Unit 1.6 Cell division Flashcards
Tell me about the cell cycle
Cellular events that repeat, in order, between one cell division and the next
Has 3 stages:
1. Interphase
a period of cell growth, synthesis and DNA replication (3 stages)
2. Mitosis
when the nucleus divides forming 2 genetically identical daughter nuclei
3. Cytokinesis
when the cytoplasm divides to form 2 daughter cells
In terms of that circle loading type shi:
- Interphase is approx 90% of the cell cycle
- Mitosis is like probably 9%
- Cytokinesis is 1% lol
In terms of a line graph:
- G1, synthesis and G2 can be easily recognised as interphase as quantity of DNA becomes doubled
- Right before the halving, mitosis and cytokinesis can be annotated near the ends
Tell me about interphase
A period of intense biochemical activity
- Apart from cell growth, DNA replicates, cell organelles replicate and ATP is built up ready for use in mitosis
- Sub-divided into 3 stages:
Growth Phase 1
- Cell is increasing in size
- Replication of organelles e.g. centrioles
- Protein synthesis
Synthesis
- DNA replicates
- Checking of DNA for errors takes place
Growth Phase 2
- Cells keep increasing in size
- Proteins synthesis
- ATP stores built up
Why is it important that DNA is checked for errors (mutations)?
- If daughter cells do not receive identical genetic information, the cells will not function
- Therefore the cell will kill itself to prevent mutations passing to daughter cells
Tell me all about chromosomes
- Only visible when cell is dividing
- Consists of a widely spread area of darkly stained material called chromatin
- Made up of mainly 3 materials:
70% protein (histones)
15% DNA
10% RNA - Once again, visible when chromatin condenses prior to cell division
- After each DNA molecule has replicated and made an exact copy of itself
- As mitosis begins, chromosomes made up of 2 strands (as DNA replicated in interphase) called sister chromatids joined by a centromere
Diagram of one chromosome:
- A really long X
- The dot in the middle is called the centromere
(where spindle attaches)
- Their “legs”, singular is called a chromatid
(one half of a duplicated chromosome)
- Together they’re called sister chromatids
- But then the whole thing is literally just called a chromosome
Chromosome numbers:
- Different species have different numbers of chromosomes
e.g. gorillas = 48, onions = 8, mice = 40, humans = 46
- Humans receive one full set from their mother and father there having 23 pairs of matching or homologous chromosomes
(just like many other organisms)
There’s also ploidy levels which is the number of complete sets of chromosomes in an organism:
(organisms with more than two sets are called polyploids)
Haploid = 1 = n = gametes
Diploid = 2 = 2n = human body cells
Hexaploid = 6 = 6n = bread wheat
Octoploid = 8 = 8n = strawberry
In terms of karyotype:
- Btw that’s like a photograph of chromosomes that are cut and pasted into a logical format in matching pairs:
1. Size is used to put them in logical format
2. Matching in here would be called homologous
3. Their 23rd pair is called sex chromosomes
5. And u can know their gender from this:
xy = male
xx = female
6. Their always displayed at bottom right so that it’s easily identified n shi
The diagram of a pair of homologous chromosomes:
1. It’s just 2 chromosomes that literally look identical
2. So that means it’s 2 x’s
3. 1 leg = chromatid
4. legs = sister chromatids
5. different legs = non-sister chromatids
6. Together = homologous pair = bivalent
- U can tell their homologous cuz they have matching:
Size
Gene
Loci?
Tell me all about mitosis
Type of cell division in which the 2 daughter cells have the same number of chromosomes and are genetically identical with each other and the parent cell, giving genetic stability
Needed in multicellular organisms for:
- Growth of tissues
- Repair/replacement of tissues
- Asexual reproduction
- Genetic stability
A continuous process and is described in 4 stages:
1. Prophase (Purple)
2. Metaphase (Mice)
3. Anaphase (Are)
4. Telophase (Tasty)
Prophase:
1. Chromosomes condense (shorten and thicken) and become visible
2. Seen as a pair of chromatids joined by a centromere
- So basically the main part is #1
3. Centrioles migrate to opposite poles of the cell
4. Starts to form a network of protein microtubules called the spindle
5. Nucleolus disappears and prophase ends with the breakdown of the nuclear envelope
Visuals:
- Within we have chromosomes
- Protected by nuclear membrane
- And we have the 2 centrioles on each side; above and below
They rely on cell cytoskeleton for movement
Real visualisation: A bunch of small lines mushed together
Metaphase:
1. Chromosomes line up at the equator
2. And are attached to the spindle apparatus by their centromere
Visuals:
- Think of it as the basketball lines
- No nuclear membrane btw
- Centromeres above and below
- 4 “spindle fibres” (the lines between centromeres) with each chromosome connected
- From their centromeres
Real visualisation: A bunch of small lines mushed together as a line
Anaphase:
1. Centromere splits and sister chromatids separate
2. Spindles contract and chromatids pulled
3. To the opposite poles of the cell, centromere first
- Movement of chromatids relies on the cytoskeleton and ATP
Visuals:
- Just like the metaphase
- However the chromosome truly is split
- And are going their separate ways (up/down, if u know u know)
Real visualisation: Very long combs moving far apart
There can also be a late anaphase/early telophase which is small combs far away
Telophase
1. Chromatids reach opposite poles on spindle
2. They uncoil and elongate = become chromosomes
3. Nuclear membrane forms around each group of chromosomes
4. Now there are 2 nuclei
Visuals:
- Think back to prophase visuals but now there’s 2 of them and are splitting
- Centriole will degenerate (the 2 dots above and below)
- Cytoplasm begins to divide
- Cleavage furrow?
Real visualisation: Small lines compactly mushed together as 2 balls
Tell me about cytokinesis
Takes place right after telophase of mitosis
1. Cell membrane constricts around the equator
2. Pinches the cell from the outside inwards
3. Forms 2 daughter cells that are genetically identical to the original cell and each other
4. Mitosis really finishes, but each daughter cell starts the interphase part of cell cycle, ready for next round of mitosis
- So it’s the actual process of division
- Cell stretching apart to become 2 new cells
- Tho it appears this stage happens at the same time of telophase
I mean they just seem the exact same…
What’s the difference in mitosis between animals and plants?
Animals:
1. Cells become rounded
2. Most cells divide by mitosis
3. Cytokinesis begins at the edge of cell, where cell membrane constricts
From outside in
Plants:
1. No shape change in plant cells
2. Mitosis occurs in growing regions called meristems e.g. root and shoot tips
3. They have no centrioles, spindle forms without them
- Cuz they use tubulin protein threads made in cytoplasm
4. Cytokinesis is different as they have cell walls;
From inside out:
- Membrane enclosed vesicles containing cell wall materials collect at mid-line of parent cell
- Vesicles join forming a cell plate
- Cell plate grows, cell walls form in centre and move outwards until they meet perimeter
- 2 distinct cell walls formed
- Spindle remains throughout until the new cell wall formation
Visuals:
1. Cell wall w/ cell membrane and inside are 2 nuclear envelopes while there are vesicles in-between them
2. So ye in the middle the vesicles form and for that it just looks like a line….
3. But because they grow, there’s now a cell wall between the 2 nuclear envelopes
Tell me about the significance of mitosis in terms of damage and disease
Cancer = uncontrolled cell division
- Genes control cell cycle
- Ensures mitosis occurs where/when it’s needed
- However in cancers:
- Cell divides by mitosis repeatedly, no control/regulation
- Irregular mass of cells formed (tumour)
- Cancer formed by damage to DNA
- Mistakes of different types build up in DNA
- Mistakes accumulate overtime (thus most arise in old people)
- Cancer = not single disease
Different types of mistakes:
1. Occur in genes that control cell division = cancer
2. Damage to a gene coding for a protein, p53
- That stops the copying of damaged DNA (prevents transcription)
3. Damage to DNA, many potential causes:
- Ionizing radiation
- Certain chemicals (mutagens) e.g. tar in tobacco
- Inheritance
There are genes that can prevent uncontrolled mitosis called tumour suppressor genes
Oncogene = a gene with the potential to cause cancer
Proto-oncogene = ^^^ before mutation
Tell me all about meiosis
Type of cell division that happens in reproductive organs to produce sex cells/gametes.
- Cells that divide this way are diploid to start with
- But sex cells that’re formed are haploids
- Cells formed by this type genetically different from parent cell & each other
Meiosis has two divisions:
Meiosis I = The homologous pairs split up
Meiosis II = Chromatids split apart, producing 4 haploid genetically different gametes
But before that, quite similar to mitosis but just a teensy bit different:
Prophase I:
1. Chromosomes condense and become visible
2. Chromosomes come together as homologous pairs (synapsis)
Each homologous pair = bivalent
3. Non-sister chromatids wrap round each other and attach at points called chiasmata (singular: chiasma)
4. They swap sections of chromatids with one another (crossing over)
5. Nucleolus disappears & nuclear envelope disintegrates
6. Centriole move to opposite poles of the cell and a spindle forms
- Almost like mitosis prophase, however it seems chiasmata swap places
Visuals:
For the chromosome mixing part:
- There’s a paternal & maternal chromosome together (homologous pair)
- Course they get together by synapsis
- And btw together they’re called a bivalent
- Then, it’s like their legs are interlocking
two legs on top each other = chiasmata
a leg = chiasma
- Finally, it appears they swapped half of their legs, if u know u know
But in terms of the whole thing:
- Within we have chromosomes:
above can be paternal homologue, with chiasma happening
below can be maternal homologue
- Protected by nuclear membrane that’s currently disintegrating
- And we have the 2 centrioles on each side; above and below/sideways
N.B. Crossing over can happen in several places along the length of the chromatids, increases genetic variation. Long term, if species is to survive in a changing environment, sources of variation are essential.
Metaphase I:
1. Bivalents lines up at equator of cell randomly, attached to spindle fibres at their centromeres
2. Allows chromosomes to independently segregate when pulled apart in anaphase I and a combination of paternal and maternal chromosomes goes into each daughter cell at meiosis I = independent assortment
Visuals:
- Practically looks exactly like metaphase in mitosis:
- Think of it as the basketball lines
- No nuclear membrane btw
- Centromeres above and below/sideways
- 4 “spindle fibres” (the lines between centromeres) with each chromosome connected (there is still chiasmata),
- From their centromeres
Showing Independent assortment:
- Basically u know how each cell has 2 homologous pairs (paternal and maternal)
- After meiosis, they can mix up in the 4 daughter cells
e.g. 1 with purely paternal and other with maternal
OR
1 with paternal and maternal and the same with the other
(Btw like, there’s like 2 different sizes the chromosome, like there’s a big maternal and small maternal, same with paternal. If u know u know)
Anaphase I:
1. Homologous chromosomes in each bivalent are separated as spindle shortens & chromosomes pulled to opposite poles
2. Centromeres do not divide (chromatids are still together)
Visuals:
- Still exactly like anaphase in mitosis:
- Just like the metaphase
- However the chromosome truly isn’t split
- And are going their separate ways (up/down/sideways, if u know u know)
Telophase I:
1. Nuclear envelope reforms around each set of haploid chromosomes
2. There’s a brief interphase and chromosomes uncoil
3. Cell now divides by cytokinesis
Visuals:
Bro just, it just looks EXACTLY like telophase….
TRANSITION
Cells now divide ‘mitosis style’
Prophase II:
1. Nuclear envelope breaks down and nucleolus disappears
2. Chromosomes condense
3. Spindles form
Visuals:
- Fr same as prophase in mitosis, but
- Centrioles replicate and move to poles
- New spindle fibres form at right angles to previous spindle axis
Metaphase II:
1 Chromosomes line up along the equator and spindle attaches to the centromere
2. Independent assortment happens because the chromatids of the chromosomes can face either pole - increasing genetic variation
Visuals:
- Fr same as metaphase in mitosis
Anaphase II:
1. Centromeres divide and chromatids are pulled to opposite poles
Visuals:
- Okay now this time, they truly split apart
- Tho it’s actually 4 chromatids yano
Telophase II:
1. Nuclear envelopes and nucleoli reform and the spindle disintegrates
2. Chromosomes lengthen and are no longer visible
3. Cytokinesis occurs
4. Four haploids, genetically different daughter cells are made
Visuals:
- So u know how the previous telophase there’s just 2
- This one has 4, each quarter type shit in a box
Compare meiosis I and meiosis II
Prophase:
Follows DNA replication: Meiosis I
Crossing over: Meiosis I
Meiosis II doesn’t….
Metaphase:
Alignment at equator:
Meiosis I - in pairs
Meiosis II - single file (individual chromosomes)
Independent assortment:
Meiosis I - between chromosomes
Meiosis II - between chromatids
Anaphase:
Separation at anaphase:
Meiosis I - chromosome pairs
Meiosis II - chromatids
Number of daughter cells:
Meiosis I - 2
Meiosis II - 4
Ploidy of daughter cells
Meiosis I & Meiosis II - Haploid
Compare mitosis and meiosis
Mitosis
Number of divisions:
- 2 diploid
Products:
- Genetically identical daughter cells
Chromosome:
- Diploid, same as parent
Do bivalents form?:
- No
Does crossing over occur?:
- No
Chiasmata?:
- No
Independent assortment?:
- No
Meiosis
Number of divisions:
- 4 haploid
Products:
- Genetically different daughter cells
Chromosome number:
- Haploid, half of parents
Do bivalents form?:
- Yes
Does crossing over occur?:
- Yes, in prophase I
Chiasmata?:
- Yes, in prophase I
Independent assortment?:
- Yes, in metaphase I & II
