D2.1 Cell and Nuclear Division Flashcards
Cytokinesis, mitosis and meiosis
Define cytokinesis.
splitting of cytoplasm
Distinguish between mitosis and cytokinesis.
Mitosis is division of the nucleus; cytokinesis is division of the cytoplasm
Compare and contrast cytokinesis in plant and animal cells.
In an animal cell, a ring of contractile actin and myosin proteins pinches a cell membrane together to split the cytoplasm. In a plant cell, vesicles assemble sections of membrane and cell wall to achieve splitting.
State two examples where unequal cytokinesis occurs.
Budding and human oogenesis.
State the role of mitosis.
Mitosis maintains the chromosome number of cells.
State the role of meiosis.
Meiosis halves the chromosome number and generates genetic diversity.
What must occur prior to both mitosis and meiosis?
DNA replication
Outline the role of histones in the condensation of DNA.
Histones package DNA into structures called nucleosomes
Each nucleosome consists of a strand of DNA coiled around eight histone proteins. Condensation occurs by the repeated coiling of the DNA molecule (supercoiling).
How are chromosomes moved during cell division?
Microtubules and microtubule motors are responsible for the movement of chromosomes during cell division.
Microtubules are tubulin fibres that form part of the cytoskeleton of the cell. They are able to lengthen and shorten in order to enable chromosome movement.
List the phases of mitosis.
Prophase
Metaphase
Anaphase
Telophase
Outline key processes in prophase.
Chromosomes condense
The two centrosomes move towards opposite poles
Spindle fibres begin to emerge from the centrosomes
The nuclear envelope breaks down into small vesicles
The nucleolus disappears
Outline key processes in metaphase
Centrosomes reach opposite poles.
Spindle fibres continue to extend from centrosomes and reach the chromosomes and attach to the centromeres.
Chromosomes line up at the equator of the spindle.
Outline key processes in anaphase
The sister chromatids separate at the centromere (the centromere divides in two).
Spindle fibres (protein microtubules) begin to shorten.
The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres.
Outline key processes in telophase.
Chromosomes arrive at opposite poles and begin to decondense.
Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes.
The spindle fibres break down
New nucleoli form within each nucleus.
Define the term ‘diploid’
contains two copies of every chromosome (one maternal copy and one paternal copy)
Define the term ‘haploid’
possessing only one copy of each chromosome
What is the result of meiosis I?
the homologous pairs of chromosomes are split up, to produce two haploid (n) nuclei
What is the result of meiosis II?
the chromatids that make up each chromosome separate to produce four haploid (n) nuclei
What is a bivalent?
A pair of homologous chromosomes
What is a chiasmata?
The crossing points where there can be an exchange of genetic material (alleles) between non-sister chromatids in the bivalent
What is crossing over?
This exchange of genetic material between non-sister chromatids
Why is meiosis I known as reduction division?
Because in meiosis I the chromosome number halves, from 2n to n, in the first division of meiosis (meiosis I), not the second division (meiosis II)
What are recombinant chromosomes?
The result of crossing over, where there is a new combination of alleles on the two chromosomes following crossing over in prophase I
Why is meiosis important for a sexual life cycle?
The halving of the chromosome number during meiosis is very important for a sexual life cycle as it allows for the fusion of gametes.
What is a zygote?
A fertilised egg cell, the result of the fusion of the nuclei of two gametes
When can non-disjunction occur in meiosis?
In anaphase I and anaphase II
What happens if non-disjunction occurs?
If the abnormal gametes are fertilized, then a chromosome abnormality occurs as the diploid cell (zygote) will have the incorrect number of chromosomes
Down Syndrome is caused by trisomy of what chromosome number?
Chromosome #21
How many chromosomes would be seen in cells of individuals with Down Syndrome?
Individuals with this syndrome have a total of 47 chromosomes in their cells as they have three copies of chromosome 21.
Outline the link between maternal age and Down Syndrome.
The risk of chromosomal abnormalities increases significantly with age. The age of the mother is particularly important in the case of Down Syndrome as non-disjunction is more likely to happen in older ova.
What is random orientation?
The random alignment and positioning of homologous or sister chromatids during meiosis.
What formula can be used to calculate the number of possible combinations produced at the end of meiosis I?
2 to the power of n can be used to calculate the number of possible combinations produced at the end of meiosis I, where ‘n’ is the haploid number.
Human cells have a diploid number of 46, and therefore a haploid number of 23. The number of possible outcomes after meiosis I is 2 to the power of 23 = 8 388 608.
What processes in meiosis generate genetic diversity?
- Random orientation of bivalents
- crossing over
