<a https://www.thestudentroom.co.uk/showthread.php?t=7467335'> Year 12s: what will be the first way you research your future options? >>

Can someone please help with Meiosis (AS)

CoolCavy

Volunteer Team

Ok at this point i think we have established i'm not great at biology lol. However tbf when we did this she wrote it in tiny letters on the board and messed it up halfway through. I just don't get what is going on and yes i have googled it but all i get is weird things about prophase 1 and all this stuff and it is all over complicated. If someone could tell me if my run through is right, or even better write one themselves (i will love you forever :adore:

1) DNA unravels and replicates to form the 2 copies of each chromosome seen in the first stage (chromatids).
2) the first arrow represents the DNA condensing to form double armed chromosomes made from 2 sister chromosomes joined by a centromere.
3) in the second stage after the arrow the chromosomes have arranged themselves into their homologous pairs.
4) The two opposite arrows then represent the Homologous pairs separating (so there is a big one so scientific ik :colondollar:

and a little on in each cell). This halves the chromosome number which is important for fertilisation so that the chromosome number is restored when the gametes fuse.
5)The four opposite arrows represents the sister chromatids that make up each homologous pair being separated as the centromere is broken. This results in four genetically different gametes as a result of independent segregation.

Can someone please check that and tell me if it is right? :redface:

even if it is wrong idm i just want to get my head around this :colondollar:

Reply 1

troubadour.

I don't remember much of this, so...

@Changing Skies?
@Ethereal World?

Reply 2

M0nkey Thunder

Original post by CoolCavy

1) DNA unravels and replicates to form the 2 copies of each chromosome seen in the first stage (chromatids).2) the first arrow represents the DNA condensing to form double armed chromosomes made from 2 sister chromosomes joined by a centromere.3) in the second stage after the arrow the chromosomes have arranged themselves into their homologous pairs.4) The two opposite arrows then represent the Homologous pairs separating (so there is a big one so scientific ik :colondollar:

and a little on in each cell). This halves the chromosome number which is important for fertilisation so that the chromosome number is restored when the gametes fuse.5)The four opposite arrows represents the sister chromatids that make up each homologous pair being separated as the centromere is broken. This results in four genetically different gametes as a result of independent segregation.Can someone please check that and tell me if it is right? :redface:

even if it is wrong idm i just want to get my head around this :colondollar:

I'm not sure what exam board you're doing, but I do think learning about prophase, metaphase, anaphase and telophase is vital.Well at least in OCR we have to be able to talk about each and identify them if given images. I'll talk about the steps I listed above if you need it and a separate bit commenting on your explanation :h:

Well first we have Meiosis 1 and Meiosis 2. Meiosis 1 is known as a reduction division as we end up halving the number of homologous chromosomes we find in each cell. Meiosis 2 is quite like mitosis, aside from the genetic variation between gametes and the production of 4 haploid cells instead of the 2 in mitosis, as they both end up producing cells with equal amount of genetic information and have more similarities I'll mention later on.

Comments on your explanation:

1-3) -You could also mention that the DNA replicates during interphase, particularly in the S phase.-As the homologous chromosomes join to each other, to form a bivalent, there's crossing over between chromatids which produces genetic variation.

4) It's important to know what causes this. Centrioles are at either end (poles) of the cell and spindle fibres join to the centromere. The spindle fibres, suspended from the centriole and centromere, shorten. This shortening ends up pulling the homologous chromosomes to either end. The nature in which this occurs is random; as you rightly said, is independent segregation/ assortment.

5) You're correct for this part; it's representative of what occurs during Meiosis 2.

Meiosis 1 and 2:

Meiosis 1:

Prophase 1: -Chromosomes condense, nucleolus disappears, nuclear envelope breaks down and homologous chromosomes pair up to form bivalents.
-The chromatids in the homolous pairs of the bivalent cross over, causing genetic variation.
-Spindle formation begins (so centrioles move to either end of the cell).

Metaphase 1: -Bivalents line along the metaphase plate (the equator of the cell).
-Spindle fibres join with the centromere, suspended from centrioles at both ends of the cell.
-The arrangement of the homologous chromosomes is random (independent segregation).

Anaphase 1: -Spindle fibres shorten and pull homologous chromosomes to either end of the cell.
-The points at which chromatids had crossed over break off and rejoin at chiasmata,
-These are recombinant chromatids.

Telophase 1: -The homologous chromosomes arrange themselves at both ends of the cell and uncoil.
-The nuclear envelope reforms and cytokinesis begins.

Meiosis 2:

Prophase 2: -So here we still have: chromosomes condensing, nucleous disappearance, nuclear envelope breakdown and spindle formation.
-Bivalents aren't formed so there's no crossing over.

Metaphase 2:- Like mitosis, the chromosomes arrange themselves along the metaphase plate, but different to mitosis, these chromomosomes are not genetically identical so independent assortment occurs producing even more genetic variation.
-Spindle fibres join to the centromere of the chromosomes.

Anaphase 2: -The chromatids are pulled to either end of the cell due to the shortening in spindle fibres.

Telophase 2: -The chromatids arrange themselves at either end of the cell and uncoil.
-The nuclear envelope reforms and cytokinesis begins.
-Produces 4 haploid cells which all have the same amount of genetic information as they 2 haploid cells produced by mitosis.

Feel free to ask any questions :smile:

(edited 7 years ago)

Reply 3

rtaylor445

It can help to remember the steps in a really simplistic sequence like this:

Please
Make
Another
Two

As prophase, metaphase, Anaphase and Telophase, and then obviously we also have interphase as the stage in which no division occurs and mostly the cell grows. Leicester uni has a really good page to help you get your head around it: http://www2.le.ac.uk/departments/genetics/vgec/schoolscolleges/topics/cellcycle-mitosis-meiosis

This animation is actually in terms of visualing the steps and understanding why they happen:
http://highered.mheducation.com/sites/0072495855/student_view0/chapter28/animation__stages_of_meiosis.html

You are much better off being able to visualise it so you can use this to help you in the exam, than to to try to learn huge chunks of text :smile:

understand the concept of what happens in each step and try to draw it out whilst explaining it to yourself or someone else, it helps me a lot to explain something to someone. Hope this helps