DNA, genes and chromosomes

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Genes and Genetic Code

Gene: a section of DNA that codes for a protein.

Base: the chemicals in DNA that carry the genetic code.

There must be a minimum of 3 bases coding for each amino acid: because there are 20 different amino acids which must be coded for - if bases were in pairs, there would only be 16 different codes possible, so bases must be in triplets.

The genetic code:

  • Triplet code: there is a sequence of 3 bases for one amino acid.
  • Degenerate code: most amino acids are only coded for by one triplet, and some triplets code for a stop codon.
  • Non-overlapping code: each base in a sequence is read only once.
  • Universal: with a few minor exceptions, triplets code for the same amino acids in all organisms.

Much of the DNA in eukaryotes does not code for a polypeptide - there are non-coding regions called introns between the coding regions called exons.

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DNA and Chromosomes

Chromosomes: thread like structures made of protein and DNA by which hereditary information is passed from one generation to he next.

Chromosomes appear as two threads joined at a single point called the centromere. Each thread is a chromatid.

How DNA is put into a chromosome:

  • DNA is a double helix. This helix is wound around histones to fix it in postion.
  • This DNA-histone complex is then coiled.
  • The coil is looped and further coiled before being packed into the chromosome.
  • In this way a lot of DNA can be condensed into a single chromosome.
  • A chromosome contains just a single DNA molecule, which has many genes along its length.

Homologous chromosomes: a pair of chromosomes, one maternal and one paternal, that have the same gene loci and therefore determine the same features.

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Allele: one of a number of alternative forms of the same gene.

Allele frequency: the number of times an allele occurs within the gene pool.

The order of bases in each allele is slightly different, so they code for slightly different amino acids, so code for slightly different versions of the same polypeptide.  

Any change in the base sequence of a gene produces a new allele of that gene, and results in a different sequence of amino acids being coded for. This can lead to the production of a different protein, which may not function correctly.

In a homologous pair, both chromosomes are the same size and have the same genes, but may have different alleles. Alleles coding for the same characteristics will be found at the same fixed position on each chromosome.

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