9.2- MEIOSIS AND GENETIC VARIATION Flashcards
In what ways can cell division occur? (2)
mitosis
meiosis
What does mitosis produce?
two daughter cells with same number of chromosomes as parent cell as each other
What does meiosis usually produce?
four daughter cells, each with half the number of chromosomes as parent cell
What do two gametes do in sexual reproduction?
two gametes fuse to give rise to new offspring
What is the diploid number of chromosomes in humans?
46
Why must the number of chromosomes be halved at some stage in the cycle?
to maintain constant number of chromosomes in adults of species
What does the halving of the number of chromosomes occur due to?
meiosis
In most animals when does meiosis occur?
in formation of gametes
In some plants i.e. ferns, how are gametes produced?
by mitosis
In the fern (type of plant) life cycle, when does meiosis occur?
in formation of spores
What does every diploid cell of organism have?
two complete sets of chromosomes: one set provided by each parent
What happens to homologous pairs during meiosis?
homologous pairs of chromosomes separate, so only one chromosome from each pair enters daughter cell
What is the haploid number of chromosomes in humans?
23
What happens when two haploid gametes fuse at fertilisation?
diploid number of chromosomes restored
How many nuclear divisions does meiosis involve?
two nuclear divisions
What happens in the first division (meiosis 1)?
homologous chromosomes pair up + their chromatids wrap around each other
equivalent portions of these chromatids may be exchanged in crossing over
What happens by the end of the first division (meiosis 1)?
homologous pairs have separated, with one chromosome from each pair going into one of the two daughter cells
What happens in the second meiotic division (meiosis 2)?
chromatids move apart
What happens by the end of the second meiotic division (meiosis 2)?
four cells have usually been formed
in humans each of these cells contain 23 chromosomes
What does meiosis produce among the offspring?
genetic variation
What can the genetic variation among offspring from meiosis lead to?
adaptations that improve survival chances
How does meiosis bring about genetic variation? (2)
independent segregation of homologous chromosomes
new combinations of maternal + paternal alleles by crossing over
What is a gene?
length of DNA that codes for polypeptide
What is the locus?
position of gene on chromosome or DNA molecule
What is an allele?
one of the different forms of a particular gene
What are homologous chromosomes?
pair of chromosomes, on maternal + one paternal that have same gene loci
What does each chromosome do during meiosis 1?
each chromosomes lines up alongside its homologous partner
In what way do each chromosome arrange themselves when they line up alongside its homologous partner during meiosis 1?
do so at random
What happens to each of the homologous pair when they line during meiosis 1?
one of each pair will pass to each daughter cell
How is where which pair of the homologous pair from meiosis 1 determined?
which one of pair goes into daughter cells, and with which one of any of other pairs, depends on how pairs lined up in parent cell
Why is the combination of chromosomes of maternal + paternal origin that go into the daughter cell at meiosis 1 a matter of chance?
since pairs lined up at random
What is the process of the chromosomes lining up in meiosis 1 and being separated called?
independent segregation
What does each member of a homologous pair of chromosomes have?
has exactly the same genes + so determines characteristics
What can be different about homologous chormosomes?
alleles of genes
What does the independent assortment of the homologous chromosomes do?
produces new genetic combinations
How do the haploid cells fuse at fertilisation?
randomly
What do the haploid cells fuse to restore at fertilisation?
restore diploid state
How is variety produced in offspring from gametes?
each gamete has different make-up + their random fusion produces variety
How is even more variety produced when gametes fuse?
gametes come from different parents two different genetic make-ups combined + so even more variety results
What happens after the chromosomes line up alongside its homologous partner in meiosis 1?
chromatids of each pair become twisted around one another
What happens during the process of chromatids of each pair becoming twisted around each other?
tensions created + portions of chromatids break off
What may happen to the broken portions when chromatids of each pair become twisted around one another and tensions created + portions of chromatids break off?
broken portions might then re-join with chromatids of its homologous partner
Usually, what is it during crossing over?
usually it’s equivalent portions of homologous chromosomes that are exchanged
What happens as the chromatids twisted, tensions created + broken portions might rejoin with chromatids of its homologous partner?
new genetic combinations of maternal + paternal alleles produced
What is the process of chromatids crossing over one another many times known as?
crossing over
What is the process of broken-off portions of chromatids recombining with another chromatid known as?
recombination
What does crossing over increase?
genetic variety
Mathematical calculation to determine number of possible combinations of chromosomes for each daughter cell?
2^n
n- number of pairs of homologous chromosomes
Mathematical calculation to determine number of possible combinations of chromosomes in offspring produced as result of sexual reproduction?
(2^n)^2
n- number of pairs of homologous chromosomes
What are the calculations to determine number of possible combinations based on?
chromosomes staying intact throughout meiosis
In practice from the calculations for number of possible combinations, what do we know?
crossing over between chromatids during meiosis I exchanges sections of chromosomes between homologous pairs in process called recombination
as recombination occurs each time gametes made, it will greatly increase number of possible chromosome combinations in gametes
