genetic variation

• gene


  a base sequence of DNA that codes for

  • the amino acid sequence of a polypeptide
  • a fundtional RNA (including rRNA and tRNA)

  a gene occupies a fixed position, called a locus, on a DNA molecule
• locus


  where a gene is located on the chromosome
• allele


  different versions of the same gene
• codon 


  3 bases code for 1 amino acid
• features of DNA

  • universal - all organisms have the same structure using the same bases (evidence for evolution)
  • non-overlapping - the codons do not overlap, and each nucleotide is only part of one codon
  • degenerate - more than one code codes for the same amino acid
• exons 


  coding sequences
• introns 


  non-coding sequences
• DNA in prokaryotes


  DNA molecules are shorter, form a circle and are not associated with protein molecules, therefore they do not have chromosomes
• DNA in eukaryotes

  DNA molecules are longer, are linear and occur in association with proteins called histones to form chromosomes. mitochondira and chloroplasts also contain DNA which is like the DNA in prokaryotes
• chromosome structure 


  two threads (chromatids) joined at a single point (centromere). the DNA in chromosomes is held together by histones, and coiled around them in a double-helix. chromosome contains just a single molecule of DNA.
• homologous pairs

  chromosomes in pairs as we receive one from each parent. (humans have 23 pairs)
• diploid number 


  total number of chromosomes
• haploid number


  half of diploid number
• maternal chromosomes


  chromosomes inherited from mother
• paternal chromosomes 


  chromosomes inherited from father
• genome


  entire set of DNA in a cell
• proteome 


  complete set of proteins expressed by an organism
• ribonucleic acid (RNA) structure 


  a polymer made up of repeating mononucleotide sub-units. it forms a single strand in which each nucleotide is made up of:

  • ribose pentose sugar
  • one of the bases adenine, guanine, cytosine and uracil
  • a phosphate group

  two types:

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

  • long strand of nucleotides in a single helix
  • base sequence determined by sequence of bases on a length of DNA in transcription
  • once formed, mRNA leaves nucleus via nuclear pores and enters cytoplasm to associate with ribosomes to act as a template for protein synthesis
• transfer RNA (tRNA)

  • small, single-stranded chain of nucleotides folded into a clover shape, with one end extending beyond the other, where the amino acid molecule can easily attach
  • at opposite end of tRNA molecule is a sequence of 3 other organic bases, known as the anticodon.
  • each molecule is specific to one amino acid
• transcription definition 


  a complementary section of part of base sequence in DNA is made in the form of a molecule called pre-mRNA
• tanslation definition 


  mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry are linked to form a polypeptide
• transcription process

  • an enzyme acts on a specific region of DNA, causing the strands to separate and expose the nucleotide bases
  • the bases on the template strand pair with their complimentary nucleotide from the pool present in the nucleus. RNA polymerase moves along the strand and joins the nucleotides to form a pre-mRNA molecule
  • the exposed bases bind to their complimentary free base, however an exposed adenine binds to uracil instead of thymine
  • as the nucleotides are added, the DNA strands rejoin behind it
  • when the RNA polymerase reaches a stop codon, it detaches
• splicing - transcription


  in prokaryotes, transcription results in the production of mRNA. in eukaryotes, it results in the production of pre-mRNA. the DNA of a gene in these cells is made up of sections called exons that code for proteins and introns that do not. these introns would prevent the synthesis of a polypeptide. the base sequences corresponding to the introns are removed and the exons are joined in splicing
• translation part 1

  • a ribosome becomes attached to the starting codon at the end of the mRNA molecule
  • the tRNA with the complimentary anticodon sequence moves to the ribosome and pairs with codon on the mRNA.
  • a tRNA with a complimentary anticodon pairs with the next codon on the mRNA
  • ribosome continues to move along the mRNA
  • the amino acids on the on the tRNA are joined by a peptide bond using an enzyme and ATP which is hydrolysed to produce the required energy
  • as the ribosome moves to the third codon and joins the amino acid, the first tRNA is released from its amino acid and is free to collect another amino acid from the pool in the cell
  • this process continues until a stop codon is reached and a polypeptide chain is built
• mutation


  any change to the quantity or base sequence of DNA or an organism
• gene mutation


  any change to one or more nucleotide bases, or a change in the base sequence, in DNA
• substitution 


  a nucleotide in DNA is replaced by another nucleotide with a different base
• substitution of bases 


  the polypeptide produced will differ in a single amino acid. the significance of this depends on the role of the original amino acid. if it is important in foring the bonds determining the tertiary structure, it may not form the same bond. the protein will be a different shape and therefore not function properly.

  the effects of the mutation are different if the new codon codes for the same amino acid. this is due to the degenerate nature of the genetic code
• deletion of bases 


  this is when a single nucleotide is lost from the DNA sequence. the polypeptide is unable to function properly as bases are read as triplets, so one deleted nucleotide caused the sequence to be read differently as each has moved to the left by one base.

  frameshift can be caused by deletion, where the entire future sequence of the protein changes
• chromosome mutations 


  changes in structure or number of whole chromosomes. can arise spontaneously and take 2 forms:

  • changes in whole sets of chromosomes
  • changes in number of individual chromosomes
• chromosome mutations - changes in whole sets of chromosomes


  occur when organisms have 3 or more sets of chromosomes, not the usual 2. this is called polyploidy, and occurs mostly in plants
• chromosome mutations - changes in number of individual chromosomes 


  sometimes individual homolohous pairs of chromosomes fail to separate during meiosi. this is non-disjunction, usually leads to a gamete have one more/less chromosome. fertilisation with a gamete with the normal complement of chromosomes, the offspring have more or less chromsomes in all body cells
• silent mutation 


  a substituted base still codes for the same amino acid due to the degenerate nature of the code. this has no effect on polypeptide ptoduction
• non-sense mutation


  a substitution leads to premature "stop codon" being coded for, creating a shorter polypeptide. successful protein synthesis is unlikely, protein would not function normally
• mis-sense mutation


  a substitution leads to a different amino acid being coded fo. the role of the amino acid in the protein is affected
• meiosis - first division


  homologous chromosomes pair up and their chromatids wrap around each other. equivalent portions of these chromatids may be exchanged in a process called crossing over. by the end of this division the homologous pairs have separated, with one chromosome from each pair going into one of the daughter cells
• meiosis - second division


  in the second division, chromatids move apart. at the end, 4 cells have usually been formed, each containing 23 chromosomes in humans
• how does meiosis bring about genetic variation

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


  a pair of chromosomes, one maternal and one paternal, that have the same gene loci