April 12, 2009 at 5:57 pm #11204greengoddessParticipant
Just to clarify:
in M1 the number of chromosomes is halved as homologous chromosomes separate.
in M2 the chromatids separate.
in terms of numbers, say for humans you’d have 46 chromosomes at the start, then 23 after M1, then 23 chromatids after M2…?
I’ve given myself a headache with this…
and what is the cell at the start? A gamete? If so wouldn’t it just have 23 chromosomes to start with.
I’m deeply confused, to say the least.
Please help me… : )
April 12, 2009 at 9:48 pm #90121PicaqParticipant
The cell at the start of meiosis is not a gamete, but a germ cell (meaning it will divide itself into gametes in the reproductive organs). So, the cell starts out as 2n= 2 sets of chromosomes= a diploid cell (46 Chromosomes). Before it enters meiosis, its chromosomes have been duplicates (DNA synthesis), which means that the chromosomes each have 2 chromatids joined together in an "X" shape. After meiosis 1, the homologous chromosomes separate, so that each of the 2 daughter cells have 23 chromosomes= n= a haploid cell (23 chromosomes), however, each chromosome still consists of 2 chromatids (still in that X shape). In meiosis 2, the haploid daughter cells’ chromosomes line up at the metaphase plate and then split into individual chromatids, that is, each chromosome in the resulting 4 gametes after meiosis 2 have 23 chromosomes, but each chromosome consists of only one chromatid— an "l" shape.
I hope this makes sense.
March 2, 2010 at 11:32 am #98040ptabbutt432Participant
This Biology book (Biology, 9th edition, Mader, Sylvia) states that a chromosome is one side of the "X" and a chromatid is the top portion of the "X". You are stating that the Chromosome is the "X" and that the chromatid is one side of the "X". The reason this is so confusing is that all of us are reading different explanations of the same thing. How do we know who is giving us the correct information? My instructor wants me to think the book is correct. Are you more prominent than Sylvia Mader, who has written a book which may or may not include the correct information? I’m not trying to argumentative, because, as we all know, anyone can write a book, but not everyone has the credentials that create believability. It’s really important to me to have the correct information because I’m writing a paper for class, and I don’t want to look stupid.
At the start of Meiosis I, the cell has 46 PAIRS or SINGLE chromosomes? When the cell divides, and at the end of telophase I, there are 2 daughter cells that contain 23 PAIRS or SINGLE chromosomes with 2 chromatids, meaning 46 chromatids? At the end of telophase II, their are 4 daughter cells, each containing 23 SINGLE Chromosomes, each with one chromatid? Or, maybe it’s 23 chromosomes, each separated, making 46 chomatids?
I feel as if this is all wrong. What am I not getting?
March 2, 2010 at 2:27 pm #98044mamoruParticipant
It sounds like you have it essentially right. Think of it like this. Imagine that A and B are a homologous pair of chromosomes in a normal ploidy germ cell.
So, the normal cell, BEFORE DNA replication is like this: AB
During the interphase leading into Meiosis I, the DNA will replicate, so going into Metaphase I, you have this:
AABB. (at this point, you can think of "AA" or "BB" as one x-shaped chromosome that you will see under a microscope)
During Anaphase I and Telophase I, the homologous pair will segregate into the daughter cells, so you get this:
AA | BB
These will then go into their respective Meiosis II phases, and end up like this after Anaphase II and Telophase II:
A | A | B | B
So, you have 4 haploid daughter cells (the gametes), each with one copy of one chromosome from a homologous pair.
So, in the case of humans, there are 23 "AB" homologous pairs. Call them whatever you want… Pair AB, CD, EF, GH, etc. if you like… the point is, each pair replicates and segregates as in the AB example above. Of course, there is some shuffling going on with crossing over, which halves of the pairs get segregated with other halves, etc. leading to a very large number of possible combinations.
Hope this helps you visualize it better. 🙂
By the way, don’t get too confused by the terms "chromosome" and "chromatid". Chromosome can refer to both 1.) the X shape you see during prophase and metaphase, which is two chromatids joined by a centromere, and 2.) a single piece of coiled DNA in a cell (so a human cell has 23 pairs of these). #2 is essentially the same as a chromatid. We call chromosomes "chromatids" only during cell division when they are duplicated, supercoiled, and the two copies (now called chromatids) are joined together by the centromere. So, "chromatid" really means "sister copy of a chromosome", and when the two copies are joined together we also call that a "chromosome".
Annoying, eh? 😉
March 6, 2010 at 7:19 pm #98147scholardParticipant
Just to confirm something….
I know humans have 46 chromosomes. Knowing as well that they have 23 pairs of chromosomes, however, are "chromosomes" the same thing as "homologous chromosomes", with the terms just being used interchangeably?
March 30, 2010 at 9:31 am #98759veenaParticipant
at the start of meiosis the cell is in diploid condition that is for humanbeings have 46chromosomes(2*23)
during meiosis-1 dna gets replicates,now we have 92chromosomes(46*2)
then chromatid formation occurs which results in 92*2(184 chromatids) each chromosme having two chromatids attached at centromere
at the end of meiosis we will have two cells having 92chromatids(46 chromosomes)
during meiosis-2 each cell having 46 chromoses divides in two cells each having 23chromosomes that is haploid cells
March 30, 2010 at 9:29 pm #98764david23Participantquote veena:
This is just wrong . Chromatid formation is the DNA replication. The two steps are the same. You can only have 46 chromosomes MAX, and 98 chromatids when it duplicates.
May 24, 2011 at 1:06 pm #105022JackBeanParticipantquote scholard:
Not really. Consider you have chromosome 1 and chromosome 5. These are not homologous 😉
July 12, 2011 at 10:16 am #105560bellyjellyParticipant
Meiosis is a special type of cell division necessary for sexual reproduction. In animals, meiosis produces gametes (sperm and egg cells), whilst in other organisms, such as fungi, it generates spores.
Whilst the process of meiosis bears a number of similarities with the the ‘life-cycle’ cell division process of mitosis, it differs in two important respects:-
the chromosomes in meiosis undergo a recombination which shuffles the genes producing a different genetic combination in each gamete, compared with the co-existence of each of the two separate pairs of each chromosome (one received from each parent) in each cell which results from mitosis.
the outcome of meiosis is four (genetically unique) haploid cells, compared with the two (genetically identical) diploid cells produced from mitosis.
Meiosis begins with one diploid cell containing two copies of each chromosome—one from the organism’s mother and one from its father—and produces four haploid cells containing one copy of each chromosome. Each of the resulting chromosomes in the gamete cells is a unique mixture of maternal and paternal DNA, ensuring that offspring are genetically distinct from either parent. This gives rise to genetic diversity in sexually reproducing populations, which provides the variation of physical and behavioural attributes (phenotypes) upon which natural selection acts, at a population level, leading to adaptation within the population, resulting in evolution.
Prior to the meiosis process the cell’s chromosomes are duplicated by a round of DNA replication, creating a maternal and paternal version of each chromosome (homologs) composed of two exact copies, sister chromatids, attached at the centromere region. In the beginning of meiosis the maternal and paternal homologs pair to each other. Then they typically exchange parts by homologous recombination leading to crossovers of DNA between the maternal and paternal versions of the chromosome. Spindle fibers bind to the centromeres of each pair of homologs and arrange the pairs at the spindle equator. Then the fibers pull the recombined homologs to opposite poles of the cell. As the chromosomes move away from the center the cell divides into two daughter cells, each containing a haploid number of chromosomes composed of two chromatids. After the recombined maternal and paternal homologs have separated into the two daughter cells, a second round of cell division occurs. There meiosis ends as the two sister chromatids making up each homolog are separated and move into one of the four resulting gamete cells. Upon fertilization, for example when a sperm enters an egg cell, two gamete cells produced by meiosis fuse. The gamete from the mother and the gamete from the father each contribute one half of the set of chromosomes that make up the new offsping’s genome.
Meiosis uses many of the same mechanisms as mitosis, a type of cell division used by eukaryotes like plants and animals to split one cell into two identical daughter cells. In all plants and in many protists meiosis results in the formation of spores: haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as binary fission.
July 22, 2011 at 1:22 am #105668nebeyuasratParticipant
well in the beginning you have a grem cell(sex cell)
1- the cell goes throught the cell cycle
#don’t forget that in the s-phase the chromosome duplicate
2-the 1st stage of miosis is the prophase I
-mitotic spindles start to be created and the nucleus starts to disintergrate
3-2nd phase is the methaphase 1
note some book have a premethaphase as one stage
-in methaphase I the duplicated homologues align at the methaphase plate
4-the 4th stage is anaphase I
in this stage the homologues migrate towards opposing poles
note that one pair goes to one end and the other homologue to the other
and while migrating the mitotic spindle depolymerize
5-the 5th stage usually consists of two stages simultaniously occuring
they are the telophase and the cytokinesis stage
in these stages the homologues are at opposing ends and the nucleus start to reapear and division of the cytoplasm occurs
now that you have two cells with a homologue inside we can basically conclude that they go through the same process as in MITOSIS(not miosis)
August 1, 2011 at 7:49 am #105747kyra13Participant
Meiosis comprises two successive nuclear divisions with only one round of DNA replication.
Four stages can be described for each nuclear division.
Interphase: Before meiosis begins, genetic material is duplicated.
First division of meiosis
Prophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing-over can occur during the latter part of this stage.
Metaphase 1: Homologous chromosomes align at the equatorial plate.
Anaphase 1: Homologous pairs separate with sister chromatids remaining together.
Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.
Second division of meiosis: Gamete formation
Prophase 2: DNA does not replicate.
Metaphase 2: Chromosomes align at the equatorial plate.
Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.
Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.
One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different.
Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes.
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