Mitosis And Meiosis➗ Flashcards
What are the three stages of a cell cycle?
- Interphase
- Mitosis
- Cytokinesis
What are the stages of mitosis?
•Prophase -chromosomes are present •Metaphase -middle •Anaphase -apart •Telophase -two nuclei
What is a histone?
A protein that DNA wraps around
What is the cell cycle?
- The sequence of events that occurs between one cell division and the next
- Mostly interphase, then m phase (mitosis and cytokinesis)
What happens at interphase?
- G1: cellular components are duplicated, excluding chromosomes
- S: each of the 46 chromosomes is duplicated by the cell by DNA replication
- G2: Cell double checks the duplicate chromosomes
- DNA is in form of chromatin
- Protein synthesis
- ATP synthesis
- DNA replication
- Organelles produced
- Centrioles replicated
What happens in mitosis?
- Nucleus division
- Genetically identical cells
- Continuous process
What happens in prophase? (Mitosis)
•Chromatin condenses to form chromosomes
-DNA becomes inactive which enables them to move easily
•Sister chromatids are seen, attached to the centromere
•Nucleoli disappear
•Centrioles move to poles and begin to form spindle fibres
•Microtubules move to poles
-some make contact with centromeres of the chromosomes
•Chromosomes move towards the equator of the cell
•Nuclear membrane breaks down
What happens in metaphase? (Mitosis)
- The centromeres of all the chromosomes are lined up on the equator
- Spindle apparatus is fully formed
- Spindle fibres attach to the chromosomes at the centromere
What happens in anaphase? (Mitosis)
•The spindle fibres contract
•The centromere separates
•Sister chromatids are drawn to opposite poles of the cell
-now referred to as daughter chromosomes
REMEMBER TO DRAW CHROMOSOMES AS V SHAPES TO SHOW THEM BEING PULLED
What happens in telophase? (Mitosis)
- Begins when the two sets of daughter chromosomes have reached the two poles
- Chromosomes unwind back to chromatin
- Nuclear membrane and nucleoli reform and chromosomes become less visible under the microscope
- At the end of telophase, the spindle apparatus disappears
When does Mitosis finish?
When two identical nuclei are formed
What happens during animal cytokinesis?
Involves formation of a cleavage furrow which pinches the cell in two
What happens during plant cytokinesis?
- Membrane enclosed vesicles containing cell wall materials collect at the midline on the parent cell
- Vesicles join to form a cell plate
- The plate grows outwards and fuses with the cell surface membrane so that two distinct cell walls can be formed, dividing the daughter cells
Where does mitosis happen in animals and plants?
- Stem cells
- Skin cells
- Meristem tissue of roots and shoots
- Cambium cells in trees
Significance of mitosis
•Repeated cell renewal and cells are genetically identical (clones)
•Asexual reproduction
•Damage and disease
-mutations in genes that control the cell cycle
-can be caused by mutagens
-can lead to uncontrolled cell division - tumour
What happens in mitosis?
•One diploid parent cell divides to form four haploid daughter cells
-genetically distinct from each other and the parent cell
•Continuous process
What happens in meiosis 1?
Homologous chromosomes are separated - diploid->haploid
What happens in meiosis 2?
Sister chromatids are separated - produce 4 haploid cells
What happens during prophase 1? (Meiosis 1)
•Chromatin condenses to form chromosomes
-2 identical chromatids joined by the centromere
•Nuclear membrane breaks down
•Nucleoli disappear
•Centrioles begin to produce microtubules which form the spindle
•Homologous chromosomes pair up to form a bivalent
•Crossing over - four chromatids intertwine and exchange information/segments
•Towards the end of prophase 1, the paired chromosomes are moved by the microtubules of the spindle apparatus
What is crossing over?
- Four chromatids (two from each chromosome) intertwine and exchange information/segments. This occurs at the chiasma.
- Source of genetic variation in meiosis
What is the chiasma?
The point at which two chromatids cross over
How is genetic variability created?
•Crossing over
What happens during metaphase 1? (Meiosis 1)
•Homologous chromosomes/bivalents line up on the equator
-this arrangement is a random distribution so it leads to independent assortment - another source of variation
•Microtubules from opposite poles attach to a centromere of one chromosome
What happens during anaphase 1? (Meiosis 1)
- Spindle fibres contract
- Homologous chromosomes/bivalents separate and move to opposite poles
- Sister chromatids of each chromosome move as a single unit
What happens during telophase 1? (Meiosis 1)
- A haploid set of chromosomes arrives at each pole
- Only one of each pair of alleles will occur in a single gamete
- Each chromosome still consists of two chromatids
- Cytokinesis usually coincides with telophase and two haploid cells are formed
What does haploid mean?
Half the number of chromosomes
What does diploid mean?
Two sets of chromosomes
What happens during prophase 2? (Meiosis 2)
- Not preceded by interphase
- No replication of DNA
- Chromosomes condense
- Spindle apparatus forms at right angles to the spindle used in meiosis 1
- Chromosomes move to the equator of the cell
What happens during metaphase 2? (Meiosis 2)
- Chromosomes line up along the equator
* Spindle fibres attach to the centromere
What happens during anaphase 2? (Meiosis 2)
- Spindle fibres contract
- Centromeres split
- Sister chromatids are separated and move to opposite poles as individual daughter chromosomes
What happens during telophase 2? (Meiosis 2)
- A complete haploid set of chromosomes gathers at each pole and is encapsulated in a nuclear membrane
- Chromosomes uncoil to form chromatin
- Nucleoli and nuclear membrane reform and cytokinesis occurs - each cell produces two cells
Why is meiosis important?
- Halves the number of chromosomes in the gametes so species diploid number maintained following fertilisation
- Produces genetic variation
What does recombinant mean?
DNA has been recombined due to crossing over
What is independent assortment?
Different genes independently separate from one another when reproductive cells develop
Controlling the cell cycle
•By proteins, e.g. tumour suppressor proteins
•Protooncogenes are genes that code for these proteins
•Mutations change the DNA sequence
-if proto-oncogenes mutate, they form oncogenes that can lead to uncontrolled cell division - a tumour
Compare mitosis in animal and plant cells
- A - centrioles / P - no centrioles
- A - in various tissues / P - meristem tissue
- A -cleavage furrows / P - cell plate grows out from centre
Homologous chromosome pairs
•Identical size and shape
•Same genes at same loci
-one from each parent
What does polyploid mean?
More than 2 sets of chromosomes (in plants)
What is ploidy level?
Number of chromosomes sets in a cell
How does meiosis create genetic diversity?
•Independent segregation
-metaphase 1 - homologous chromosomes
-metaphase 2 - chromatids
•Genetic recombination by crossing over in prophase 1
Explain how four different haploid sperm are produced at the end of meiosis
- Diploid parent divides twice
- During prophase 1, crossing over/independent assortment allows the exchange of genes
- Only one chromatid from each pair of chromosomes in each daughter cell
What must be done to tissues to enable chromosomes to be seen more clearly?
Be stained
Limitations of calculating mitotic index via squashing technique
- Cells might not be stained enough
- Cells could be damaged
- Inconsistent counting of cells, i.e. counting/discounting of those partly in
- Layers If cells overlap
Why are nuclei only visible in some cells and where they are visible, why they appear to be different sizes?
- Angle of section may not include a nucleus
* Where nuclei are visible, it may have been cut at different levels/planes
Explain how the chromosome number is halved during meiosis
- Homologous chromosomes (pairs)
* One of each pairs goes to each daughter cell/opposite poles
Chromosome structure
- Consists of DNA and a protein
- Genes are specific sections of the DNA in chromosomes that code for one polypeptide
- Each chromosome is made up of two identical chromatids joined by a centromere
- Chromosomes form homologous pairs
- Each chromosome in a pair contains the same genes but in different forms
How many chromosomes do human diploid cells contain?
46 chromosomes, forming 23 homologous pairs
How many chromosomes do human haploid cells contain?
23 chromosomes
What is a tumour?
An abnormal mass of tissues created by uncontrolled cell division
What is produced by meiosis?
- Produces gametes
- 4 haploid daughter cells are produced which are genetically different from each other and from the original cell
- As the male and female gametes produced are haploid, when they fuse during fertilisation the zygote formed has the full diploid number of chromosomes
- As each gamete is genetically different, meiosis introduces genetic variation into sexual reproduction
What are the sources of genetic variation in meiosis?
•Crossing over
-during prophase 1, homologous chromosomes in a bivalent swap genes at the chiasmata
•Independent assortment
-during metaphase 1, homologous chromosomes in bivalents arrange themselves randomly at the equator
•Mixing of parental chromosomes at fertilisation
-chromosomes of both parents (the genotypes) are mixed to form the diploid zygote
