gene expression in eukaryotes

• prokaryotes contain between 500 and 4000 genes
• eukaryotes contain between 6000 and 30,000 genes
• prokaryotes organise genome into 


  circular DNA and have small genomes
• eukaryotes organise genome into

  linear DNA into chromatin and have large genomes
• eukaryotic genomes are so large because

  • number of genes
  • amount of non-coding DNA
• eukaryotes have more genes which reflects the greater complexity of the organisms
• in prokaryotes, most DNA codes for protein
• in eukaryotes, most DNA doesn't code for protein or RNA at about 


  97% in humans
• non-coding sequences consist of

  • gene regulatory sequences such as promoters
  • introns which are non-coding sequences within genes
  • sequences of no known function
• promoter is part of the gene that controls

  transcription
• exon is the 


  transcribed sequence that is represented in the final mRNA
• intron is the 


  intervening sequence in the transcribed region that is not represented in the final mRNA
• 2 types of repetitive sequences

  1. interspersed repetitive DNA
  2. tandemly repetitive (satellite) DNA
• interspersed repetitive DNA

  • repeated units throughout genome
  • single unit 100-10,000 bp
  • copies not identical, closely related instead
  • make up 25-40% mammalian genome
• interspersed repetitive DNA tends to be close to protein coding genes
• tandem repetitive DNA

  • regular 100,000-10 million bp per site
  • mini-satellite is 100-100,000 bp per site
  • micro satellite is 10-100 bp per site
• satellite DNA is often found at

  telomers and centromeres
• satellite DNA has a

  structural role
• genetic disorders can be caused by abnormally long stretches of

  repetitive sequence within a gene (e.g. Huntington's disease)
• chromatin structure

  • intricate form of packaging for DNA
  • heterochromatin is highly condensed during interphase, not actively transcribed
  • euchromatin is less condensed during interphase, able to be transcribed
• chromosomes are composed of chromatin (protein and DNA)
• DNA has to be accessible for transcription and replication
• DNA is associated with histones and formation of chromatin for transcription and replication
• eukaryotic chromatin is _ complex than prokaryotic chromatin

  more
• histones are 


  proteins with positively charged amino acids that bind to the negatively charged DNA
• nucleosome is 


  the basic unit of chromatin
• during interphase, most of the chromosome is the _ form 


  euchromatin
• during meiosis and mitosis the chromatin folds further (to condense)
• highly condensed chromatin also occurs in interphase in some regions of the chromosome known as

  heterochromatin
• chromatin structure

  labels

  A) euchromatin

  B) heterochromatin

  
• long term control of gene expression during

  cellular differentiation
• regulation of genes

  labels

  A) DNA unpacking

  B) transcriptional control

  C) RNA controlling process

  D) RNA transport and localised control

  E) mRNA degradation control

  F) translational control

  G) protein activity control

  
• DNA methylation is associated with

  gene silencing
• histone acetylation is assoicated with

  gene activation
• methylated DNA, histones not acetylated so chromatin is closed
• 3 types of RNA polymerases

  • type 1 transcribes ribosomal RNA
  • type 2 transcribes messenger RNA
  • type 3 transcribes small RNA e.g. tRNA
• transcription begins when the RNA polymerase binds to a promoter
• promoter is the DNA sequences adjacent to the gene (upstream)
• TATA box is part of the promoter, provides site of initial binding, located upstream (10-35bp) of transcription start site
• TATA box is made up of T's and A's