Cell Division: Mitosis & Meiosis

[SilverStallion]

How do we grow? Where does the sperm and egg come from?

In this thread, you will understand more about 2 different types of cell division, mitosis (mai-toes-sis) and meiosis (may-o-sis). Mitosis is responsible for our growth where as meiosis is the initial process in the formation of the sex cells (also called gametes).

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Mitosis

Cells in the body, excluding some type of cells in the reproductive system, undergo mitosis in order to multiply. During a certain stage in the cell life cycle, the cell will gain enough mass and initiates mitosis to take place.

Mitosis occurs into our bone marrow (giving rise to new red blood cells) and also the liver tissue (allowing the liver to have a regenerative ability).

Below is a simplified diagram of the different stages during mitosis.



Interphase. During this phase, the nucleus of the cell contains double the number of DNA as a result of DNA replication. Once all mechanisms is in working order, the will give the "go" button and proceeds into the next phase.

Prophase is where the cell division begins.

• Chromatin undergoes a series of coiling process to produce a visible bar-like structures called chromosomes. Each chromosome consist of a pair of sister chromatids which is held together by a centromere.
• The centrioles will migrate to opposite ends (poles) and starts to assemble the mitotic spindles. These spindles will be the one that provides attachment and movement of the chromosomes.
• At the end of prophase, the nuclear envelope will start to break down and disappear, allowing the spindles to attach themselves to the centromere of each chromosome.

Metaphase

• The mitotic spindles aligned the chromosomes at the center of the cell, thus a row of chromosomes is formed.

Anaphase

• The centromere splits and the chromosome is seperated into 2 sister chomatids.
• The chromatids will start to move towards opposite ends "pulled" by the spindles.
• Once the chromatids stop moving, it marks the end of the anaphase.
• During late anaphase, cytokinesis begins.

Telophase, this phase is generally the reverse of prophase.

• The chromatids (now called chromosomes) will uncoil into chromatins again.
• The nuclear envelope forms back around the chromatins.
• The mitotic spindles will break down.
• Cytokinesis ends here.

Cytokinesis is the division of the cytoplasm of the cell. Thus at the end of mitosis, 2 daughter cells will be form. These cells will continue into the cell cycle to gain mass and mitosis will repeat itself once more.

Next, I will explain to you about meiosis.
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Meiosis

Meiosis itself consist of 2 sub-divisions, meiosis I and meiosis II.

Meiosis mainly takes place in the reproductive organs which results in sperm production (spermatogenesis) and egg production (oogenesis).

Below is a simplified diagram of the different stages of meiosis.



Meiosis I is similar to mitosis, but one big different is that during anaphase I, the centromere remains intact. Which means that no spliting of chromosomes during meiosis I. At the end of meiosis I, 2 daughter cells will be produce, each having the same number of chromosome as a parent cell.

Once meiosis I is complete and no damage is found, the 2 daughter cells proceeds into meiosis II. Meiosis II is like a second round of mitosis. This time, the centromere splits during anaphase II and at the end of meiosis II, 4 daughter cells (now called gametes) will form, but each only have half the number of chromosomes as a parent cell.

From here, depending whether the organism is a male or female, the gametes will either enters into spermatogenesis or oogenesis to produce mature sperms or eggs.
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Here, I end the subject about cell division.

Feel free to discuss with me about the topic. I will try my best to answer any doubts.

And lastly, the pictures above is either taken or drawn by me, so do ask for permission or credit it of you're gonna use it. Thanks.

Angel

[thee_boss]

the sister chromatids is separated during mitosis.

during meiosis 1, sister chromatids also gets separated.

meiosis 2 is when the each homologus copy of the diploid chromosome get separated from each other which results in an autosomic cell.

i cannot rem if the centromere hold the diplody DNA or sister chromatids or both?

[SilverStallion]

Originally Posted by thee_boss

meimei, u forgot to mention one very impt thing in the meiosis.... sister chromatids.
and their splitting...

It's in already.

Once meiosis I is complete and no damage is found, the 2 daughter cells proceeds into meiosis II. Meiosis II is like a second round of mitosis. This time, the centromere splits during anaphase II and at the end of meiosis II, 4 daughter cells (now called gametes) will form, but each only have half the number of chromosomes as a parent cell.

er... or did i really miss something?

[thee_boss]

i'm the one who missed it hehe...

i've edited liao... so how? centromere hold what together???

[SilverStallion]

Originally Posted by thee_boss

i'm the one who missed it hehe...

i've edited liao... so how? centromere hold what together???

they hold the pair of sister chromatids together.

[cwtan12]

Reminds me of Biology I took in A-levels 13 yrs ago. Something nice to refresh my memory!!

[thee_boss]

Originally Posted by SilverStallion

they hold the pair of sister chromatids together.

so if the centromere doesn't get split during anaphase of meiosis1, then wat does this anaphase separated? the corresponding homologus chromosome?

[SilverStallion]

Originally Posted by thee_boss

so if the centromere doesn't get split during anaphase of meiosis1, then wat does this anaphase separated? the corresponding homologus chromosome?

If you recall, before meiosis I, the cell is actively producing DNA. which means, for a short period of time before meiosis I starts, there is 2 times (4n) the amount of chromosomes present in the parent cell. During anaphase I, the chromosomes is shared to the 2 sister cells (now have 2n each).

then each sister cell (still 2n) will enter into meiosis II. During anaphase II, the amount of chromosomes will split again, this time the centromere will break. Resulting each sister cell have (n) number of chromosomes.

[thee_boss]

Originally Posted by SilverStallion

If you recall, before meiosis I, the cell is actively producing DNA. which means, for a short period of time before meiosis I starts, there is 2 times (4n) the amount of chromosomes present in the parent cell. During anaphase I, the chromosomes is shared to the 2 sister cells (now have 2n each).

then each sister cell (still 2n) will enter into meiosis II. During anaphase II, the amount of chromosomes will split again, this time the centromere will break. Resulting each sister cell have (n) number of chromosomes.

yeah, after some reading up, comfirmed liao.. meiosis is the splitting of homologus chromosome, for example, sex chromosome, Y from it's homologue X.

then in meiosis two, the sister chromotids split. then u got haploid cells le... yeah!

in contrast for mitosis, the sister chromotids breaks while each of the resultant daughter cells maintain the two homologues thus resulting in autosomal cells(diploidy)..yeah!

thanks man!