Chromosomes – so what exactly are they? Well, chromosomes are made from DNA simply.
The bad news is there is quite a lot to learn about DNA, the good news is that once you learnt it, that’s a big topic out the way, and it’s also pretty interesting.
So let’s get stuck in! DNA and chromosomes may seem like completely separate things, but in fact they’re not. All chromosomes are individual DNA molecules coiled and twisted around, because DNA is huge! (As in compared to other cells, not so much to an elephant).
Human DNA is stored in our chromosomes. We have 46 chromosomes in our body cells, this differs from eukaryotic organisms.
A pair of matching chromosomes are called homologous. We have two copies of every chromosome in our cells, one maternal and one paternal copy. These pair up when the cell divides.
Human sperm cells have 23 chromosomes. Sex cells, or gametes, have half the number of normal chromosomes. This is to ensure offspring have the correct number of chromosomes post-fertilisation.
Now that we have a better understanding of chromosomes, let’s look at genes.
A gene which is positioned on a particular strand of DNA is called a locus. A homologous pair of chromosomes have the same genes and the same loci.
A gene is a length of DNA that codes for a particular polypeptide. Every organism has a different number of genes. Each gene codes for a particular polypeptide or protein. Not every gene is always switched on all of the time in every cell in your body. (That might get a bit tiring).
DNA bases make up genetic codes. The order of the base codes form the order of amino acids in a polypeptide chain. If the base sequence changes this is called a mutation.
Got the basics about genes – let’s move on to alleles.
Different forms are genes are called alleles. An example of this is the eye colour gene. Every human has the gene coding for eye colour, but we have different alleles. Some people have the brown allele whilst other people have the blue allele.
If a section of DNA does not code for a polypeptide that is located within a gene this is called an intron.
There are many introns in our DNA, much more than the amount of DNA that actually codes for proteins. These introns are removed during the process of protein synthesis.
DNA in bacteria differs from that found in humans. Bacteria DNA is found in a circular structure which is suspended in the cytoplasm as bacterial cells do not have a nucleus.
In bacteria DNA does not wind around any proteins, instead the DNA condenses and coils around itself in order to fit into the cell.
In a eukaryote the protein which winds around the DNA is called histone. These proteins give the DNA more stability and support.
An example of a eukaryote is a plant as they have a nucleus in their cells.
The final produce made from DNA is a protein. DNA does code for RNA but the final product produced is protein. The tertiary structure of the protein is governed by the order of the bases in the DNA.
Proteins are made on a ribosome. Ribosomes are found in the cytoplasm and on the rough endoplasmic reticulum.
That was a lot to take in, but just remember chromosomes are made from DNA, genes are found on strands of DNA and alleles are different types of genes.