Lecture 10

Question 1

What are the desitines of cells

Answer 1

• Live and function without dividing (e.g. neurons - many cells in brain - and most muscle cells)
• Grow and divide (e.g. skin or liver cells)
• Die (without apoptosis can end up with cancer)

Destinies depend on cell signalling

Question 2

What are somatic cells

Answer 2

Cells within the body, undergo mitosis

They undergo mitosis (somatic cell division) for growth and development, tissue renewal (especially skin and liver). Not all divide, and some a lot more (brain and muscle cells don’t divide heaps).

Most of the time somatic cells are going about their function - in interphase

Question 3

What are reproductive cells

Answer 3

They are what produce germ cells/gametes, undergo meiosis (reproductive cell division)

Question 4

What are diploid cells

Answer 4

Cell that have 2n chromosomes (the normal amount), for us this means 46 chromosomes

Contain DNA from both parents

Question 5

What are Haploid cells

Answer 5

Cell that have 1n chromosomes (half the normal amount), for us this means 23 chromosomes

Contain DNA from one parent

Question 6

Describe homologous chromosomes (non-sisters)

Answer 6

Chromosomes from two different parents that code for the same genes

Question 7

Describe sister chromatids

Answer 7

Identical copies of DNA that are still part of one chromosome - connected by a centromere

These are made during interphase, line up on different sides during anaphase, and are split during telophase

The DNA is from the same parent

Question 8

What are the two main phases of the eukaryotic cell cycle

Answer 8

Interphase (G1,S,G2) and Mitotic phase

Question 9

Describe interphase

Answer 9

G1/growth or gap phase 1:
Most cellular activities are occurring here.
Growth
Duration variable - cell type specific

S/Synthesis of DNA:
DNA replication occurs; strands are separated at the hydrogen bonds holding the bases together, a new strand of DNA is synthesised opposite to each of the old strands (and these two strands join together to form 2x double helix of DNA identical to prior DNA). Protein involved is DNA polymerase

G2: Growth or Gap phase 2
Checks for correct DNA synthesis, prepares for the mitotic phase (synthesis of proteins and enzymes required, gathering of reactants), replication of centrosomes is completed (Centrosome replication occurs as needs one per side of cells)

Sometimes see G0 (e.g. neurons that don’t divide)

Question 10

Describe the mitotic phase

Answer 10

Prophase:
Mitotic spindle forming (centrosomes separate as spindles form).
Chromosomes in chromatin form - made of two identical/sister chromatids - condense
Nuclear envelope disappears, and spindle are able to interact with centromere of chromosomes (spindles are microtubules)

Metaphase:
Condensed chromosomes line up in the centre of the cell (alignment not important as one of each sister chromatid will go into each cell anyway) - due to dynamic-ness of microtubules

Anaphase:
Chromosomes are separated such that sister chromatids separate.

Telophase and cytokinesis:
Cell splits and nuclear envelope reforms. Daughter cells are identical to parent cell

Question 11

Describe cell cycle checkpoints

Answer 11

Points at which decision need to be made regarding whether the cycle should continue - if not apoptosis will occur (not too important)

Question 12

Describe G1 checkpoints

Answer 12

Number of signal molecules to ‘tell’ the cell whether it should continue. Important as you don’t want more dud cells, or don’t want to grow when barely surviving
- Is the DNA undamaged
- Is the cell size and nutrition OK
- Appropriate signals present

If not passing these things, may exit to G0

Also have a G2 checkpoint (explained in the next lecture)

Question 13

Describe M checkpoints

Answer 13

• Are all chromosomes attached to spindles? (during the M phase)

Question 14

Describe meiosis

Answer 14

Occurs in the gonads (ovaries and testes) produces gametes which are haploid (a single set of 23 chromosomes) fertilisation then restores the diploid number of chromosomes (2n) produces cells genetically different from the parent cell. Preceded by DNA replication

Question 15

What are the phases of Meiosis

Answer 15

Meiosis I and Meiosis II

Question 16

Describe Meiosis I

Answer 16

Prophase I
Nuclear envelope disappearing
Homologous pairs are attached (called Synapsis) to form tetrads - sometimes they are joined by things called chiasmata that exchange genetic material
Crossing over/recombination between non-sister chromatids occurs (every chromosomes does this on average once, and the regions where this occurs is called chiasmata)
Spindle microtubules forming

Metaphase I
Homologous Pairs/Tetrads line up at the cell equator
INDEPENDENT ASSORTMENT - as each homologous chromosome can go in either direction

Anaphase I
Homologous chromosomes/Tetrads separate such that the sister chromatids - now genetically different - are kept together

Telophase I
Cell divides and nuclear envelopes reform

Question 17

Describe Meisois II

Answer 17

Prophase II
Nuclear envelope disappears
Metaphase II
Sister chromosomes line up in the centre
Anaphase II
Sister chromosomes are separated
Telophase II
Cells divide and nuclear envelop reforms

No DNA replication beforehand

Question 18

What are the sources of variation in meiosis

Answer 18

• Independent assortment at metaphase I (2^23/over 8 million possible combinations)
• Crossing over at prophase I (~1-3 crossover events per pair)
• Fusion between two gametes (more than 2^23 x 2^23/8 million squared combinations)

Question 19

Compare Mitosis versus Meiosis:

Answer 19

Mitosis:
DNA replication occurs during interphase before mitosis begins
Starts with chromosomes with two sister chromatids (synapsis/tetrads doesn’t occur)
One division
Produces 2x 2n/diploid cells that are identical.

Meiosis:
DNA replication happens during interphase before meiosis I but not meiosis II
Starts with tetrads (synapsis occurs at Prophase 1)
Two divisions
Produces 4x 1n/haploid cell that are different from initial parent cell and from each other