cell at birth. Once puberty is reached, sperm patagonia start being generated and parilla proliferation occurs.
So we end up with this mitotic cell division in the testes, occurring continuously. So we start with 25 days of mitotic cell division to create primary spermatocytes. They then undergo meiosis 1 to produce secondary spermatocytes and then meiosis 2 to produce mature sperm.

Question 24

Human Oogenesis

Answer 24

Before birth a lot of this happens, we start with the primordial germ cell and repeats of cell division. This
then creates an oogonium and some of these become Granulosa cells. The oogonium become primary oocytes (in which there are 1 to 2 million) and sit there in prophase I until puberty and menstruation in which meiosis I is completed. After the completion of meiosis I this produces the first polar body and a secondary oocyte which is then arrested in metaphase II. This sits in the follicle until we have ovulation. Release of this oocyte which is in metaphase II allows for fertilisation and completion of meiosis II. Fertilisation produces a second polar body and ovum which becomes a zygote.

Question 25

Fertilisation

Answer 25

DNA transfer and source of variation
Further source of species variation is influenced by mate choice and sperm competition.