Gene Expression

Gene Expression

PROTEINS
1. Importance of Proteins
  1. all enzymes are proteins
  2. chemical messengers of the body
  3. make up important structures of the body
    1. e.g. hair, muslce
  4. carry O2 around the body
    1. i.e. haemoglobin
  5. fight disease
    1. e.g. antibodies antioxins
  6. make up important parts of the cell membrane
2. Structure of Proteins
  1. main elements making up amino acids:
    1. carbon, hydrogen, oxygen and nitrogen
  2. amino acids join together in a long chain forming a protein
    1. there are 20 diff amino acids
      1. held together by peptide bonds
        many amino acids joined together = polypeptide chain
  3. proteins are polymers
    1. i.e. large molecule made of many repeating units
  4. can be either:
    1. long and stringy
      1. fibrous proteins
    2. folded into almost circular balls or globs
      1. globular proteins
  5. divided into 4 diff stages
    1. primary structure:
      1. amino acids joined to form polypeptide strands
      2. secondary structure:
        polypeptide strands form alpha helixes or Beta pleated sheets
        Tertiary structure
        polypeptides fold, forming specific shapes
        Quarternary structure
        two or more folded polypeptides assemble to form larger protein molecules
3. a functional unit made of 1 or more polypeptide chains
4. produced from instructions of genes
5. gene: length of DNA that codes for a ppolypeptide
PROTEINSYNTHESIS
1. RNA
  1. Ribonucleic acid
  2. mRNA
    1. messenger RNA
    2. carries genetic info from DNA to ribosomes so that photosynthesis can occur
    3. long single stranded chain (molecule) made in nucleus
    4. long string of nucleotides grouped in sets of 3 bases called codons
      1. each codon codes fora specific amino aid
  3. tRNA
    1. picks up amino acids from cytoplasm and carries it to ribosomes
      1. amino acids are joined together to make a polypeptide chain at ribosomes
    2. clover leaf shaped molecule
      1. has a 3 base anticodon
        complementary to codon on mRNA
        and an amino acid attachment site at other end
    3. each tRNA molecule is able to pick up only 1 type of amino acid
  4. rRNA
    1. makes up part of the structure of the ribosomes
    2. holds together the mRNA and tRNA so that peptide bond b/w amino acids can be formed
    3. rRNA made in nucleolus of the cell
2. Transcription
  1. a section of DNA unwinds and mRNA strand is produced using the template strand
    1. using the base-pairing rule except Thymine is replaced by Uracil
      1. When RNA polymerase reaches the terminator sequence, transcription is completed
        mRNA strand detaches and 2 DNA strands re-join and helix reforms
  2. where DNA's code is transcribed onto the mRNA in the nucleus
3. Translation
  1. mRNA molecule latches onto a ribosome and protein synthesis is initiated by a start codon
    1. through complementary base pairing, tRNA attaches anticodons on the codons
      1. as amino acids on the other end of the tRNA bind together (by peptide bonds) forming a polypeptide chain
        as more amino acids are added, the next tRNA falls into place.
        the tRNA molecule separates and a functioning protein is formed when a stop codon is reached on the DNA
        these polypeptide chains fold into 3D shapes to function
      2. each tRNA molecule is able to pick up only 1 type of amino acid
THE GENETIC CODE
1. living things use 20 amino acids to produce proteins
  1. combination of amino acids is controlled by 4 diff bases
    1. Adenine, Thymine, Guanine, Cytosine
2. as there are 20 amino acids, each must be coded for by 3 nucleotides
  1. this is the triplet code on DNA and has a corresponding 3 nucleotides on mRNA called codons
3. there are 64 different codons and only 20 amino acids, most amino acids are coded for by more than 2 codon
  1. This redundancy in the code is called degeneracy.
    1. it buffers the effect of mutations
      1. if only 1 base changes, in the 3rd position,
        it doesnt necessarily change the amino acid picked up, the same amino acid in coded for
4. genetic code is universal
  1. the same codons code for the same amino acids in plants, fungi, bacteria and viruses
5. 1 codon codes for amino acid methionine
  1. (AUG) start signal
6. termination/stop codons
  1. UAA
  2. UAG
  3. UGA
MUTATION
1. permanent change in the DNA
2. mutagenic agents: something that causes a mutation
  1. can be chemicals, or forms of energy
3. Point mutations
  1. small changes in the base sequences.
  2. can happen in DNA replication just before meiosis or by mistakes made during crossing over
  3. affect only a single gene
    1. alters one nucleotide
  4. can be:
    1. neutral
      1. have no effect on functioning of polypeptide chain produced
        due to degeneracy of the code
        change in a single base usually in the third position of triplet
        may result in same amino acid being coded for
        if a new amino acid is coded for at a position of polypeptide chain
        that does not form part of the active site of the enzyme
        if shape of active site is unchanged, enzyme is likely to function
    2. beneficial
      1. change of an amino acid may alter shape of protein allowing it to function better than before
    3. harmful
      1. if the change in shape of polypeptide chain changes active site
        may decrease the functioning of enzyme or stop it working at all
        may introduce a stop codon and prematurely stop the formation of polypeptide chain
        result in necessary protein not being formed
        as a result, a function critical to life may not occur
  5. Types of point mutations
    1. Deletion
      1. one base is removed from the DNA sequence
        causes a reading frame shift
        i,e. every amino acid coded for after this mutation will be different
      2. usually produce a non-functioning protein
        harmful to the organism

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	Created by CChristine about 10 years ago