Topic 4 — B: Diversity and Selection Flashcards
What do most cells contain?
exactly the same genetic information.
- do not contain same gametes (cells involved in
sexual reproduction.)
What number of chromosomes do normal body cells contain?
diploid number (2n) of chromosomes
What are diploid cells?
each cell contains two of each chromosome (a pair)
- one from the mum and one from the dad.
Sizes of chromosomes in diploid cells?
chromosomes that make up each pair are the same size and have the same genes but could have different versions of those genes
What are alleles?
an alternative form of a gene
What are homologous pairs?
pairs of matching chromosomes in diploid cells
How many homologous pairs do humans have?
- 23 pairs and 46 chromosomes in total
- diploid number for humans is 46
What are gametes?
the sperm cells in males and egg cells in females
What number of chromosomes do gametes have?
Have a haploid (n) number of chromosomes
What are haploid cells?
only contain one copy of each
chromosome in a homologous pair.
- The haploid number for humans is 23.
What happens to gametes in sexual reproduction?
two gametes join together at fertilisation to
form a zygote, which divides and develops into a new organism.
What happens at fertilisation?
haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes
- Half these chromosomes are from the father (the sperm) and half are from the mother
(the egg).
What sperms can fertilise eggs?
fertilisation is random.
- During sexual reproduction, any sperm can fertilise any egg
What does random fertilisation produce?
produces zygotes with different combinations
of chromosomes to both parents.
What does mixing of genetic material in sexual
reproduction do?
increases genetic diversity within a species
What is Meiosis?
a type of cell division
Where does Meiosis take place?
place in the reproductive organs of multicellular, eukaryotic organisms.
Cells at the beginning of meiosis?
- diploid to start with, but the cells that are formed from meiosis are haploid — the chromosome number halves.
What does meiosis produce in humans and other mammals?
produces gametes directly.
- other organisms (e.g. some insects and plants)
it produces haploid cells which later divide by mitosis to become gametes.
Without meiosis:
you’d get double the number of chromosomes when the gametes fused.
Step 1 of meiosis:
Before meiosis starts, the DNA unravels and replicates so there are two copies of each chromosome, called chromatids.
step 2 of meiosis:
The DNA condenses to form double-armed chromosomes, each made from two sister chromatids.
The sister chromatids are joined in the middle by a centromere.
Step 3 of meiosis:
Meiosis I (first division) — the chromosomes arrange themselves into homologous pairs
Step 4 of meiosis:
These homologous pairs are then separated, halving the chromosome number.
Step 5 of meiosis:
Meiosis II (second division) — the pairs of sister chromatids that make up each chromosome are separated
(the centromere is divided).
Step 6 of meiosis:
Four haploid cells that are genetically
different from each other are produced.
What in meiosis leads to genetic variation?
- Crossing over of chromatids
- Independent segregation of chromosomes
How does Crossing over of chromatids occur?
During meiosis I, homologous chromosomes come together and pair up. The chromatids
twist around each other and bits of chromatids swap over.
The chromatids still contain the same genes but now have a different combination of alleles
What does crossing over of chromatids in meiosis I creat?
four daughter cells formed from meiosis II
contain chromatids with different alleles
Meiosis I in crossing over:
- homologous chromosomes pair up
- chromatids cross over
- one chromosome from each homologous pair ends up in each cell
Meiosis II in crossing over:
- each cell has a different chromatid so a different set of alleles which increases genetic variation in potential offspring
What is each homologous pair of chromosomes in your cells is made up of?
- one chromosome from your mum (maternal) and one chromosome from your dad (paternal).
Independent segregation of chromosomes:
When the homologous pairs are separated in meiosis I, it’s completely random which chromosome from each pair ends up in
which daughter cell.
- so independent segregation (separation) of the chromosomes happens
- ‘shuffling’ of chromosomes leads to
genetic variation in any potential offspring.
What is independent segregation?
four daughter cells produced by meiosis
have completely different combinations of those maternal and paternal chromosomes
