Question

Explain meiosis (in humans) as thoroughly as possibly, especially to address the how there comes to be 92 chromatids at some point. I just cannot seem to wrap my mind around this, maybe I'm overthinking it and that's causing me to not fill in my own gaps of understanding. Anyway, please start just from the beginning of course with maternal 15 chromosomes and paternal 15 chromosomes and use clear pictures if possible too, to highlight your written explanations. I just need to follow step by step and stage by stage the numbers, chromosomes, and chromatids to really visualize and finally get this!

Please don't answer with simple textbook like definitions or wiki quick answers. I've been all over the internet, so I'm looking to see how someone else can really break this all down for me to understand. Thanks to whoever can help.

Answer 1

I am writing the question in three parts -

◆ Purpose of meiosis

◆ Stages of meiosis

◆ Number of chromosomes, chromatids and ploidy level in different stages of meiosis in a human cell

Purpose of meiosis -

Meiosis is reductional division. It take place in germline cells of an organism to give rise to gamtes. It shows to karyokinesis, meiosis 1 and meiosis 2. Karyokinesis means nuclear division. It begins with a diploid parent cell and ends with the formation of four haploid cells. The two most important events taking place in this process are crossing over which takes place during prophase 1 and independent assortment which takes place during metaphase 1. Both of these events give rise to haploid cells which are different from each other and also from the parent cell. This leads to variation seen between parents and progenies and among progenies of same parent.

Stages of meiosis -

G1 (gap 1 phase) - cell prepares itself to divide, duplicate its organelles and cytoplasm

S (synthesis phase) - replication of DNA

G2 (gap 2 phase) - prepares itself to divide, grow in size

Prophase 1 - crossing over between non sister chromatids of homologous chromosomes, disintegration of nuclear membrane and condensation of chromatin into chromosomes

Metaphase 1 - alignment of chromosomes at middle of the cell into double equatorial plate

Anaphase 1 - separation of homologous chromosomes and their movement towards opposite poles of the cell

Telophase 1 - reappearance of nuclear membrane and loosening of chromosomes into chromatin

Cytokinesis 1 - formation of two haploid daughter cells

Prophase 2 - disintegration of nuclear membrane, condensation of chromatin into chromosome

Metaphase 2 - alignment of chromosomes at single equatorial plate

Anaphase 2 - separation of sister chromatids and movement towards opposite poles of the cell

Telophase 2 - reappearance of nuclear membrane, decondensation of chromosome into chromatin

Cytokinesis 2 - formation of four haploid daughter cells

Overview of stages -

Stage	#chromosomes	#chromatids	amount of DNA	ploidy
G1	46	46	2c	2n
S	46	92	2c to 4c	2n
G2	46	92	4c	2n
P1	46	92	4c	2n
M1	46	92	4c	2n
A1	46	92	4c	2n
T1	23	46	4c	2n
C1	23	46	2c	n
P2	23	46	2c	n
M2	23	46	2c	n
A2	46	92	2c	n
T2	23	46	2c	n
C2	23	46	c	n

1 chromatid = 1 DNA molecule

In humans, 2n = 46, n = 23

This is the best possible answer and very complete answer to understand meiosis. Please give this answer a very good rating.