section 7 - genetics and inheritance

• Gene


  Part of DNA
• Genome


  An organism's genes
• Phenotype


  The physical characteristic/appearance
• Allele


  Different versions of a gene
• Dominant allele


  Always expressed
• Recessive allele


  Not expressed
• Homozygous


  2 of the same alleles
• Heterozygous


  2 different alleles
• Codominant alleles


  Neither allele is recessive so you end up showing a combination of both. Eg blood type.
• DNA


  • Double helix strand
  • 4 bases: A-T, G-C
• Codon


  3 bases that code for 1 amino acid
• There are 64 different codons to code for 20 amino acids, reducing risk of mutations
• DNA codons are non-overlapping: each codon codes for one amino acid and bases are not shared between amino acids
• A change in the order of bases

  Can lead to a different codon and amino acid in the protein
• Some mutations may have little effect on the phenotype as it may code for the same amino acid or be a recessive allele
• Carcinogens increase the risk of mutations
• Differences between DNA and RNA


  • DNA is double stranded, RNA is single stranded
  • DNA has thymine, RNA has uracil
  • DNA is long, RNA is short
  • DNA has deoxyribose, RNA has ribose
• Protein synthesis


  DNA->transcription->mRNA->translation->amino acid chain
• Transcription


  1 strand of DNA is copied to mRNA in nucleus
• Translation


  1. mRNA attached to ribosome in cytoplasm

  2. mRNA is a template

  3. Ribosomes move along the mRNA strand

  4. tRNA binds with mRNA

  5. Amino acid chain formed
• Differences between transcription and translation


  • Transcription is in nucleus, translation is in cytoplasm
  • Transcription uses DNA to make RNA, translation involves mRNA and tRNA
  • Transcription makes mRNA, translation makes protein
• Haploid cells


  Have 23 chromosomes - egg cell, sperm cell and RBC
• Diploid cells


  Have 46 chromosomes - all other cells
• Mitosis


  1. Cell has 4 chromosomes

  2. Chromosomes double

  3. Chromosomes line up down the centre

  4. Two copies split to opposite sides

  5. Cell splits down the middle forming two identical cells

  
• Meiosis


  1. 1 and 2 are the same as mitosis

  2. 4 cells with 2 chromosomes in each. So 1 diploid cell makes 4 haploid cells

  
• Mitosis makes genetically identical cells, used for growth and repair
• Meiosis makes cells that are not genetically identical

  
• Monohybrid cross


  • Number of heterozygous: 4
  • Number of homozygous: 3
• Physical factors


  Can be affected by genetics, diet, environment, gender
• Genetic factors


  Depend on genes
• The theory of evolution by Charles Darwin is natural selection
• Natural selection theory


  1. Each species reproduces and the offspring show variation

  2. There is competition between offspring for food, space

  3. The ones with the best adaptations for their environment survive

  4. The survivors pass on their beneficial alleles to their offspring