Log in Spope 8 years agoPosted 8 years ago. Direct link to Spope's post “What is the difference be...” What is the difference between a chromosome and a chromatid. We often see pictured the 23 pairs of chromosomes in a human Karyotype. Do BOTH of the copies of each chromosome participate in Mitosis? So in Interphase are their actually TWO pairs of each chromosome, giving a total of 92 Chromosomes ( in a human)? Is this correct? This is the part that has always been the most difficult for me to grasp. • (15 votes) Tyler Hall 6 years agoPosted 6 years ago. Direct link to Tyler Hall's post “I think another way to th...” I think another way to think about it is remembering the difference between "sister chromatids" and "hom*ologous chromosomes". In a regular somatic cell (before DNA is replicated in the S phase), there are 46 chromosomes - 23 of each kind as well as their hom*ologous opposite. I.e. - you have an 'A' chromosome (1 'A' chromatid) and an 'a' chromosome (1 'a' chromatid), 'B' and 'b', 'C' and 'c' and so on, each coming from a different parent. 'A' and 'a' are still 2 different chromosomes, though they are hom*ologous to each other (code for the same genes). When DNA is replicated, you now have 2 copies of the 'A' chromosome (or 2 'A' chromatids) and 2 copies of the 'a' chromosome (2 'a' chromatids), 2 'B' and 2 'b', and so on. However, the 2 'A' chromatids are still linked together by the hip, and thus are considered to still be only one chromosome. In this way, you do have 92 chromatids, but still only 46 chromosomes. Or, you could say you have 46 pairs of sister chromatids, but still 23 pairs of hom*ologous chromosomes. This might help a little more: Somatic cell (before S phase): 46 chromosomes, 46 chromatids, 23 pairs of hom*ologous chromosomes, 0 pairs of sister chromatids For a male, this would look like: A a, B b, C c, D d, E e, F f, G g, H h, I i, J j, K k, L l, M m, N n, O o, P p, Q q, R r, S s, T t, U u, V v, W w, X Y Somatic cell (after S phase, before mitosis/meiosis): 46 chromosomes, 92 chromatids, 23 pairs of hom*ologous chromosomes, 46 pairs of sister chromatids In a male this would look like: AA aa, BB bb, CC cc, DD dd, EE ee, FF ff, GG gg, HH hh, II ii, JJ jj, KK kk, LL ll, MM mm, NN nn, OO oo, PP pp, QQ qq, RR rr, SS ss, TT tt, UU uu, VV vv, WW ww, XX YY Germ cell (after meiosis II): 23 chromosomes, 23 chromatids, 0 pairs of hom*ologous chromosomes, 0 pairs of sister chromatids. This could look like: A, b, C, D, e, f, g, H, i, j, K, L, M, n, o, p, q, R, s, T, U, v, w, X The most important thing to remember that will clear up the confusion, I think, is that after DNA is replicated in S phase, the 2 sister chromatids (or 2 copies of each chromosome) are still linked together and still considered to be just 1 chromosome. But hopefully the rest helps clear up some things as well. (60 votes) stpeterlauren 5 years agoPosted 5 years ago. Direct link to stpeterlauren's post “The misconception in many...” The misconception in many of the comments below is that the article, and its diagrams, are depicting meiosis, when they are actually describing MITOSIS. The whole point of mitosis is to make exact copies of the parent’s cells, so that each cell produced has the same genetic information as its parent cell. In humans, the case is that the each of the 46 chromosomes will be present in the daughter cells after mitosis. For our nematodes example, each cell after MITOSIS should contain four chromosomes given that its diploid number in four. One of the most confusing aspects of the cell cycle is terminology, and you have to be very careful when using it. The words chromosome and chromatid can be referring to several different stages of DNA strands throughout the cell cycle. So, make sure to know the exact state of the DNA strand you are describing. During MITOSIS, the parent, diploid (2n), cell is divided to create two identical, diploid (2n), daughter cells. This occurs by undergoing DNA replication (in S phase during interphase) where the monovalent chromosome is duplicated so that it will have two DNA strands that are replicas of each other. These two strands are each now called a sister chromatid, and the two sister chromatids make up a divalent chromosome. This parent cell has a diploid number of 4 because there are four chromosomes present in an autosomal cell. During meiosis I, however, the parent, diploid (2n), germ cells are divided to create two haploid (n) daughter cells. This means that in nematodes, the parent cells will contain 4 total chromosomes, but the daughter cells will only have 2. Important terminology here is hom*ologous pairs chromosomes, or hom*ologues. hom*ologues consist of two sets of chromosomes, one from the mother and the other from the father. These hom*ologues are similar in shape, size and type of genetic information they contain, but are not identical in the alleles they carry. The nematodes have 2 sets of hom*ologous chromosomes (for a total of 4 chromosomes), whereas humans have 23 hom*ologues (for a total of 46 chromosomes). • (9 votes) whosgonnacarrytheboats205 7 months agoPosted 7 months ago. Direct link to whosgonnacarrytheboats205's post “Thank you” Thank you (1 vote) ff142 9 years agoPosted 9 years ago. Direct link to ff142's post “In the meiosis diagrams, ...” In the meiosis diagrams, two groups of two tetravalent chromosomes are shown, not two groups of two bivalent chromosomes. The diagram should show two sets of hom*ologous chromosomes, not four, and hom*ologous chromosomes should not be connected to each other by a centromere. Each cell after meiosis I should have two bivalent chromosomes with the chromosome numbers 1 & 2, not two tetravalent chromosomes with different chromosome numbers for the different cells (1 and 2 for one cell and 3 and 4 for the other cell), whatever organism it is wouldn't be able to survive in that case. • (7 votes) rhyshannaford93 8 years agoPosted 8 years ago. Direct link to rhyshannaford93's post “I understand this, but if...” I understand this, but if someone could explain this conceptual problem it would be very much appreciated. • (3 votes) Autumn Nelson 8 years agoPosted 8 years ago. Direct link to Autumn Nelson's post “So in meiosis there are t...” So in meiosis there are two divisions. The first division there are still 2 copies of each chromosome. During the second division, they split so there is only one copy of each chromosome, each one not identical to the other. A normally body cell (humans is 46) contains 2 copies of each chromosome, gametes contain 1 copy of each, therefore has half the chromosomes. Because multiplication happens in both meiosis and mitosis, in meiosis you end up with 4 cells, each containing different genetic information but one of each chromosome and in mitosis you get 2 cells containing identical information with pairs of chromosomes. Does that help? (3 votes) chase.booth 3 years agoPosted 3 years ago. Direct link to chase.booth's post “how many cells in a human...” how many cells in a human body are going through mitosis nonstop • (4 votes) Abdulrahman Abazid a year agoPosted a year ago. Direct link to Abdulrahman Abazid's post “There are an estimated 37...” There are an estimated 37.2 trillion cells in the average adult human body. (0 votes) Tadeniji.durls 7 years agoPosted 7 years ago. Direct link to Tadeniji.durls's post “What is interphase part i...” What is interphase part in the replication and division of the cell • (3 votes) aditi 3 years agoPosted 3 years ago. Direct link to aditi's post “Interphase, in very simpl...” Interphase, in very simple terms, is cell growth. during interphase, the cell prepares for cell division by producing new organelles, replicating the DNA, and preparing for mitosis/meiosis. Interphase doesn't have a part in the division of the cell. Replication is one part of interphase. (1 vote) shreypatel0101 8 years agoPosted 8 years ago. Direct link to shreypatel0101's post “Can anyone explain me the...” Can anyone explain me the last part of the article i.e down syndrome? • (1 vote) ♪♫ Viola ♫♪ 8 years agoPosted 8 years ago. Direct link to ♪♫ Viola ♫♪'s post “So, see how the product o...” So, see how the product of meiosis is 4 gametes which have one copy of each chromosome (monovalent)? In other words, gametes are not supposed to have two sisters chromatids for each chromosome. You're supposed to end up with four little eggs with one copy of chromosome 21, so that the sperm can complete it with its one copy of chromosome 21. In trisomy 21, one gamete contributes 2 copies of ch.21 while the other contributes 1, so you get a zygote with 3 copies. In down syndrome, during the step of meiosis where sister chromatids in one cell are pulled apart to form two cells with one chromatid each [the last step in this image: https://biologypost.files.wordpress.com/2013/01/meiosis.jpg], the two sisters do not separate! So one of the cells will get no copy of chromosome 21 while one cell gets 2 copies of chromosome 21 (bivalent). The one with no chromosome 21 is not viable at all. But if a sperm comes along and fertilizes the one with the 2 copies of chromosome 21, then it adds its own copy to the egg, thus the zygote now has 3 copies of chromosome 21, hence the name "trisomy 21". Honestly, forget about the monovalnt and bivalent wording. I've never seen it be used in my textbooks, so it's probably not common terminology. Here's what I just explained in a simple picture: http://4.bp.blogspot.com/_JI1iL2842nw/SR2FnYjLKYI/AAAAAAAAAEQ/fJSQc5bUfpE/s320/nondis_m22964.gif (4 votes) jsanc153 9 years agoPosted 9 years ago. Direct link to jsanc153's post “I'm still confused about ...” I'm still confused about Mitosis. You start with 46 chromosomes (92 chromatids) and then the chromatids replicate and make 46 pairs of chromosomes which will eventually divide through the rest of mitosis making 2 daughter cells, each with 46 chromosomes (23 pairs )? Am I understanding this correctly? • (2 votes) kenipa29 9 years agoPosted 9 years ago. Direct link to kenipa29's post “Wait you are thinking of ...” Wait you are thinking of Meiosis. In Mitosis cell divide and gave us 46 chromosome in each two daughter cells. Meiosis divide in 4 cells in that chromosomes divide in 23 pair each. I guess this will helpful. (0 votes) purpleowl123 7 years agoPosted 7 years ago. Direct link to purpleowl123's post “What exactly does the ter...” What exactly does the terminology tetrad mean? • (1 vote) Naidel Montano 6 years agoPosted 6 years ago. Direct link to Naidel Montano's post “The prefix tetra meaning ...” The prefix tetra meaning four is referring to the chromatids. In meiosis a tetrad is when two hom*ologous chromosomes align next to each other in prophase I. Since each hom*ologous chromosome has 2 chromatids 2x2=4 and that is why we call it a tetrad. (1 vote) RY4N 5 years agoPosted 5 years ago. Direct link to RY4N's post “Why is this so interestin...” Why is this so interesting? • (1 vote)Want to join the conversation?
Skipping to anaphase, the four sets of divalent chromosomes, which were lined up during metaphase, are each split at the centromere as the spindle fibers pull the now four monovalent chromosomes to each end of the cells. Before cytokinesis, there is a total of eight monovalent chromosomes in one cell, with four chromosomes on each end of the cell. After cytokinesis, the ploidy of the daughter cells remains the same because each daughter cell contains 4 chromatids, as the parent cell did. These daughter cells contain single stranded chromatid, but that does not affect the ploidy (as I commonly confused) because eventually, these cells will undergo DNA replication and have double stranded chromosomes- which are sister chromatids.
The process by which meiosis I occurs is different than mitosis because hom*ologous pairs of chromosomes (called tetrads) are lined up during metaphase I, rather than single divalent chromosomes. These hom*ologous pairs are split apart, and the maternal hom*ologue goes to one pole, while the paternal hom*ologue goes to the other. Therefore, after anaphase I, the daughter cells will contain only one of the two hom*ologous chromosomes, ultimately reducing the overall number of chromosomes present in the daughter cells. It is then during meiosis II, where the newly formed haploid (n) daughter cells are equationally divided by separating the sister chromatids of the hom*ologue, similar to mitosis. The end result is four haploid daughter cells, called gametes.
If the division of chromosomes in meiosis allows some chromosomes to be in some gametes cells and others in other gamete cells (as shown in the first couple diagrams of the meiosis section), then how can gametes posses the correct type and number of 26 chromosomes. Basing off the diagram, it seems that 2 and 4 chromosomes are in one gamete whilst lacking 1 and 3.
Check this artical https://sitn.hms.harvard.edu/art/2018/mitosis/
It says that bivalent chromosomes during meiosis II seprate, but there are no bivalent chromosomes (a bivalent is also called tetrad, that is a hom*ologous chromosome is called is called bivalent)
Please help..!
See Also
7.1: Mitosis and the Cell Cycle