unit 4 - genetic variation

• eukaryotic DNA

  - linear

  - double stranded helix

  - found in the nucleus

  - associated with histones/proteins

  - no plasmids

  - presence of introns

  - longer than prokaryotic
• prokaryotic DNA

  - circular

  - double stranded

  - found floating free in the cytoplasm (no nucleus)

  - not associated with proteins

  - has plasmids

  - no introns

  - shorter
• what is a gene

  a sequence of DNA bases that codes for a polypeptide or a functional RNA
• what is functional RNA either

  ribosomal RNA or transfer RNA
• what is a locus

  location of a gene on a particular DNA molecule
• what are alleles

  different forms of a gene

  = different alleles of a gene have slightly different nucleotide sequences but still occupy the same loci on the chromosome
• what does a chromosome consist of

  a long DNA molecule that codes for several different proteins

  - chromosomes occur in pairs with genes for the same features (alleles) at the same loci

  
• what is a chromosome

  a long strand of DNA that consists of numerous genes
• chromatids

  - sister chromatids are exact replicas produced by DNA replication

  - homologous chromosomes are not replicas but the chromosomes with the same genes at the same loci
• how do you count how many chromosomes are present

  count centromere
• what is a karyotype

  a picture of all the chromosomes from a single cell
• karyotype

  - homologous pairs can be identified due to distinctive banding after straining

  - generally, 2 chromosomes are displayed to one side

  = sex chromosomes

  - all other chromosomes are called autosomes
• homologous pair of chromosomes

  - same genes in same loci

  - may have different alleles
• XY


  male sex chromosomes
• XX


  female sex chromosomes
• what is a genome

  complete set of genes in a cell
• what is a proteome

  full range of proteins that a cell can produce

  = coded for by the cells DNA/genomex
• what codes do the genetic code contain

  - triplet

  - degenerate

  - non overlapping

  - universal
• triplet code

  a sequence of 3 bases (called a codon) codes for a specific amino acid

  = with 3 bases there are 64 codons
• degenerate code

  - some amino acids are coded for by more than 1 codon as there are 64 different triplet combinations, yet only 20 different amino acids

  = 64 is more than enough combinations to code for all 20

  = 4n is mathematic formula

  
  

  - 3 triplets (called stop codons) do not code for any amino acid + are used to make the end of the polypeptide chain

  = the start of the sequence is always the same triplet code
• what are stop codons

  UAA, UAG, UGA
• non overlapping code

  - each base in the sequence is read only once

  = eg. 123456 is read as 123, 456

  = each codon is read as a discrete unit (each 3 bases is separate to the others)

  
  

  - the code is always read in one direction along the DNA strand
• universal code

  it is the same in all organisms

  = ie. the same triplet code. for the same amino acids in all organisms
• DNA amino acid codon table

  
• protein synthesis summary

  1. transcription - forming pre messenger RNA in the nucleus

  
  

  2. splicing (eukaryotic cells only) - splicing of 'pre-mRNA (removes introns) to form messenger RNA, which leaves the nucleus

  
  

  3. translation - binding of mRNA to a ribosome. tRNA brings amino acids to mRNA

  = polypeptide is formed
• what are the characteristics of genetic code

  1. triplet code:

  - sequence of 3 bases (codon) codes for a specific amino acid

  - with 3 bases coding ti allows us to have 64 codons available

  
  

  2. degenerate code:

  - some amino acids are coded for by more than 1 codon as there are 64 different triplets, but only 20 amino acids

  - 3 triplets (called stop codons) don't code for any amino acid - stop codons stop translation and separate the polypeptide chain from the ribosome

  - are used to mark the end of the polypeptide chain

  
  

  3. non overlapping - each base in the sequence only reads once eg. 123456 is read as 123, 456, not 123, 345, 567

  = the code is read in one direction alongside the DNA strand

  
  

  4. universal code - its the same in all organisms

  = ie. same triplet code for the same amino acids
• what is a sequence of 3 bases called

  a codon or triplet
• what are introns

  non-coding regions of DNA

  = prokaryotic don't have introns
• why do we remove introns

  we get rid of them to stop the molecule getting too big so it can pit out the nuclear pores
• DNA helicase

  used to break the H bonds to separate the double helix
• information about mRNA


  - used to transfer the DNA code from the nucleus into the cytoplasm

  - complimentary to the DNA code

  - small enough to leave the nuclear pores

  - once in the cytoplasm, mRNA associates with the ribosomes

  - mRNA is used to determine the sequence of amino acids during protein synthesis

  - easily broken down + only exists whilst it is needed to make a protein

  - sequence of nucleotides in the mRNA is referred to as the genetic code
• how is mRNA formed in transcription


  - DNA helicase unwinds + separates the polynucleotides of DNA by breaking H bonds between the bases in the region of one gene

  - one strand of DNA ('non coding' strand) acts as a template strand for the formation of pre mRNA, the other strand is not transcribed ('coding' strand)

  - free RNA nucleotide line up along the template strand

  = form H bonds with the complimentary DNA nucleotides (C-G/A-U)

  - RNA polymerase catalyses the formation of the covalent. phosphodiester bonds between adjacent nucelotides

  = joins RNA nucleotides together to form pre mRNA

  - when the RNA polymerase reaches a "stop" codon the chain is terminated and the newly formed pre mRNA detaches
• which type of cells involve splicing in protein synthesis

  eukaryotic
• splicing

  - in eukaryotes, portions of the mRNA called introns are removed + the remaining extrons sections join together in splicing

  
  

  - splicing does not occur in prokaryotes

  = no introns to be removed
• what are extrons

  base sequence coding for polypeptide

  = functional, coding regions
• key points of DNA replication

  - uses DNA polymerase to synthesise new DNA molecules

  
  

  - creates identical DNA molecules before cell division

  
  

  - both strands act as templates
• key points of transcription

  - uses RNA polymerase to synthesise mRNA

  
  

  - coverts DNA into mRNA for protein synthesis

  
  

  - only one strand acts as a template
• transfer RNA

  - small molecule

  
  

  - single stranded + folded into a clover leaf shape with one end of the chain slightly longer

  = longer section is attached to an amino acid

  
  

  - each tRNA can carry a different amino acid

  
  

  - 3 bases (codon) at the apposite end of the tRNA

  = anticodon

  = each amino acid has a different anticodon

  
  

  - the anticodon pairs with the complementary codon on the mRNA

  
• what is translation


  the formation of the polypeptide (protein) using mRNA code in a ribosome
• translation - part 1 start - detailed

  - the mRNA attaches to the ribosome

  
  

  - 2 mRNA codons are exposed in the ribosome at the same time

  
  

  - mRNA is used as a template to which complimentary tRNA molecules attach

  
  

  - H bonds form between the start codon + a tRNA with a complimentary anticodon

  = this tRNA has an amino acid attached to it

  
  

  - a second tRNA with an anticodon complimentary to the next codon forms H bonds with the next 3 bases