Genetics and Variation

• Process of translation

  1. Ribosome attaches to starting codon on mRNA

  2. tRNA with complementary anticodon pairs with codon on mRNA, carrying amino acid

  3. Next tRNA with complementary anticodon pairs with next codon, carrying another amino acid

  4. Ribosome brings together two tRNA molecules, joining amino acids with peptide bond

  5. Ribosome moves to third codon, linking two amino acids

  6. Process continues until ribosome reaches stop codon
• gene 


  section of DNA that codes for a polypeptide chain and functional RNA
• degenerate

  more than one codon will code for the same amino acid
• universal

  same codon will always code for the same amino acid
• non-overlapping

  each codon is only read once
• exons

  coding sequences
• introns

  non-coding sequences
• prokaryotic DNA

  shorter

  circular

  not associated with histones
• eukaryotic DNA

  longer

  linear

  associated with histones
• mitochondria and chloroplast DNA

  like prokaryotic DNA
• chromosome structure

  two sister chromatids

  joined by the centromere
• locus

  the specific position of a gene on a DNA molecule
• diploid

  full number of chromosomes
• homologous pair

  two chromosomes that carry the same genes but not necessarily the same alleles of the gene
• haploid

  half the number of chromosomes
• allele

  one of a number of alternative forms of a gene
• codon

  the sequence of three bases that codes for a single amino acid
• genome

  the complete set of genes in a cell
• proteome

  the full range of proteins that a cell can produce
• ribonucleic acid (RNA) structure

  nucleotides are made up of:

  • the pentose ribose sugar
  • an organic base (adenine, guanine, cytosine and uracil)
  • a phosphate group

  two types:

  • messenger RNA (mRNA)
  • transfer RNA (tRNA)
• structure of mRNA

  single strand

  contains codons

  linear

  larger
• structure of tRNA

  smaller

  clover-leaf shaped

  amino acid binding site

  anticodon
• process of transcription

  DNA helicase breaks hydrogen bonds between complementary base pairs causing the two strands to separate

  one of the two strands acts as a template strand

  the template strand pairs with free complementary RNA nucleotides

  RNA polymerase moves along the strand to join nucleotides together

  when RNA polymerase reaches a stop codon it detaches

  pre-mRNA is produced
• splicing

  occurs to remove introns from pre-mRNA

  splicing does not have to occur in prokaryotic cells as they do not contain introns
• mutation

  change to the base sequence of DNA of an organism
• substitution of bases

  a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base
• deletion of bases

  when a nucleotide is lost from the normal DNA sequence
• chromosome mutations

  • changes in whole sets of chromosomes - when an organism has 3 or more sets of chromosomes. called polyploidy - occurs mostly in plants
  • changes in the number of individual chromosomes - homologous pairs can fail to separate during meiosis - non-disjunction - offspring will have fewer or more chromosomes than normal
• the process of meiosis

  1. in the first division homologous chromosomes pair up and their chromatids wrap around each other. equivalent portions of these chromatids may be exchanged in crossing over. by the end of the division the homologous pairs have separated, with one chromosome from each pair going into one of the two daughter cells.
  2. in the second division the chromatids move apart, at the end four cells have been formed
• how does meiosis bring about genetic variation?

  • independent segregation of homologous chromosomes
  • new combinations of maternal and paternal alleles by crossing over
• independent segregation of homologous chromosomes

  during meiosis 1 each chromosome lines up alongside its homologous partner.

  when they arrange themselves in this line they do so at random.

  one of each pair will goes into the daughter cell, this depends on how the pairs are lined up.

  the combination of chromosomes of maternal and paternal origin that go into the daughter cell at meiosis 1 is also a matter of chance
• genetic recombination by crossing over

  each chromosomes lines up alongside its homologous partner in meiosis 1.

  • the chromatids of each pair become twisted around one another.
  • during this twisting process tensions are created and portions of the chromatids break off.
  • these broken portions might then rejoin with the chromatids of its homologous partner.

  the broken-off portions of chromatid recombine with another chromatid, called recombination.
• all members of the same species have the same genes
• genetic diversity

  the total number of different alleles in a population
• population

  a group of individuals of the same species that live in the same place and are able to interbreed.
• species

  consists of one or more populations
• natural selection

  • within any population of a species there is a gene pool containing a wide variety of alleles.
  • random mutation of alleles within this gene pool may result in a new allele of a gene.
  • the new allele may be advantageous
  • these individuals with the new allele will be better adapted and therefore more likely to survive.
  • they are more likely to reproduce
  • they will pass on their alleles to the next generation
  • over many generations the number of individuals with the advantageous allele will increase
  • the allele frequency will increase
• directional selection

  favour individuals that vary in one direction from the mean of the population
• stabilising selection

  selection may favour average individuals
• natural selection results in species that are better adapted to the environment they live in. these adaptations may be

  anatomical

  physiological

  behavioural