DNA + protein synthesis

• a gene is a section of dna that contains the coded information for making polypeptides or functional RNA
• the gene locus is the particular position on the dna molecule where a particular gene is located
• specific sequence of bases along the DNA polypeptide determines the primary structure of the protein
• The order of bases on DNA is called the genetic code which consists of triplets of bases, with each triplet of bases coding for a particular amino acids known as a codon#
• the non-coding sections of DNA are called introns and the coding regions are called exons.
• an allele is a different form of the same gene
• chromosomes are tightly coiled up DNA within the nucleus (eukaryotic)
• structural differences in the DNA found in eukaryotes and prokaryotes:

  • Eukaryotic DNA is linear, prokaryotic DNA is circular
  • Eukaryotic DNA is associated with histone proteins (dna wound tightly to fit in nucleus as chromosome), prokaryotic DNA is not associated with histones
  • Eukaryotic DNA is longer than prokaryotic DNA
• mitochondrial DNA and chloroplast DNA are similar to prokaryotic dna:

  • short
  • circular
  • not associated with histones (no chromosomes)
• start codon are 3 bases at the start of every gene which initiates translation (coding)
• stop codon are 3 bases at the end of every gene to mark the end of a polypeptide causing translation to stop and ribosome to detach
• Features of the genetic code:

  • Triplet code = 3 bases code for an amino acid [4 bases + 20 amino acids ----> 4^3 = 64 enough to satisfy 20]
  • Degenerate = most amino acids are coded for by more than one triplet
  • Non-overlapping = codons do not overlap so each base in the sequence is read only once
  • Almost universal = each triplet codes for the same amino acid in all organisms
• being degenerate is an advantage as if it mutates some mutations will have no effect on the organism since the same protein will still be produced
• being universal is indirect evidence for evolution and makes genetic engineering possible
• a genome is an organisms complete set of dna in a cell
• a proteome is the full range of proteins that a cell is able to produce - depending on which proteins are currently needed
• sections of the DNA (genes) molecules which code for polypeptides are transcribed onto a single stranded messenger RNA and leave to ribosome out of nuclear pores
• in dna 3 bases are known as triplets while in mRNA three bases are known as codons
• complete proteome is the full range of proteins produced by the genome
• structure of RNA includes a single strand made up of a nucleotide:

  • pentose sugar ribose
  • one nitrogenous base (A, G, C, U)
  • a phosphate group
• mRNA:

  • polynucleotide chain arranged in a single helix
  • Produces during transcription carries the genetic code from the nucleus to the cytoplasm (has instructions for making a protein on the ribosome in translation)
  • carry info in the form of codons
• tRNA:

  • relatively small and single stranded
  • folded into a clover leaf shape
  • Contains an amino acid binding site at one end and an anticodon at the opposite end (bind to complementary codons on mRNA to sequence into a protein’s primary sequence)
  • Carries amino acids to the ribosome during translation
• transcription is the process of making (pre) mRNA using part of DNA as a template
• Transcription:

  1. The enzyme dna helicase breaks the hydrogen bonds between bases for dna helix to unzip exposing bases to act as a template (only one chain)
  2. Free mRNA nucleotides in nucleus align opposite to exposed complementary dna bases RNA polymerase to move along strand joining nucleotides together forming phosphodiester bond
  3. when RNA polymerase reaches stop triplet code it detaches forming pre-mRNA molecule
  4. SPLICING = pre-mRNA has to be modified to become mRNA so introns (non-coding bases) are spliced out (exons are spliced together) forming mRNA
• • In prokaryotes, transcription results directly in the production of mRNA from DNA
  • In eukaryotes, transcription results in the production of pre-mRNA; this is then spliced to form mRNA.
• Translation is the production of of polypeptides from the sequence of codons carried by the mRNA
• Translation:

  1. mRNA leaves the nucleus attaching to start codon at ribosome
  2. The tRNA molecule with the complementary anticodon pairs up with the codon on the mRNA (carrying a specific amino acid)
  3. the ribosome moves along the mRNA so other complementary tRNA anticodon can attach to next codon (tRNA carries another amino acid)
  4. the amino acids (delivered by the tRNA) are joined by a peptide bond [requires an enzyme + ATP for energy]
  5. once tRNA releases amino acid it leaves - continuing as ribosome moves across until ribosome reaches stop codon at end and ribosome detaches - TRANSLATION ENDS