Genetics

• What 2 strands does DNA consist of?

  Polynucleotide strands
• What direction does the second polynucleotide chain run in?


  Opposite
• What does RNA do?


  Transfers genetic information from DNA to ribosomes
• What is messenger RNA?

  A single stranded copy of one genes DNA.
• What does mRNA do?

  Takes a copy of the gene out of the nucleus to the ribosome where it is used for protein synthesis.
• What is transfer RNA?

  Single stranded molecule thats folds using complementary base pairing.
• What does tRNA do?

  Carries an amino acid to the ribosome to be made into a protein.
• How many tRNA molecules are needed for 20 different amino acids?

  20
• What is rRNA?


  Single stranded molecule which folds using complementary base pairing
• What structure does rRNA form part of?

  Ribosome
• What is a gene?

  Section of DNA on a chromosome that controls a feature by coding for formation of a specific protein.
• Allele


  Alternative forms of a particular gene with different base sequences, coding for different versions of the same protein
• Structure of chromosomes


  • DNA molecule
  • DNA combined with histone
  • DNA-histone complex called nucleosome
  • Nucleosomes form loops
  • Loops pack together to form chromosomes
• Formation of chromosomes


  1. DNA wrapped around histone proteins to form nucleosomes

  2. Nucleosomes form chromatin loops

  3. Chromatin loops wrapped around each other to form full chromosome
• Chromosomes are located within the nucleus and only visible as distinct structures when a cell is dividing
• Chromosome structure during cell division


  • Appear as 2 threads (chromatids) joined by a single point (centromere)
  • Each thread is called a chromatid because the DNA has already replicated to give 2 identical DNA molecules
• Chromosome


  • Has 2 short arms (parms)
  • Has longer arms (franger arms)
  • Has a centromere holding it all together at the lenne
• Telomeres


  Sections of DNA at the ends of each chromosome
• Homologous chromosomes


  The number of chromosomes is always the same for individuals of the same species
• Humans always have 46 chromosomes arranged into 23 pairs
• Genes on a chromosome


  • Paternal - 13 different forms of the same gene
  • Maternal - 9 different forms of the same gene
• Genes are always located at the same portion (locus) on the same chromosome
• Alleles


  Slight differences in DNA structure that cause variation in genes
• A karyotype is the full chromosome complement, and illustrates the number, size and shape of the chromosomes
• The human male karyotype has 22 autosomes and sex chromosomes plus X,Y
• The human female karyotype has 22 autosomes and sex chromosomes plus XX
• In human ordinary body cells, humans have 46 chromosomes - the diploid number
• In gametes (sex cells) the number of chromosomes is 23, the haploid number
• Features of the genetic code


  • It is a triplet code
  • 3 bases = 1 amino acid
  • There are 20 different amino acids
  • 4 DNA bases (adenine, guanine, cytosine, thymine) are used to achieve this
• If there are 2 bases, it codes for only 16 amino acids
• If there are 3 bases, it codes for 1 amino acid
• Advantage of triplet code


  Allows for mistakes to be made in DNA sequence
• Genetic code


  • It is a degenerate code
  • Most amino acids are coded to by more than one triplet
  • Only 2 amino acids have just one triplet code (methionine and tryptophan)
• The start codon is always methionine, but the first methionine molecule doesn't form part of the final polypeptide chain and is later removed
• There are 3 stop codons (do not code for any amino acid), and therefore mark the end of the polypeptide chain
• Advantage of degenerate code


  If certain bases are changed, the same amino acid is produced
• Genetic code


  • It is a non-overlapping code
  • Each base appears only once in one triplet
  • Can only read the base once
• Advantage of non-overlapping code


  If a mistake is made, it only affects one amino acid
• The genetic code is universal, it is the same in all living things
• Advantage of universal code


  Allows insulin producing bacteria to take the human insulin gene and produce human insulin