Chromosomes become visible during prophase (the first stage of nuclear division). They appear as two threads (chromosomes) joined at the centromere. Each thread is called a chromatid and they are identical as the DNA has already replicated (during interphase). The chromosome structure is held in place by histones (see above, a type of protein). Since the DNA is highly coiled around histones a considerable length of DNA can be found in each cell.
A gene is a section of DNA that codes for a polypeptide sequence. It technically first codes for an mRNA strand, so a gene is also a section of DNA that codes for a functional RNA. The position of a gene on a section of DNA is known as the locus. An allele is one of a number of alternate forms of a gene (e.g a gene will code for eye colour, so an allele will code for blue and an allele will code for brown etc etc).
A gene can code for a polypeptide sequence because each DNA triplet (sequence of three DNA bases) codes for a single amino acid. This is known as a degenerate code. It is important to realise that the triplets are read in one direction only, each base is only read once (triplets are non-overlapping), the start of a gene always codes for the same amino acid (methionine), and stop codons do not code for an amino acid (they mark the end of the sequence). There are 64 possible triplet combinations but only 20 regularly occurring different amino acids. This means that some triplet codes code for the same amino acid. The code is also universal meaning each triplet codes for the same amino acid in (pretty much) every organism.
That being said, there also a few other reasons why a DNA sequence may not code for an amino acid (along with stop codons). In eukaryotic organisms, between genes there are non-coding parts of the DNA . Furthermore, within genes there are sections that are non-coding. These are known as introns. The coding parts are known as exons.
As I mentioned earlier, genes can also code for RNA sequences. RNA stands for ribonucleic acid. It is single stranded and made up of a phosphate group, ribose sugar, and a nitrogenous base. The two RNA molecules that are used to for proteins (polypeptide sequences).
- mRNA
- longer stranded
- arranged in a single straight helix
- base sequence determined by the base sequence of the length of DNA is was transcribed from
- tRNA
- smaller than mRNA (only around 80 molecules long)
- clover-leaf shaped
- one end extends beyond the other (this is the bit where an amino acid attaches)
- On the opposite side of the tRNA molecule is the anticodon (this is complimentary to codons on the mRNA molecule
- Each tRNA molecule is specific to one coding triplet, and therefore one amino acid
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