W5 Sexual reproduction and meiosis/ Lec 10 Flashcards
How many pairs of chromosomes in humans?
Humans have 22 homologous chromosomes and a pair of sex chromosomes
What do sex chromosomes define?
They participate in gender determination at birth
XX= female
XY= male
X is much larger than Y and they carry different genes
What are gametes?
What are examples?
Diploid or haploid?
Reproductive cells involved in sex reproduction.
e.g. egg and sperm cell
How are gametes formed?
1) Spermatozoa and egg (n) are produced by
meiosis from diploid germ-line cells (2n)
➢ maternal and paternal chromosome sets
are partitioned into the single
chromosome sets of the gametes
How is the diploid structure of somatic cells regenerated?
Fertilisation (fusion) of gametes regenerates the diploid structure of the somatic cells (zygote) with homologous chromosomes from both)
What is meiosis.
What is produced?
What cells does it occur in?
Used for sexual reproduction.
Gametes, 4 non-identical daughter cells
Specialised germ-line cells that reside in the ovaries or testes
What are the differences between mitosis and meiosis?
Meiosis
* Used only for sexual reproduction
* Requires two nuclear divisions (Meiosis I & II)
* Halves chromosome number (2n to n)
* Produces four daughter nuclei
* Produces daughter cells genetically different
from parent and each other
Mitosis
▪ Used for asexual reproduction and growth
▪ Requires one nuclear division
▪ Preserves chromosome number (2n to 2n)
▪ Produces two daughter nuclei
▪ Produces daughter cells genetically identical
to parent and to each other
What are the 3 main events of meiosis?
- Interphase, constituting in 3 phases: G1, S , G2
In S phase, DNA is replicated (sister chromatids)
2.Meiosis I (1st division), with 4 stages + cytokinesis I
- Meiosis II (2nd division) with four stages + cytokinesis II
What happens during meiosis l (overview not stages)
➢ Separation of the pairs of homologous chromosomes, after the process of pairing of the duplicated homologous chromosomes along their entire length
➢ Homologous chromosomes are similar but not identical
What happens during mitosis ll? (overview not stages)
Four stages + cytokinesis II
➢Separation of the sister chromatids (copies of the same chromosome)
Meiosis l - Prophase l
The meiotic spindle forms from the cell poles
(centrosomes)
* Nuclear envelope breaks down
* Duplicated chromosomes (with sister chromatids) homologs (and the sex chromosomes) condense and are brought together (lined up)
during a process called PAIRING (or synapse)
➢ Each pairing forms a structure called bivalent, which give rise to a unit of four sister chromatids stuck together (tetrad)
When does crossing over occur?
Prophase l meiosis l
What occurs during crossing over?
➢Genetic exchange between DNA
segments of the non-sister chromatids
within tetrads of homologous chromosomes
➢One or more cross-over events can occur
per tetrad at sites called chiasmata (s. chiasma)
➢the chromatids break in the same place and
sections of chromosomes (alleles) are
swapped (based on similar sequences)
How does crossing over contribute to genetic variation?
➢ Allele reassortment
➢ This creates chromosomes that are unique mosaics (mixture) of the maternal and paternal homologs from
which they arise
➢ Offspring will have different set of
alleles to their parents
What occurs in metaphase l?
Metaphase I
* The spindle microtubules from both poles
capture each tetrad (through kinetochores), and
move them towards the centre of the meiotic
plate (at the equatorial plate)
* Homologous chromosomes are aligned on either side
of the meiotic spindle
➢ the orientation of the homologous chromosomes
on the metaphase plate is random (independent
assortment)
* As in mitosis, there is at late metaphase I checkpoint
to control the correct attachment between spindle
microtubules and kinetochores of chromosomes
