inheritance, variation and evolution

• DNA (deoxyribonucleic acid) is often called the blueprint of life. it is found in the nucleus as threadlike structures called chromosomes. it is a very long polymer made of millions of nucleotides. the basic shape is a double helix.
• chromosomes are made up of many genes. they are long polymers of DNA. they are tightly coiled around proteins (histones). after DNA replication they coil up to form chromatids, of which there may be two identical copies joined at the centromere. they contain many genes.
• a gene is a small piece of DNA that codes for a specific sequence of amino acids, to make a specific protein, eg enzymes, antibodies, haemoglobin, hormones
• a genome is all of the genetic material in an organism.
• a nucleotide

  • the backbone of the DNA molecules is alternating phosphate and deoxyribose sugars
  • the teeth are nitrogenous bases

  A) phosphate

  B) nitrogenous base

  C) pentose sugar (ribose or deoxyribose)

  
• • a DNA molecule is made up of 2 strands
  • DNA has 4 different bases
  • cytosine, guanine, adenine and thymine
  • the base pairing rule says that cytosine and guanine are complimentary in shape so always join together
  • adenine and thymine are complimentary in shape so always join together

  
• the DNA or genetic code

  • proteins are made up of a chain of amino acids
  • the 'genetic' language only has 4 letters - ATCG
  • the 'protein' language has 20 letters as there are 20 different amino acids that make up protein
  • a sequence of three nucleotide bases in a strand of DNA is called a codon
  • one codon = one amino acid
  • the DNA or genetic code is therefore a triplet code, degenerate (each amino acid has more than 1 codon), non-overlapping, and universal
• DNA replication

  • occurs just before a cell undergoes mitosis or meiosis
  • strands of double helix separate (H bonds break)
  • each strand acts as a template for new DNA
  • an enzyme called DNA polymerase assembles nucleotides into 2 new strands using the base pairing rule
  • two identical DNA molecules are formed

  
• • a mutation is an unexpected random change in the DNA of a cell and they occur continuously
  • many are repaired, some make no difference (silent mutations), some are dangerous (can cause genetic diseases), some are beneficial and are the basis of evolution
  • gene mutation occurs in individual genes during DNA replication where bases can be deleted, added, duplicated or substituted
  • chromosome mutation occurs in whole chromosomes where large pieces of DNA can be changed or damaged
  • things that cause mutations are called mutagens and include UV light, carcinogens and ionising radiation
• types of mutation:

  • substitution - a base is replaced with the wrong one
  • addition - one or more extra bases are added
  • duplication - a base or bases get repeated
  • deletion - a base or bases get left out
  • inversion - some of the sequence is back to front
  • some mutations are worse than others and completely change the meaning of the gene
  • other mutations don't matter if they are neutral or silent
  • the frequency of some mutations is increases by some chemicals, eg tar in cigarette smoke
• protein synthesis is the process of making proteins from amino acids. the DNA is too large to leave the nucleus to get to the ribosomes so transcription and translation occur.
• transcription is RNA synthesis

  • the 2 strands of double helix are unzipped by breaking hydrogen bonds between base pairs and a helicase enzyme causes the helix to unwind
  • the enzyme RNA polymerase attaches to the DNA in a non coding region just before the gene
  • RNA polymerase moves along the DNA strand, free RNA nucleotides bond with the exposed DNA strand nucleotides by complimentary base pairing to form a strand of mRNA (same bases as DNA but thymine is replaced with uracil)
  • the strand of mRNA is now ready to leave the nucleus and travel to the ribosome
• translation in protein synthesis

  • the mRNA travels through the cytoplasm and attaches to the ribosome
  • for every mRNA codon, the ribosome lines up one complimentary molecules of tRNA
  • tRNA molecules transport specific amino acids to the ribosome which they leave behind after lining up opposite the DNA
  • there are 3 mRNA bases for each tRNA molecule so this is called a triplet code
  • used/'empty' tRNA molecules exit the ribosome to collect another specific amino acid
  • a chain of several hundred amino acids in the correct order according to the original DNA is then made, called a polypeptide
• not all of the bases in DNA code for an amino acid, some are non-coding regions called junk. the mRNA contains both coding and non-coding regions. the non-coding regions are removed before the mRNA leaves the nucleus.