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Genes

Biology: IB Diploma Higher Level, topic 13.3.1

Genes

DNA consists of millions of genes. Genes are a section of DNA that code for a polypeptide or RNA molecule.

Base sequence

Base sequence

  • DNA is made from four different bases (A, T, C, G).
  • A gene is a sequence of bases in DNA.
  • Each gene has a specific base sequence.
Locus

Locus

  • The specific location of a gene in the DNA is called the locus.
  • The locus is a fixed position.
Codons

Codons

  • Three bases in a gene are called a triplet or a codon.
  • A specific codon encodes a specific amino acid.
  • The codons present in a gene and the sequence of the codons are translated into a sequence of amino acids.
  • Together these amino acids form a polypeptide.
mRNA

mRNA

  • The sequence of bases in a gene is made into a polypeptide by copying DNA into a molecule called messenger RNA (mRNA).
  • mRNA is then copied into the corresponding polypeptide.
Functional RNA molecules

Functional RNA molecules

  • Some genes encode a functional RNA molecule instead of a polypeptide.
  • Functional RNA molecules are necessary for protein synthesis.
  • E.g. Transfer RNA (tRNA) helps find the correct amino acids for a specific codon.
  • E.g. Ribosomal RNA (rRNA) are a part of ribosomes, the centre of protein synthesis.

Non-Coding Genes

A large proportion of the genes in eukaryotic DNA do NOT code for any polypeptides. These genes have no biological function and are referred to as non-coding genes.

Exons

Exons

  • The coding regions in a gene are called exons.
  • Sequences of coding genes are interrupted at regular intervals by non-coding regions.
Introns

Introns

  • The non-coding regions in a gene are called introns.
  • When a gene is translated for protein synthesis, the introns are removed from the sequence.
Multiple repeats

Multiple repeats

  • There are also regions of non-coding multiple repeats in eukaryotic DNA.
  • Multiple repeats are random sequences in the DNA that will be repeated many times (e.g. CGCCGCCGCCGC).

The Genetic Code

The genetic code is the combination of triplet codes in the genome. There are three important features of the genetic code:

Degenerate

Degenerate

  • The genetic code is degenerate.
  • Some amino acids are encoded by more than one codon (e.g. tyrosine is encoded by TAT and TAC).
  • There are 64 possible triplet codons and only 20 amino acids.
Non-overlapping

Non-overlapping

  • The genetic code is non-overlapping.
  • The non-overlapping nature of DNA means that there is no overlap between triplet codes.
  • Each triplet code is separate from the other triplets in the chromosome.
Universal

Universal

  • The genetic code is universal.
  • The same four bases (A, T, G, C) are used in the DNA of every organism.
  • The same codons encode the same amino acids in every organism.
  • Codons in DNA are transcribed into mRNA and translated into amino acids in every organism.

Test yourself on Genes

  • Which RNA molecule copies the DNA sequence into a polypeptide?

  • Which of the following statements about genes are true?

  • Which of the following statements about non-coding genes are true?

  • What is the name given to coding genes?

  • Which of the following statements about the genetic code are true?

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