Topic 4

• Gene: a section of DNA that contains a code for making a polypeptide and functional RNA

  
• Locus: the location of a particular gene on a chromosome

  
• Allele: a different version of the same gene
• Chromosome:

  • threadlike structure
  • composed of tightly coiled DNA wrapped around histones (if it is a eukaryotic cell)

  
• Homologous chromosome: a pair of chromosomes that have the same genes therefore, they are the same size

  
• Eukaryotic DNA:

  • DNA is stored as chromosomes inside the nucleus
  • linear chromosomes
  • DNA is tightly coiled and wrapped around proteins called histones
• Prokaryotic DNA:

  • DNA molecules are shorter and circular
  • DNA is not wound around histones. Instead, it supercoils to fit in the cell
• Codon: 3 bases on mRNA that code for an amino acid
• Start codon: 3 bases at the start of an mRNA sequence which help to initiate translation
• Genetic code: an amino acid is coded for by 3 DNA bases which are described as the "triplet code"
• What is the advantage of the genetic code being universal?:

  • genetic engineering is possible
  • a human gene can be inserted into another organism
  • e.g human gene for insulin inserted into bacteria to make insulin
• What is the advantage of the genetic code being non-overlapping?: if a point mutation occurs, it will only affect one codon and therefore one amino acid
• Triplet code: an amino acid is coded for by 3 bases
• Mutation:

  • a change in the DNA
  • can be a gene or chromosome mutation
• What is meant by 'the genetic code is universal?'

  
  

  the same triplet of bases codes for the same amino acid in all organisms
• What is meant by 'the genetic code is non-overlapping?:

  • each base in a gene is only part of one triplet of bases that codes for one amino acid
  • therefore each codon is read as a discrete unit
• What is splicing?:

  • post-transcription modification
  • removing introns

  
• Genome: the complete set of genes in a cell
• Proteome: The full range of proteins that a cell is able to produce
• Anticodon: 3 bases on the tRNA which are complementary to the codon on mRNA.
• tRNA structure:

  • single-stranded, folded to create a cloverleaf shape
  • held in place by hydrogen bonds
  • has an anticodon and amino acid binding site

  
• mRNA function:

  • a copy of a gene from DNA
  • created in the nucleus, and it then leaves the nucleus
  • to carry the copy of the genetic code of one gene to a ribosome in the cytoplasm
• tRNA function:

  • a specific amino attaches at the binding site
  • transfers this amino acid to the ribosome to create the polypeptide chain
• Transcription:

  • the first stage in protein synthesis
  • one gene in DNA is copied into mRNA
  • occurs in the nucleus

  
• Translation:

  • the second stage in protein synthesis
  • the polypeptide chain is created using both the mRNA base sequence and the tRNA
  • occurs on ribosomes in the cytoplasm
• Which enzymes are involved in transcription?

  • DNA helicase
  • RNA polymerase
• RNA polymerase: joins adjacent RNA nucleotides together forming a phosphodiester bond
• pre-mRNA: mRNA in eukaryotes that still contains the introns
• How is premRNA modified?

  
  

  • the introns are removed by a protein called a spliceosome
  • this leaves just the exons
• What is ATP used for in translation? forming the peptide bond between amino acids
• Haploid: one copy of each chromosome in a cell
• Diploid: two copies of each chromosome in a cell
• Meiosis:

  • cell division that creates genetically different gametes
  • there are two nuclear divisions in this process
  • results in four haploid daughter cells
• Independent segregation:

  • homologous pairs of chromosomes randomly line up opposite each other at the equator of the cell
  • when they separate it creates a large number of possible combinations of chromosomes in the daughter cells produced
• Crossing over:

  • homologous pairs of chromosomes line up opposite each other at the equator in meiosis 1
  • parts of the chromatids twist, break and recombine with another chromatid
  • results in new combinations of alleles in the gametes

  
• Gametes: sex cells (sperm and egg)
• What is a frameshift?:

  • the removal of one base changes all of the subsequent codons
  • all the bases shift back one position
• Chromosome mutation:

  • change in the number of chromosomes
  • occurs during meiosis
• Deletion mutation:

  • a gene mutation
  • a base is removed from a sequence
  • causes a frameshift
• Substitution mutation:

  • a gene mutation
  • a base is swapped for a different one