Lecture 10 Flashcards
What are the desitines of cells
- 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
What are somatic cells
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
What are reproductive cells
They are what produce germ cells/gametes, undergo meiosis (reproductive cell division)
What are diploid cells
Cell that have 2n chromosomes (the normal amount), for us this means 46 chromosomes
Contain DNA from both parents
What are Haploid cells
Cell that have 1n chromosomes (half the normal amount), for us this means 23 chromosomes
Contain DNA from one parent
Describe homologous chromosomes (non-sisters)
Chromosomes from two different parents that code for the same genes
Describe sister chromatids
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
What are the two main phases of the eukaryotic cell cycle
Interphase (G1,S,G2) and Mitotic phase
Describe interphase
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)
Describe the mitotic phase
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
Describe cell cycle checkpoints
Points at which decision need to be made regarding whether the cycle should continue - if not apoptosis will occur (not too important)
Describe G1 checkpoints
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)
Describe M checkpoints
- Are all chromosomes attached to spindles? (during the M phase)
Describe meiosis
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
What are the phases of Meiosis
Meiosis I and Meiosis II
Describe Meiosis I
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
Describe Meisois II
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
What are the sources of variation in meiosis
- 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)
Compare Mitosis versus Meiosis:
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
