I hope everyone reading this blog knows the definition of a gene.  It is one of the few things in the iGCSE course that it is worth learning by heart.

“A gene is a sequence of a DNA molecule that codes for a single protein“.

In human cells, every nucleus contains about 23,000 genes.  Remember there is about 1.5m of DNA inside each nucleus.  For most of the life-cycle of the cell, this DNA is in a tangled web called chromatin.  Chromatin is DNA molecules loosely associated with some scaffolding proteins.  The scaffolding proteins are shown in the second level down of this excellent diagram as “beads on a string”.

But this tangled web of DNA in chromatin poses a problem for the nucleus.  For the cell to divide by mitosis, it is essential that the nucleus replicates into two identical nuclei, one for each new cell.  The DNA molecules in the nucleus will make a copy of themselves by semi-conservative replication but how then can you ensure that each daughter nucleus gets exactly one copy of each DNA molecule if they are all tangled up….?  This is where chromosomes come in!

Each chromosome is a physical structure formed by supercoiling of the DNA round the scaffold proteins.  The DNA coils, then folds back on itself, then coils again until each DNA molecule is so tightly coiled up that a visible chromosome appears in the nucleus.  Chromosomes only become visible just before mitosis starts as for the rest of the time, the DNA is much more loosely coiled and so cannot be seen.

This also explains why each chromosome always looks X shaped.  When chromosomes become visible the DNA has already replicated, so one chromosome is now made of two identical sister chromatids joined at a region called the centromere.

So the picture on the left shows a chromosome made as a single structure comprising one DNA molecule wrapped around the scaffold proteins.  Then DNA replication occurs (in the S phase of the cell cycle) and now each chromosome is made of two identical chromatids joined at the centromere.  Then the two chromatids are separated in mitosis and the chromosome returns to the structure it had at the start.

How many chromosomes are there in human cells?

The key idea here is that chromosomes are found in pairs in all body cells apart from gametes,  These pairs of chromosomes  (called homologous pairs) have exactly the same genes in the same locations on the chromosome.  They are inherited one from each parent so one member of each pair will come from your father, one from your mother.

Different species have different numbers of pairs of chromosomes.  For humans you should know that we have 23 pairs of chromosomes in the nucleus of every body cell (making a total of 46).  Cells with chromosomes found in pairs are called diploid cells.  Every cell in the body is diploid apart from the gametes.  Gametes only have one member of each homologous pair and are called haploid cells.

Which of the following cells are diploid, which are haploid?

1. Zygote
2. Skin cell
3. Sperm cell
4. Liver cell
5. Pollen grain
6. Egg cell

If you are not sure, ask me by leaving a comment below….

Finally for this post, chromosomes determine the sex of a human.  You can see in the picture above that the 23 pairs of chromosomes can be divided into pairs 1 to 22 – these are called autosomes and play no role in determining your sex.  But the 23rd pair of chromosomes are called the sex chromosomes.  Males have one large X chromosome and one tiny Y chromosome as their 23rd pair whereas females have two large X chromosomes.

Gametes are haploid so only have one member of each pair.  So when a man makes sperm cells (by meiosis) 50% of his sperm cells will contain his X chromosome, 50% his Y chromosome.  A woman’s egg cell will always contain one X chromosome. (Why is this?) So I hope you can see that at the moment of fertilisation, the babies sex is determined depending on whether it is a Y-containing sperm cell that happens to fertilise the egg or an X-chromosome containing sperm…  If the former, the baby is male, if the latter female.

I might explain this more fully in a post some other time….

Final thing for this post.  If you have got to the end of this and understand everything in the text above, you are in a tiny minority of school students. Well done!  This is a tricky topic and if you really understand chromosomes, you stand a chance of understanding cell division and genetics.