Variation 3.4 Flashcards
What happens in early prophase?
DNA condenses; chromatin coils tightly to form dense, visible chromosomes.
DNA has replicated, so that the
chromosomes now have their 2 identical chromatids, joined by a centromere in the familiar “X” shape.
What happens in prophase?
The nuclear envelope disintegrates and forms vesicles.
Nucleolus is no longer visible.
The centrioles move to opposite
poles of the cell, and form a spindle of microtubules.
What happens in metaphase?
Chromosomes attach to the spindle at their centromere.
Chromosomes are lined up on the equator of the spindle.
One chromatid from each chromosome is attached, by the microtubules, to each pole.
What happens in anaphase?
The centromeres divide
The spindle contracts.
The separated chromatids move
towards their respective poles.
Poles move further apart
What happens in telophase?
The daughter chromosomes are now at the poles of the cell.
The spindle disappears.
Nuclear envelope reforms.
Chromosomes unravel to form
chromatin
What is meiosis?
Meiosis is a type of cell division which is essential to produce gametes for sexual reproduction
Where does meiosis occurs?
It occurs in plants - in the anthers and ovules - and in animals, in the testes (male) and ovaries (female).
Why does meiosis occur?
So that genetic information is halved (from diploid 🡪 haploid). This is necessary to ensure that there is correct number of chromosomes at fertilisation.
What happens during prophase of meiosis I?
During prophase of meiosis I,
homologous pairs of chromosomes form bivalents
(meaning they pair up closely)
Chromatids twist around each
other. The point where they join
are called chiasmata.
Fragments of non-sister
chromatids swap over and so
genes are exchanged.
What is genetic diversity?
The number of different alleles of genes in a species or population.
What is allele frequency?
Allele frequency is how often a particular allele occurs within a
population.
Why is genetic diversity important?
If a population has a low genetic diversity, it might not be able to
adapt to change in an environment, for example a disease or climate change.
How can genetic diversity in a population be increased?
Mutations causing new alleles;
Migration from other populations – this is called gene flow.
What is genetic drift?
The process whereby an allele becomes more common in a
population due to chance.
What is genetic bottleneck?
Ecological events may reduce population sizes dramatically e.g. earthquakes, floods, fires.
Disasters that are unselective .
Small surviving populations are unlikely to be representative of the original population.
By chance alleles may be over-represented among survivors, some may be eliminated completely.
