Chapter 6.2 Meiosis Flashcards
What is sexual reproduction
Where two sec cells (gametes) containing half the genetic material HAPLOID fuse in fertilisation to make a zygote which is diploid and divides by mitosis, making an embryo which is the start of an organism
What is the difference between normal body cells and gametes (homologous ?)
2) normal body cells are diploid, containing 2n amount of chromosomes , a pair one from both sides. The pair of chromosomes have the same genes (but not the same type of gene) and size, thus are HOMOLOGOUS pair of chromosomes. This also produced by mitosis
- a sex cell contains n amount of chromosomes haploid so that in fertilisation the right amount of chromosomes are made
This also produced by meiosis
Why is Meoisis considered reduction division?
Mitosis produces two daughter cells genetically identical and has same amount of chromsomes, however meiosis produces 4 genetically different with HALF THE AMOUNT OF CHROMOSOME, as this has reduced, it is reduction division
What is an allele , relate to homologous chromosomes (where are they)
In a homologous chromosome you will have same gene and size, but the VARIANT OF THE GENE IS DIFFERENT (might be), this is how variation happens .
An allele is a different version of the same gene
2) as homologous chromosomes have the same genes in same positions, they will be same length and centromeres be at same position too.
Remember meiosis starts at interphase, what happens here (same for mitosis
DNA duplicates and chromatin now takes on the form of chromatids, where two sister chromatids are genetically identical held at centromere
Which out of meiosis 1 or 2 is reduction division
It is meiosis 1, here the cells go fro, being diploid (idk) to haploid as homologous pairs of chromosomes are separated into two cells.
Think of diploid being 2 n (like twice chromosomes even though it’s 4)
What happens in prophase 1?
New then old
HOMOLOGOUS CHROMOSOMES(1 with 1) pair up, and this forms BIVALENTS. This results in cross over and change of genetic information leading variation When broken later
- here the chromatin still condenses
- Nucleolus still breaks down
- nuclear membrane eventually breaks down
- and the centrioles move to opposite poles while spindle fibre is formed which extends to the centromeres
What are bivalents?
These are what you call when homologous chromosomes chromatids entangle , leading to crossing over of genetic information
Wherever there is crossover a chasima is formed
-this genetic information is exchanged when these points break later in anaphase
What happens in metaphase 1?
Same / different)
(What is independent assortment and hoe leads to genetic variation)
(Similar to mitosis)
- spindle fibre connected to centromere causes chromosomes to line up along the metaphase plate
- however this time the HOMOLOGOUS PAIRS LINE UP TOGETHER IN THR MIDDLE (before it didn’t matter)
- here the ORIENTATION OF THE PAIRS IS RANDOM AND INDEPENDENT, this is called independent assortment
- the direction one allele may face isn’t determined by anything , so
- as a result when they split up, MANY DIFFERENT combinations of alleles facing poles are formed , this creates MORE GENETIC VARIATION
What happens in anaphase 1
What is formed and how is genetic variation happen here
- Instead of spindle fibre contracting causing chromatids to split, this time they stay together and the homologous chromosomes are pulled to opposite ends
- the points of dna where the chromatids were entangled , CHIASMATA now break off and rejoin, forming RECOMBINANT CHROMATIDS,
- this was an exchange of DNA as genes may have exchanged , combination of alleles have changed and lead to genetic variation
What happens in telophase 1 and cytokinesis
Similar to mitosis
= chromosomes are at poles nuclear membrane begins to reform and Nucleolus
- chromosomes uncondesne
Cytokinesis happens again
- here cleavage furrow happens which the cytoskeleton pinches the cell surface membrane until it can fuse with itself to create two cells
Finally reduction division is complete, from diploid to haploid
What happens in prophase 2
Same to mitosis
- chromosomes condense again , but have two chromatids still
- nuclear membrane breaks and Nucleolus too
- spindle fibre starts being made as centrioles are moved
What happens in metaphase 2
Remember…
Now chromosomes line up on metaphase plate again by movement of spindle fibre
- here independent assortment happens again, and the combination of alleles facing the poles are random
- this will lead to more genetic variation (however no crossover happens )
What happens in anaphase 2
Spindle fibres contract and pull chromatids apart (these are not the same )
- it is different to anaphase 1 as thst had the chromatids still together, but this is similar to anaphase of mitosis as centromere is divided and chromatids of chromosomes pulled to separate sides
Telophase 2 and cytokinesis ?
- chromosomes uncoil again
- nuclear envelope starts to form and Nucleolus too around each set of chromosomes which assembled at opposite poles
Cytokinesis happens by cleavage furrow firmed where cytoskeleton pinches membrane to fuse
- as a result you now have 4 daughter cells, haploid, genetically different
