8+9 DNA, Genes and Protein Synthesis + Genetic Diversity

• Eukaryotic and Prokaryotic DNA Similarities:

  • Identical nucleotide structure
  • Adjacent nucleotides joined by phosphodiester bonds
  • Complementary bases joined by hydrogen bonds
  • DNA in mitochondria/chloroplasts have similar structure to DNA in prokaryotes - Short, circular, not associated with proteins
• Nucleotide structure - Deoxyribose sugar attached to phosphate and a nitrogenous base
• Eukaryotic and Prokaryotic DNA Differences:

  • Eukaryotic DNA is longer
  • Eukaryotic DNA is linear whereas prokaryotic is circular
  • Eukaryotic DNA is associated with histone proteins whereas prokaryotic DNA is not
  • Eukaryotic DNA contains introns whereas prokaryotic DNA does not
• A chromosome is long, linear DNA and its associated histone proteins located in the nucleus of eukaryotic cells
• A gene is a section of the base sequence of DNA (nucleotide bases) that codes for the amino acid sequence of a polypeptide or a functional RNA.
• A locus is a fixed position that a gene occupies on a particular DNA molecule
• Describe the nature of the genetic code:

  1. Triplet code - A sequence of 3 DNA bases (called a triplet) codes for a specific amino acids
  2. Universal
  3. Non-overlapping
  4. Degenerate
• The genetic code is universal - The same base triplets code for the same amino acids in all organisms
• The genetic code is non-overlapping - Each base is part of only one triplet so each triplet is read as a discrete unit
• The genetic code is degenerate - An amino acid can be coded for by more than one base triplet
• Non-coding base sequences are sections of DNA that do not code for amino acid sequences.
• Non-coding base sequences of DNA within genes are called introns.
• In eukaryotes, much of the nuclear DNA does not code for polypeptides.
• Introns are sections of the base sequence of DNA in a gene that do not code for amino acids, found in eukaryotic cells
• Exons are sections of the base sequence of DNA in a gene that code for amino acid sequences
• Prokaryotic DNA is NOT single stranded
• RNA is single stranded
• The genome is the complete set of genes in a cell, including those in the mitochondria/chloroplasts
• The proteome is the full range of proteins that a cell can produce, coded for by the cell's DNA/genome
• Transcription involves the production of messenger RNA from DNA, in the nucleus
• Translation involves the production of polypeptides from the sequence of codons carried by mRNA, at the ribosomes
• Fill in the blanks:

  A) DNA

  B) mRNA

  C) Polypeptide

  D) Transcription

  E) Translation

  
• Both tRNA and mRNA are composed of a single polynucleotide strand
• tRNA is short for transfer RNA
• tRNA vs mRNA structural differences:

  1. tRNA is folded into a clover leaf shape whereas mRNA is linear
  2. tRNA has hydrogen bonds between paired bases whereas mRNA does not
  3. tRNA is a shorter, fixed length whereas mRNA is a longer, variable length
  4. tRNA has an anticodon whereas mRNA has codons
  5. tRNA has an amino acid binding site whereas mRNA does not
• tRNA and mRNA structure:

  A) binding site

  B) Hydrogen bonds

  C) Anticodon

  D) Codon

  
• Describe how mRNA is formed by transcription in eukaryotic cells

  1. DNA helicase unwinds the DNA by breaking the hydrogen bonds between bases.
  2. Only one DNA strand acts as a template
  3. Free RNA nucleotides align next to their complementary bases on the template strand - In RNA, uracil is used in place of thymine (pairing with adenine in DNA)
  4. RNA polymerase joins adjacent RNA nucleotides
  5. This forms phosphodiester bonds via condensation reactions
  6. Pre-mRNA is formed and this is spliced to remove introns, forming (mature) mRNA
• Transcription diagram

  A) DNA

  B) mRNA

  C) RNA polymerase

  D) Coding strand

  E) Template strand

  F) mRNA

  G) Phosphodiester

  H) RNA nucleotide

  
• Pre-mRNA is produced in eukaryotic cells but not in prokaryotic cells because genes in prokaryotic cells do not contain introns so splicing does not occur.
• Describe how translation leads to the production of a polypeptide

  1. mRNA attaches to a ribosome and the ribosome moves to a start codon (AUG)
  2. tRNA brings a specific amino acid
  3. tRNA anticodon binds to complementary mRNA codon
  4. Ribosome moves along to next codon and another tRNA binds so 2 amino acids can be joined by a condensation reaction forming a peptide bond, using energy from hydrolysis of ATP
  5. tRNA released after amino acid joined polypeptide
  6. Ribosome moves along mRNA to form the polypeptide, until a stop codon is reached
• The role of ATP in translation:

  • Hydrolysis of ATP to ADP + Pi releases energy
  • So amino acids join to tRNAs and peptide bonds form between amino acids
• The role of tRNA in translation:

  • Attaches to and transports a specific amino acid, complementary to its anticodon
  • tRNA anticodon complementary base pairs to mRNA codon, forming hydrogen bonds
  • 2 tRNAs bring amino acids together so peptide bond can form
• The role of Ribosomes in translation:

  • mRNA binds to ribosome, with space for 2 codons
  • Allows tRNA with anticodons to bind
  • Catalyses the formation of a peptide bond between amino acids, held by tRNA molecules
  • Moves along mRNA strand to the next codon - Translocation
• A gene mutation is a change in the base sequence of DNA that can arise spontaneously during DNA replication (in interphase)
• A mutagenic agent is a factor that increase the rate of gene mutation e.g. UV light or alpha particles
• Explain how a mutation can lead to the production of a non-functional protein or enzyme

  1. Changes sequence of base triplets in DNA (in a gene) so changes sequence of codons on mRNA
  2. So changes sequence of amino acids in the polypeptide
  3. So changes position of hydrogen / ionic / disulphide bonds (between amino acids)
  4. So changes protein tertiary structure (shape) of protein
  5. Enzymes - active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form
• Substitution mutation:

  1. Base in DNA is replaced by a different base
  2. This changes one triplet so it changes one mRNA codon
  3. So one amino acid in the polypeptide changes
• In a substitution mutation, the amino acid may not change due to the degenerate nature of the genetic code OR if the mutation occurs in an intron
• This is an example of substitution mutation

  
• Deletion mutation:

  1. One base is removed from the DNA sequence
  2. This changes the sequence of DNA triplets from the point of mutation (frameshift)
  3. This changes the sequence of mRNA codons after the point of mutation
  4. This changes the sequence of amino acids in the primary structure of a polypeptide
  5. This changes the position of the hydrogen/ionic/disulphide bonds in the tertiary structure of a protein
  6. This changes the tertiary structure, so the shape of a protein