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Hi, I was completing the mitosis and meiosis content review videos and am a little confused on some (what I thought was) simple vocab. I know that humans have 46 total chromosomes and that 44 are autosomes/somatic and 2 are germ cells. However, on the intro quiz to the meiosis video (question 4), the explanation showed that human somatic cells have 46 chromosomes (23 pairs of homologous chromosomes) - I thought somatic cells were all cells except for germ cells? Why is it 46 instead of 44? Thanks!
You may need to un-know this. (Jokes aside, this is a very common area of confusion)I know that humans have 46 total chromosomes and that 44 are autosomes/somatic and 2 are germ cells.
Let's clean our understanding of this up a bit:
All human cells have 23 pairs of homologous chromosomes*. 22 of those are autosomes, and 1 is a sex chromosome.
A homologous chromosome is one that has all the same genes, but may have different alleles (versions of those genes).
After S phase, humans have 2 identical copies of genetic information within each of their 46 chromosomes - these are called sister chromatids. Unlike homologous chromosomes, they are completely identical down to the last base (hopefully).
Germ line cells are no different in this regard. Their only differentiating factor is that they can undergo meiosis. During meiosis I, they separate their pairs of homologous chromosomes, but retain their sister chromatids together. These sister chromatids are separated during meiosis II.
The sex chromosomes can, in humans, be either X or Y. The Y chromosome is heavily degenerate (it is tiny and contains less information). In order for the right amount of transcription and subsequent protein expression to still happen, a mechanism called dosage compensation exists - where an entire X chromosome in females is basically not transcribed. This works differently in other species.
But either way, all cells definitely do have the same amount of genetic information (with the distinction between G1 and G2+ phases, where cells in G1 do not have two identical copies of each chromatid yet). I hope this helps a bit!
Yes, it does! Thank you!