Unit 4

• define gene

  A gene is a section of DNA that is located at a particular position called a locus on a DNA molecule. The gene is a base sequence of DNA that codes for:

  • amino acid sequence of a polypeptide
  • or a functional RNA, including ribosomal RNA (rRNA), mRNA and tRNA.
• what are features of the genetic code ?

  1. degenerate - amino acids can be coded for by more than one triplet
  2. non-overlapping - each base is only read once
  3. universal - each triplet code is codes for the same amino acid in all organisms.
• describe the characteristics of prokaryotic DNA:

  • single stranded
  • circular
  • not associated with protein molecules (therefore do not have chromosomes)
  • contains not introns
• describe the characteristics of eukaryotic DNA: linear, double-stranded, associated with proteins and enclosed in a nucleus
• Define genome

  Complete set of genes present in a cell
• define proteome

  full range of proteins that a cell is able to produce.
• Describe transcription:

  [occurs in the nucleus]

  1. DNA helical breaks hydrogen bonds between DNA molecule.
  2. This exposes the DNA bases.
  3. Free activated RNA nucleotides pair up via hydrogen bonds with their complementary base pairs
  4. RNA polymerase joins the nucleotide bases together.
  5. pre-mRNA is then spliced to remove introns.
  6. mRNA then leaves the nucleus via nuclear pores.
• describe translation:

  [occurs in the cytoplasm]

  1. mRNA molecule attaches to a ribosomes.
  2. A tRNA molecule with the complementary anticodon pairs with complementary triplet codon on the mRNA molecule.
  3. the tRNA carries a specific amino acid.
  4. ribosome molecule moves along mRNA strand.
  5. peptide bonds forms between amino acids ( this requires energy from ATP provided by mitochondria within cell)
  6. this continues the ribosomes reaches a stop codon on the mRNA strand.
• define gene mutation

  a change in the sequence of base pairs in a DNA molecule.
• insertion mutation

  a mutation that occurs when a base is randomly inserted into the DNA sequence.

  • has a knock-on effect also known as a frameshift mutation.
• deletion mutation

  a mutation that occurs when a base is randomly deleted from the DNA sequence.

  • this has a knock on effect also know as a frame shift mutation.
• substitution mutation

  a mutation that occurs when a base is randomly swapped for a different base: there are three types of substitution mutations:

  1. silent mutations - the mutation does not alter the amino acid sequence of the polypeptide chain. as the codon may be degenerate.
  2. missense mutation - the mutation alters a single amino acid.
  3. nonsense mutation - mutation creates a permanent stop codon.
• What are the two sources of genetic variation in meiosis

  Crossing over - the process by which non-sister chromatids exchange alleles

  idependent assortment - production of different combination of alleles in daughter cells due to random aligment of homologous pairs along the equator of the spindle during metaphase 1.