Cell Division, Cell Diversity & Cellular Organisation Flashcards
Whats the cell cycle
Phases in the cell cycle
Interphase (G1, S, G2 & sometimes G0)
Mitosis (M)
Cytokinesis
What occurs in the G1 phase
Cell growth
Biosynthesis (protein/lipid)
Organelles duplicate
What goes on in the S phase
DNA rapidly replicates
Short phase (to reduce chance of spontaneous mutations)
What happens in the G2 phas
More cell growth
Replicated DNA is checked for mutations / errors OR to check if it has replicated correctly
If damaged DNA is detected, the cell cycle stops and cell tries to repair damage, or moves into what phase
G0
What happens into G0 phase
Cells undergo senescence / apoptosis or differentiation
-> muscle + neurons remain here for a long time
Senescence-> ages + stops, but won’t die
Mitosis vs cytokinesis
Nuclear division by mitosis
Cell division by cytokinesis (division of cytoplasm, by constriction from the edge of the cell)
Why is the cell cycle regulated
- prevent uncontrolled cell division (which would lead to tumour formation)
- detect & repair damage to DNA
- ensure cycle only occurs in 1 direction
- ensure DNA is only replicated once per cycle
How is the cell cycle regulated
Regulated by checkpoints between the G1 & S phase, and between the G2 and mitosis
Key proteins, cyclin-dependent kinases & cyclins, control the process.
Cyclins are produced at specific times, activating the kinases and allowing progression through the cell cycle
What is mitosis
The nuclear division which produces 2 generically identical diploid cells (nuclei)
Maintains the chromosome number
Main stages in mitosis
P rophase
M etaphase
A naphase
T elophase
What occurs in prophase
-> chromosomes condense + become visible (supercoil)
-> nuclear envelopes breaks down
-> spindle fibres break down
What occurs during Metaphase
Chromosomes line up at the equator
Chromosomes attach by their centromeres to spindle fibres
What occurs in Anaphase
Centromeres divide: sister chromatids separate at centromere
Motor proteins pull sister chromatids to opposite poles in a V shape -> centromeres travel first
What happens in telophase
Nuclear envelope reforms around both sets of chromosomes at opposite poles
Spindle fibres break down
2 genetically identical nuclei formed
Significance of mitosis in life cycles
- tissue growth & repair
- production of new stem cells
- maintain chromosome number
- asexual reproduction
- development of body plan
- proliferation of white blood cells (clonal expansion)
- producing gametes from haploid cells
Significance of meiosis
Produces haploid gametes which are randomly fertilised
Increases genetic variation -> through independent assortment
How is genetic variation produced
-> Independent assortment of homologous chromosomes in metaphase 1 & chromatids in metaphase 2
-> crossing over in prophase 1
- mutation
- non-disjunction
- random fusion of gametes
Increased genetic variation in prophase 1?
Crossing over
Alleles are swapped between non-sister chromatids
- chromatids now have a new combination of alleles
How is there increased variation in metaphase 1
-> independent assortment of homologous chromosomes
- these line up across the centre of the cell, & each of has different alleles, therefore lots of combos
How does increased variation occur through non-disjunction
Homologous chromosomes do not separate in metaphase 1
One more or less chromosome present
How does increased variation occur through mutations
Changes nucleotide base sequence
DNA checks don’t recognise the damage
Changes amino acid sequence
How does increased variation occur through random fertilisation of gametes
Gametes not genetically identical
Produces large number of allele combinations
What are homologous chromosomes
Chromosomes which have the same gene at the same genes loci. They may contain different alleles
What is meiosis
Nuclear division which produces 4 haploid cells, which are genetically different to the parent cell
Half the chromosome number
What is independent assortment
How the inheritance of 1 chromosome does not affect the inheritance of others
What is crossing over
When the 2 chromosomes come together, align, & them recombine & swap parts with each other
This results in a mix of / the creation of new alleles
Stages of Meiosis
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2
What happens in Prophase 1 (Meiosis)
Chromosomes condense & supercoil / becoming visible
Chromosomes arrange in homologous pairs
Crossing over of non-sister chromatids occurs
Nuclear envelope breaks down
Spindle fibres form
What happens in metaphase 1 (meiosis)
Homologous chromosomes line up across the equator along spindle fibres
Independent assortment of homologous chromosomes occurs
Maternal or paternal chromosome can end up facing either poles
What happens in anaphase 1 (meiosis)
Homologous chromosomes separate
Motor proteins pull chromosomes to opposite poles
What happens in telophase 1 (meiosis)
Nuclear envelope reforms around each set of chromosomes
Spindle fibres break down
THEN
cytokinesis occurs
What happens Prophase 2
Chromosomes condense & supercool
Nuclear envelope breaks down
Spindle fibres form
What happened in metaphase 2
Chromosomes line up along the equator
Independent assortment of chromatids
What happened in anaphase 2
Chromatids pulled to opposite poles by motor proteins
Centromeres divide
What happens in telophase 2
Nuclear envelope forms around each of haploid nuclei
Then, cytokinesis occurs to form 4 haploid cells
