3.4.1 DNA, genes and chromosomes

• DNA

  A complex molecule containing the genetic information that makes up the chromosomes.
• The DNA in prokaryotic cells is significantly different from


  from the DNA found in eukaryotic cells
• DNA is

  double-stranded
• Prokaryotes possess one or more plasmids


  Plasmids are very small circular DNA molecules They usually only contain a few genes Plasmids are more accessible for proteins required for gene expression and therefore contain genes that are required often , quickly and / or in emergencies The genes for antibiotic resistance are often found in plasmids
• The DNA within which cells are associated with proteins (histones)


  Eukaryotic DNA
• Chromosomes The main proteins present are the

  Histones - their role is to organise and condense the DNA tightly so that fits into the nucleus
• chromatin


  The tightly coiled combination of DNA and proteins

  Makes up chromosomes
• S phase


  DNA replicates to create two identical strands of DNA called chromatids joined together by the centromere
• The two chromatids that make up the double structure of a chromosome are known as

  sister chromatids
• Chromatids are identical because


  They form 2 separate daughter cells after mitosis so this ensures that the daughter cells are genetically identical
• Mitochondria


  The site of aerobic respiration within eukaryotic cells

  Has a double membrane with the inner membrane folded to form cristae. The matrix formed by the cristae contains enzymes needed for aerobic respiration producing ATP
• Chloroplast


  Chloroplasts, larger than mitochondria, are surrounded by a double membrane, with membrane-bound compartments called thylakoids which contain chlorophyll-stacked structures called grana, joined by lamellae.

  They are the site of photosynthesis, with light-dependent stages occur in the thylakoids.

  Light independent stage (Calvin Cycle) occurs in the stroma.
• lamellae


  thin and flat thylakoid membranes
• gene


  is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA molecule
• Functional RNA molecules are required for protein synthesis

  mRNA the base sequences on messenger RNA molecules are used by ribosomes to form polypeptide chains tRNA - amino acids are carried to the ribosome by transfer RNA molecules rRNA - ribosomal RNA molecules form part of the structure of ribosomes
• The shape and behaviour of a protein molecule depends on the

  the exact sequence of these amino acids ( the initial sequence of amino acids is known as the primary structure of the protein molecule
• primary structure


  the initial sequence of amino acids is
• The genes in DNA molecules therefore control protein structure


  control protein structure ( and as a result , protei function ) as they determine the exact sequence in which the amino acids join together when proteins are synthesised in a cell
• One gene codes for one mRNA molecule which codes for one polypeptide This


  known as the central dogma of molecular biology
• each chromosome in a human cell nucleus contains one very long DNA molecule This DNA molecule is made up of


  thousands of specific nucleotide sequences called genes that code for specific proteins . Even though these genes are all found within the same DNA molecule and are therefore all linked up the cell knows where individual genes start and stop This ensures the cell reads the DNA correctly and can produce the correct protein molecules that requires to function properly
• The DNA contained within chromosomes is essential

  for cell survival
• Every chromosome consists of a long

  consists of a long DNA molecule that codes for several different proteins %
• gene


  A length of DNA that codes for a single polypeptide or protein FORMS
• locus ( plural loci )

  The position of gene on chromosome
• Each gene can exist in two or more different forms called

  alleles
• Different alleles of a gene have slightly different nucleotide sequences but they still


  they still occupy the same position ( locus ) on the chromosome
• gene


  is a sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids that in turn make up a specific polypeptide ( protein )
• The DNA nucleotide base code found within a gene is a three multiple


  three - letter , or triplet , code
• Triplets (3 bases) are known as codons

  Each codon codes for a different amino acid
• Genetic code is degenerate

  There are 64 different triplets possible, yet there are only 20 amino acids.

  This results in multiple codons coding for the same amino which can limit the effect of mutations.
• Genetic code is universal


  The same codons code for the same amino acids in all living things.

  There are 64 triplet codons which make up 20 amino acids.
• Eukaryotic cells contains introns


  Non - coding sections of DNA which does not code for any amino acids
• Introns


  Non-coding regions of DNA
• Pre-mRNA to mRNA


  Before the pre - mRNA exits the nucleus the introns are removed and the exons are joined together in a process called splicing.

  This forms mRNA.

  Only occurs in eukaryotic cells