B1.1 Genomes

• Cell types


  • Eukaryotic (animals and plants)
  • Prokaryotic
• Eukaryotic cell


  • Contains a nucleus and membrane-bound organelles
• Prokaryotic cell


  • Does not contain a nucleus and membrane-bound organelles
• Gametes


  Reproductive cells (e.g. egg and sperm cells) that contain a single copy of each chromosome
• Observation of eukaryotic cell structure


  Using a light microscope
• Sexual reproduction in terms of chromosome number


  1. Two gametes with a single copy of each chromosome fuse

  2. Resulting embryo has two chromosomes for each gene and two copies of each allele

  
• Genetic information storage in eukaryotic cell


  Within the nucleus, arranged in chromosomes
• Homozygous


  Having two identical alleles of a gene e.g. FF or ff
• Genetic information storage in prokaryotic cell


  Found free within the cytoplasm as a single large loop of circular DNA and plasmids
• Heterozygous


  Having two different alleles of a gene e.g. Ff
• Plasmids


  Small, circular loops of DNA found free in the cytoplasm and separate from the main DNA, carry genes that provide genetic advantages e.g. antibiotic resistance
• Genome


  The entire genetic material of an organism
• Dominant allele


  Describes an allele that is always expressed, represented with a capital letter e.g. F
• Chromosome


  A long, coiled molecule of DNA that carries genetic information in the form of genes
• Recessive allele


  An allele that is only expressed in the absence of a dominant allele, represented with a small letter e.g. f
• DNA


  A double-stranded polymer of nucleotides, wound to form a double helix
• Female homozygous recessive for cystic fibrosis (ff) has a child with a heterozygous male (Ff)

  • Punnett square
• PKU is a recessive condition. Two heterozygous parents have offspring.


  • Punnett square
  • 75% chance of normal phenotype
  • 25% chance of PKU phenotype
• Gene


  A section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein
• Most characteristics are controlled by multiple alleles rather than just one

  
• Sex chromosomes


  A pair of chromosomes that determines sex: Males have an X and a Y chromosome, Females have two X chromosomes

  
• Alleles


  Different versions of the same gene
• Genotype


  An organism's genetic composition, describes all alleles
• Inheritance of a Y chromosome


  Embryo develops into a male
• A couple have a child


  • Punnett square
  • 50% chance of female (XX)
• Phenotype


  An organism's observable characteristics due to interactions of the genotype and environment (which can modify the phenotype)
• Other ways to represent single gene inheritance


  Using a family tree
• DNA nucleotides


  • Common sugar
  • Phosphate group
  • One of four bases: A, T, C or G
• How Mendel's work helped develop understanding of genetics


  1. Mendel studied inheritance of different phenotypes in pea plants

  2. Established correlation between parent and offspring phenotypes

  3. Noted inheritance was determined by 'units' passed on to descendants

  4. Devised terms 'dominant' and 'recessive' using gene crosses
• Formation of DNA molecule


  1. Sugar and phosphate molecules join to form a sugar-phosphate backbone in each DNA strand

  2. Base connected to each sugar

  3. Complementary base pairing: A pairs with T, C pairs with G
• Genome sequencing


  Finding out the order of nucleotides in the DNA of an organism, enabling the function and interaction of genes to be assessed
• Protein synthesis


  1. In the nucleus, DNA is used as a template to form mRNA

  2. mRNA exits the nucleus, moving into the cytoplasm where it attaches to a ribosome

  3. The ribosome joins amino acids in a specific order, dictated by mRNA to form a protein
• Importance of genome sequencing


  • Allows comparison of genomes of healthy individuals with patients who have a disease
  • Potential disease-causing alleles are identified
  • Individuals can then undergo genetic testing for these alleles
• Difference between mRNA and DNA


  • mRNA is single stranded whereas DNA is double stranded
  • mRNA uses U whereas DNA uses T
• DNA is too large to leave the nucleus so cannot reach the ribosome
• Mutation


  A random change to the base sequence of DNA which results in genetic variants
• Types of gene mutation


  • Insertion
  • Deletion
  • Substitution
• Effect of gene mutation in coding DNA

  • If a mutation changes the sequence of amino acids, protein structure and function may change
  • If a mutation does not change the sequence of amino acids, there is no effect on protein structure or function
• Non-coding DNA


  DNA which does not code for a protein but instead controls gene expression
• Effect of gene mutation in non-coding DNA


  • Gene expression may be altered, affecting protein production and the resulting phenotype