Chromosomes a level biology

I don’t understand how chromosomes exist in a cell.
Like during mitosis they replicate and exist as two sister chromatids joined at centromere. The result is a single chromatid right?

In meiosis, by the end there are half the no of chromosomes and chromosomes are again existing as a single chromatid

So when we say there are 46 chromosomes in a cell, they exist as two chromatids joined at centromere.
I don’t understand how

Reply 1

toothysmile

I got this confused at first as well! here is the explanation (pay attention to numbers in brackets - differentiate between chromatids and chromosomes!):

So chromosomes exist as genetic material in general loose - in metaphase they condense and form 'arms'. In interphase these arms duplicate to form an X shaped - two chromatids attached to a centrometer. These sister chromatids are just the same chromatids joined in the middle

In meiosis a diploid (46 chromosomes) divide to form haploid gametes (23). Each chromosome number (1..23) has a maternal and paternal chromatid (so 46 in total). Each of these replicate to form 92 chromatids (but still 46 chromosomes) . They divide once in mitosis 1 to form 46 chromatids (still 46 chromosomes, 1 chromatid for each chromosome in diploid cell) in one cell and then again in mitosis 2 to form 23 chromatids (one chromatid for each chromosome in haploid cell).

sorry for the long explanation, hope this helps!

Reply 2

Ghostlady

Original post by Aeshakhan

My daughter did biology alevel (OCR). I could see she was struggling with some bits in year 12, so having done the alevel myself we got together found some more better ways to remember things than just writing it down or reading.
One of the ways of remembering things are videos. Its such a god send compared to the days I did biology. No wonder I did rubbish lol. So Ameoba sisters below are well worth watching on mitosis, meiosis, biochemistry, protein structures and folding, ATP and enzymes. I quite enjoy the silly little cartoons, they do seem to make it stick better. Then from chapter 8 for the heart, transport in animals etc you will want ''Bio Rach'' shes really good in explaining things. Respiration ''snap revise'' is a good set of videos. He also does flash cards which we bought and they are really great in testing you. **edit** shes just finished her year 13 and her exam and she feels shes done alright. She needs a B for uni so she feels shes done Bs in paper 1 and 2 and A for paper 3.