DNA, genes, chromosomes

• prokaryotic DNA

  posses single, circular chromosomal DNA molecule

  usually have plasmids

  • small circular DNA molecules
  • contain few genes
  • more accessible for proteins required for gene expression
  • genes for antibiotic resistance found in plasmids

  DNA in prokaryotic cells not associated with proteins
• eukaryotic DNA

  linear DNA exits as chromosomes, each made of a long molecule of DNA.

  chromosomes found in the nucleus

  they are wound up so the can fit into the nucleus

  DNA molecule wound around proteins called histones

  histone also help support DNA

  DNA is tightly coiled to mke a compact chromosome
• • The tightly coiled combination of DNA and proteins is called chromatin – this is what chromatids, and therefore chromosomes, are made of
• label the making of a chromosome

  A) chromatid

  B) chromosome

  C) chromatin

  D) histones

  
• • The site of aerobic respiration within eukaryotic cells, mitochondria are just visible with a light microscope
  • Surrounded by double-membrane with the inner membrane folded to form cristae
  • The matrix formed by the cristae contains enzymes needed for aerobic respiration, producing ATP
  • Small circular pieces of DNA (mitochondrial DNA) and ribosomes are also found in the matrix (needed for replication)

  A) inner membrane

  B) outer membrane

  C) cristae

  D) matrix

  
• • A 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 exact sequence of these amino acids (the initial sequence of amino acids is known as the primary structure of the protein molecule)
  • The genes in DNA molecules, therefore, control protein structure (and as a result, protein function) as they determine the exact sequence in which the amino acids join together when proteins are synthesised in a cell
• Genes, loci and alleles

  • The DNA contained within chromosomes is essential for cell survival
  • Every chromosome consists of a long DNA molecule that codes for several different proteins
  • A length of DNA that codes for a single polypeptide or protein is called a gene
  • The position of a gene on a chromosome is its locus (plural: loci)
  • 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 occupy the same position (locus) on the chromosome
• • A 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-letter, or triplet, code
• • Each sequence of three bases (in other words each triplet of bases) codes for one amino acid
• The triplets of bases are known as codons (each codon codes for a different amino acid – there are 20 different amino acids that cells use to make up different proteins)
• Triplets of bases

  Code for start (TAC - methionine) and stop signals
• There are four bases so there are 64 different triplets possible (43), yet there are only 20 amino acids that commonly occur in biological proteins
• Degenerate code

  Multiple codons coding for the same amino acids, can limit the effect of mutations
• The genetic code is universal, meaning that almost every organism uses the same code (there are a few rare and minor exceptions)
• The same codons code for the same amino acids in all living things (meaning that genetic information is transferable between species)
• • Some triplets of bases code for start (TAC – methionine) and stop signals
  • These signals tell the cell where individual genes start and stop
  • This ensures the cell reads the DNA correctly (the code is non-overlapping) and can produce the correct sequences of amino acids (and therefore the correct protein molecules) that it requires to function properly
• The genome within eukaryotic cells contains many non-coding sections of DNA
• Non-coding DNA


  Does not code for any amino acids
• Locations of non-coding DNA

  • Between genes, as non-coding multiple repeats
  • Within genes, as introns
• Non-coding DNA contains the same base sequences repeated multiple times
• Transcription in eukaryotic cells

  1. Transcribe the whole gene (all introns and exons) to produce pre-mRNA molecules

  2. Remove the non-coding sections (introns)

  3. Join the coding sections (exons) together in a process called splicing
• The coding exons can be separated by one or more introns