books and websites are giving different answers to this which is really confusing me. first off, the definition of chromosome itself is tripping me up because I don't know when to call 1 chromatid a chromosome or the pair of sister chromatids a chromosome. Also when does the cell actually become haploid in terms of chromosomes? A website says "if you consider sister chromatids to each be a chromosome, then meiosis I doesn't halve the chromosome count but separates the homologous chromosomes". I get the fact that homologous chromosomes are separated but what do they mean by "consider"? is it for us to decide whether we refer to identical sister chromosomes collectively as a chromosome? I'm so confused
A chromosome is one long strand of condensed DNA. During interphase, the DNA replicates two produce two copies of each chromosome. When you have two identical copies of a chromosome, each copy is called a sister chromatid. I think a lot of the confusion arises from the fact that if the two sister chromatids are joined by a centromere, they are known as a chromosome.
This video is great for explaining this: https://www.youtube.com/watch?v=gcz1FOWw0Cg&t=115s
In terms of haploid vs diploid, a diploid cell will have pairs of homologous chromosomes- one from each parent. This means that they have two sets of chromosomes, denoted by 2n.
A haploid cell will have just one set of chromosomes that are not paired.
Recall that during fertilisation, the two haploid gametes fuse to produce a diploid cell; the sperm carries the set of paternal chromosomes, and the egg holds the set of maternal chromosomes. This means that in the fertilised cell, there will be two sets of homologous chromosomes, so will be diploid.
I hope this makes sense. This is from my own understanding, so if anyone notices any mistakes I've made I would really appreciate any corrections!
The answer by Caviat [meant to taste good! - sorry! ] is very good - does not need corrections, but I would add the following tips:-
1. Please ignore what you read on the website you quote to keep it simple and clear. think ONLY of WHOLE chromosomes as being two chromatids joined up at the centromere, and as a chromatid being almost like "half a chromosome".
2. Look at the first division of meiosis [Meiosis I] as the one that halves the chromosome number from diploid to haploid because WHOLE CHROMOSOMES MOVE APART to the two poles, half of them to one pole and the other half to the other pole e.g. the housefly [Musca domestica [only an example - don't try to memorize!] has a diploid number of 8, so 4 WHOLE chromosomes will move to one pole at this stage [=Meiosis I] and the other 4 WHOLE chromosomes to the other pole, happy with that?
After that look at the second division [Meiosis II] as just about the same as mitosis, yeah? i.e. the CHROMATIDS of each chromosome separate and move to each pole e.g. in the housefly, 4 chromatids move to one pole and 4 to the other pole, still with me? You ask "Why 4 and not 8?" OK: simple answer [the number was halved in Meiosis I, reemember? See start of point 2 above. - Good - well done! So the daughter cells here will have ONLY 4 chromosomes each [after DNA replication]
3. Check out my own teaching file [at-d] to clarify and to apply Araldite so that it sticks in your mind!! "OMG" he/she goes "Now this guy wants to contaminate my brain!" - jk!
NOW RUSH TO THE KITCHEN - your mum has cooked sausage, egg & bacon for you - yummy dw I will not nick your breaky! - I had mine 3 hours ago!
Pay special attention to pics!