cell biology II

• translation initiation

  • 40s ribosomal subunit binds to mRNA
  • an initiator tRNA with the anticodon UAC base pairs with AUG start codon
• the AUG codon changes the structure of the ribosome once it binds to the trna

  locks the trna to the ribosome
• translation initiation:

  • the 60s ribosomal subunit completes the initiation complex
  • gets energy from GTP
• translation elongation:

  • the A site will be the landing site for the next tRNA. with anticodon complementary for the exposed codon on mRNA
• translation elongation:

  • the peptide bond connects one amino acid to another

  the methionine from the first tRNA transfers into the amino acid of the second tRNA in the A site
• trna and amino acid bond are broken at P site
• after the peptide bond is formed

  • mRNA pulled onward through the ribosome by exactly one codon
  • allows the first empty trna to exit via e site
  • also exposes new codon in the A site and cycle repeats
• translation termination:

  • termination happens when a stop codon in the mRNA enters A site
• stop codons are recognised by release factors which fit into the A site
• translation termination:

  release of 2 tRNAs and 2rRNA subunits

  hydrolysis of bound GTP-GDP
• polyribosome:

  a cluster of ribosomes linked together by a molecules of a messenger and forming the site of protein synthesis
• structure of DNA:

  alternation groups of sugar and phosphate groups form a DNA strand
• 2 strands wind around one another like a twisted ladder and form dna double helix
• spatial location of dna in eukaryotic cells:

  plastid dna sequences can be found in chloroplast
• chloroplast is a plasmid in green plant cells which contains chlorophyll where photosynthesis takes place
• dna is packaged into chromosomes
• naked dna molecules are unstable within the cell and packaging it protects dna from damage
• compaction allows long dna molecule to fit within a cell
• allows efficiency of dna replication and transfer to daughter cells
• organisation of genome allows for regulation of gene expression
• dna replication:

  semi-conservative replication

  produce 2 copies that each contained one of the original stands and one new strand
• process of dna replication:

  • dna unwinds at the origin of replication (replication fork)
• helicass separates the dna strands
• primase synthesises rna primer to initiate synthesis by dna polymerase
• dna polymerase adds new bases to the complementary parental strands
• one new strand is made continuously in the 5’-3’ direction leading strand
• other strand is made in 3’-5’ direction in short stretches of dna which is okazaki fragments - lagging strand
• once replication is done

  rna primers are replaced by dna nucleotide
• dna is sealed with dna ligase
• gene expression in prokaryotes:

  • prokaryotes only transcribe genes that their end proteins are needed at time
  • save energy and increase efficiency
• regulation of gene expression depends on immediate environment, like presence and absence of nutrients
• operons

  • genes devoted to single metabolic goal mostly found in continuous array in DNA
  • operates as unit from single promoter
• transcription of operon produces a continuous strand of mRNA contains details for related series of proteins
• 1 promoter activated more than 1 gene
• general structure of operons:

  • promoter
  • operator
  • genes
• only 3 types of polymerase in bacterial cells
• promoter

  • recognised by rna polymerase which initiates transcription
• operator

  • a segment of DNA that a regulator bind to and halt transcription
• genes

  • encoded by operon
• repressor

  • dna binding protein that inhibits the expression of genes by binding to operator