Daughter cells resulting from mitosis are diploid, while those resulting from meiosis are haploid. Daughter cells that are the product of mitosis are genetically identical. In addition, in meiosis I, the chromosomal number is reduced from diploid 2n to haploid n during this process. It is very simple to count number of DNA molecules or chromosome during different stages of cell cycle. In meiosis, however, you start with a diploid cell that divides twice to produce four haploid cells.
In other words a diploid cell that has 2n chromosomes produces four cells, each of which contains n chromosomes. Meiosis mainly takes place in sperm cell male and in egg cell female. In the male, meiosis takes place after puberty. Diploid cells within the testes undergo meiosis to produce haploid sperm cells with 23 chromosomes. The number of chromosomes is reduced from 46 23 pairs to 23 during the process of meiosis. Because they have only half the total chromosomes in a somatic cell, they are termed haploid n.
In a human egg or sperm, there are 23 chromosomes , one of which is an X or Y. Mitosis occurs in every cell of the body except in germ cells which are produced from meiotic cell division. What is an unreplicated chromosome? Category: science genetics. An unreplicated chromosome contains one double strand -DNA molecule. A replicated chromosome contains two identical double strand -DNA- molecules, the chromatids, that are joined at their centromere. Are chromatids DNA? Is a chromatid half a chromosome?
Why are chromatids important? Functions of Sister Chromatids. How many chromatids do humans have? How do you count chromatids? Let me delete some of that stuff over here. Delete that stuff right there. So you have this double helix. They were all connected. They're base pairs. Now, they separate from each other. Now once they separate, what can each of these do? They can now become the template for each other.
If this guy is sitting by himself, now all of a sudden, a thymine base might come and join right here, so these nucleotides will start lining up. So you'll have a thymine and a cytosine, and then an adenine, adenine, guanine, guanine, and it'll keep happening. And then on this other part, this other green strand that was formerly attached to this blue strand, the same thing will happen. You have an adenine, a guanine, thymine, thymine, cytosine, cytosine.
So what just happened? By separating and then just attracting their complementary bases, we just duplicated this molecule, right? We'll do the microbiology of it in the future, but this is just to get the idea. This is how the DNA makes copies of itself. And especially when we talk about mitosis and meiosis, I might say, oh, this is the stage where the replication has occurred. Now, the other thing that you'll hear a lot, and I talked about this in the DNA video, is transcription.
In the DNA video, I didn't focus much on how does DNA duplicate itself, but one of the beautiful things about this double helix design is it really is that easy to duplicate itself. You just split the two strips, the two helices, and then they essentially become a template for the other one, and then you have a duplicate.
Now, transcription is what needs to occur for this DNA eventually to turn into proteins, but transcription is the intermediate step. And then that mRNA leaves the nucleus of the cell and goes out to the ribosomes, and I'll talk about that in a second. So we can do the same thing. So this guy, once again during transcription, will also split apart. So that was one split there and then the other split is right there.
And actually, maybe it makes more sense just to do one-half of it, so let me delete that. Let's say that we're just going to transcribe the green side right here.
Let me erase all this stuff right-- nope, wrong color. Let me erase this stuff right here. Now, what happens is instead of having deoxyribonucleic acid nucleotides pair up with this DNA strand, you have ribonucleic acid, or RNA pair up with this.
And I'll do RNA in magneta. So the RNA will pair up with it. And so thymine on the DNA side will pair up with adenine. Guanine, now, when we talk about RNA, instead of thymine, we have uracil, uracil, cytosine, cytosine, and it just keeps going.
This is mRNA. Now, this separates. That mRNA separates, and it leaves the nucleus. It leaves the nucleus, and then you have translation. The transfer RNA were kind of the trucks that drove up the amino acids to the mRNA, and this all occurs inside these parts of the cell called the ribosome. But the translation is essentially going from the mRNA to the proteins, and we saw how that happened.
You have this guy-- let me make a copy here. Let me actually copy the whole thing. This guy separates, leaves the nucleus, and then you had those little tRNA trucks that essentially drive up. So maybe I have some tRNA. Let's see, adenine, adenine, guanine, and guanine.
This is tRNA. That's a codon. A codon has three base pairs, and attached to it, it has some amino acid. And then you have some other piece of tRNA. Let's say it's a uracil, cytosine, adenine. And attached to that, it has a different amino acid. Then the amino acids attach to each other, and then they form this long chain of amino acids, which is a protein, and the proteins form these weird and complicated shapes. So just to kind of make sure you understand, so if we start with DNA, and we're essentially making copies of DNA, this is replication.
You are transcribing the information from one form to another: transcription. Now, when the mRNA leaves the nucleus of the cell, and I've talked-- well, let me just draw a cell just to hit the point home, if this is a whole cell, and we'll do the structure of a cell in the future. If that's the whole cell, the nucleus is the center. That's where all the DNA is sitting in there, and all of the replication and the transcription occurs in here, but then the mRNA leaves the cell, and then inside the ribosomes, which we'll talk about more in the future, you have translation occur and the proteins get formed.
So mRNA to protein is translation. You're translating from the genetic code, so to speak, to the protein code. So this is translation. So these are just good words to make sure you get clear and make sure you're using the right word when you're talking about the different processes.
Now, the other part of the vocabulary of DNA, which, when I first learned it, I found tremendously confusing, are the words chromosome. I'll write them down here because you can already appreciate how confusing they are: chromosome, chromatin and chromatid. The difference between a duplicated chromosome and a chromatid, strictly speaking, is that a chromosome contains two chromatids that are joined at a structure called a centromere.
A duplicated chromosome therefore includes two identical strands joined along their length at corresponding lengths of DNA. Chromosomes are nothing more than distinct chunks of a substance called chromatin , which consists of very long molecules of DNA wrapped around special proteins known as histones. Different organisms have different numbers of chromosomes.
Humans, for example, have In most prokaryotes i. In sexual reproduction in humans, a sperm cell carries half of the complete set of the father's DNA, and an egg cell holds half of the complete set of the mother's. When these fuse in the process of fertilization , a chromosome zygote is formed, which soon becomes an embryo and then a fetus. In essence, the first identifiable thing about you was the DNA in your freshly assembled chromosomes, unique in human history unless you have an identical twin.
Humans have 46 chromosomes, 23 from each parent; 22 these come in distinct pairs, meaning that the copy of chromosome 1 you inherited from your mother is structurally identical to the copy of chromosome 1 you inherited from your father, and so on for the other 21 "matched" chromosomes. The 23rd chromosome in each parental set is a sex chromosome , either X or Y.
These structurally identical chromosomes in pairs are called homologous chromosomes. Differences between these homologous chromosomes occur only at the level of the nucleotide base sequences of the DNA of each chromosome. Recall that DNA is double-stranded.
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