Conceptual questions about chromosomes

kenny1999

Books say that we have 46 (or 23 pairs) of different chromosomes in human which determines how we look like and our sex. I now have some conceptual questions about chromosomes


1. I know that chromosomes exists as pairs in body cells except sperm cell and egg cell, but is the chromosome identical to their paired chromosomes?
How about sex chromosomes? Does male contain a pair of sex chromosome that the individual one is different while female contains a pair of sex chromsomes such that they are the same?


2. Does one cell alone in our body already contains all these 46 chromosomes? or do different body cells have different chromosomes? For example, such type of chromosomes might exists in liver cell but not heart cell etc...

Thanks

Ygggdrasil

No. As you said, humans have two versions of each chromosome in their body, one which they inherited from their father and one which they inherit from their mother. The DNA sequences on these chromosomes, while almost entirely identical, do contain differences, and these differences can lead to different traits.

The same applies with the pair of sex chromosomes. Males have an X-chromosome (inherited from their mother) and a Y-chromosome (inherited from their father), while females contain one X-chromosome inherited from their mother and one X-chromosome inherited from their father.

Nearly all of the cells in our body contain identical chromosomes. The 46 chromosomes in a liver cell will be identical to the 46 chromosomes in a brain cell from the same individual.

There are, of course, a few exceptions (for example, sperm and egg cells have only 23 chromosomes, and the DNA in each sperm/egg cell will be distinct from the DNA contained in other sperm/egg cells). Furthermore, cells can accumulate mutations in their DNA, so there will be small differences in the DNA sequences between the chromosomes of cells within the same individual. But, for the most part, the DNA in most cells in our body is identical.

Simon Bridge

[edit]Ygggdrasil beat me to it :)

1. a chromosome in a pair is not going to be identical to it's partner. The obvious comparison would be the paired XY in men.

In women, whose sex pair is XX, the two X chromosomes have different histories - one comes from her father and the other from her mother. They play the same role in determining sex but chomosomes are more complicated than that - being composed of DNA wrapped around protein. The DNA from each parent will be different.

2. Each cell in your body contains 23 unique pairs of chromosomes in it's nucleus.
They get them by reproduction from the stem-cells, which come from the original fertilized egg. Which type of cell they differentiate into depends on factors in the embryology. Once differentiated, liver cells provide quite a different working environment to a heart cell (to use your examples) - and each cell operates in a different environment too - it is this working environment, in conjunction with the DNA, that builds the different cell types.

The whole topic is very big though.
This is not going to be very complete - I'd expect a biologist would want to fill in a few of the more misleading gaps :)

Also found:
http://learn.genetics.utah.edu/conte...hromosome.html
http://ghr.nlm.nih.gov/handbook/basics/chromosome

kenny1999

hi

man has a pair of sex chromosomes XY while female has a pair of sex chromosome XX. Does it mean that being a male is a dominant gene character?

Ryan_m_b

It's a little bit more complicated than that as sex determination is more complicated than simply male or female, especially as there are both biological and social implications.

On the Y chromosome is the SRY gene which is necessary for male sex development. If this gene is damaged or not present you do not get an organism developing into a female in the same sense as if you had an organism with two X chromosomes. Instead you get a woman with Swyer syndrome. It's also possible for the SRY gene to be translocated (cut and pasted) to an X chromosome resulting in an XX male with de la Chapelle syndrome.

Simon Bridge

Sex determination is one of the biggies.

... general "birds and bees" approach:
http://www.nature.com/scitable/topic...ermination-314

... in terms of "dominant/recessive":
http://www.biology-online.org/2/6_sex_chromosomes.htm

Then there's the XXY and XYY stuff.

One of the things I noticed about High School biology (in NZ anyway) is that the relationship between DNA, genes, alleles, and chromosomes, species etc is not all that clear - at least, not to the students.