Gene Expression

Gene Expression

1. PROTEINS

   • Importance of Proteins
     • all enzymes are proteins
     • chemical messengers of the body
     • make up important structures of the body
       • e.g. hair, muslce
     • carry O2 around the body
       • i.e. haemoglobin
     • fight disease
       • e.g. antibodies antioxins
     • make up important parts of the cell membrane
   • Structure of Proteins
     • main elements making up amino acids:
       • carbon, hydrogen, oxygen and nitrogen
     • amino acids join together in a long chain forming a protein
       • there are 20 diff amino acids
         • held together by peptide bonds
           • many amino acids joined together = polypeptide chain
     • proteins are polymers
       • i.e. large molecule made of many repeating units
     • can be either:
       • long and stringy
         • fibrous proteins
       • folded into almost circular balls or globs
         • globular proteins
     • divided into 4 diff stages
       • primary structure:
         • amino acids joined to form polypeptide strands
         • 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
   • a functional unit made of 1 or more polypeptide chains
   • produced from instructions of genes
   • gene: length of DNA that codes for a ppolypeptide

2. PROTEINSYNTHESIS

   • RNA
     • Ribonucleic acid
     • mRNA
       • messenger RNA
       • carries genetic info from DNA to ribosomes so that photosynthesis can occur
       • long single stranded chain (molecule) made in nucleus
       • long string of nucleotides grouped in sets of 3 bases called codons
         • each codon codes fora specific amino aid
     • tRNA
       • picks up amino acids from cytoplasm and carries it to ribosomes
         • amino acids are joined together to make a polypeptide chain at ribosomes
       • clover leaf shaped molecule
         • has a 3 base anticodon
           • complementary to codon on mRNA
           • and an amino acid attachment site at other end
       • each tRNA molecule is able to pick up only 1 type of amino acid
     • rRNA
       • makes up part of the structure of the ribosomes
       • holds together the mRNA and tRNA so that peptide bond b/w amino acids can be formed
       • rRNA made in nucleolus of the cell
   • Transcription
     • a section of DNA unwinds and mRNA strand is produced using the template strand
       • using the base-pairing rule except Thymine is replaced by Uracil
         • When RNA polymerase reaches the terminator sequence, transcription is completed
           • mRNA strand detaches and 2 DNA strands re-join and helix reforms
     • where DNA's code is transcribed onto the mRNA in the nucleus
   • Translation
     • mRNA molecule latches onto a ribosome and protein synthesis is initiated by a start codon
       • through complementary base pairing, tRNA attaches anticodons on the codons
         • 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
         • each tRNA molecule is able to pick up only 1 type of amino acid

3. THE GENETIC CODE

   • living things use 20 amino acids to produce proteins
     • combination of amino acids is controlled by 4 diff bases
       • Adenine, Thymine, Guanine, Cytosine
   • as there are 20 amino acids, each must be coded for by 3 nucleotides
     • this is the triplet code on DNA and has a corresponding 3 nucleotides on mRNA called codons
   • there are 64 different codons and only 20 amino acids, most amino acids are coded for by more than 2 codon
     • This redundancy in the code is called degeneracy.
       • it buffers the effect of mutations
         • 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
   • genetic code is universal
     • the same codons code for the same amino acids in plants, fungi, bacteria and viruses
   • 1 codon codes for amino acid methionine
     • (AUG) start signal
   • termination/stop codons
     • UAA
     • UAG
     • UGA

4. MUTATION

   • permanent change in the DNA
   • mutagenic agents: something that causes a mutation
     • can be chemicals, or forms of energy
   • Point mutations
     • small changes in the base sequences.
     • can happen in DNA replication just before meiosis or by mistakes made during crossing over
     • affect only a single gene
       • alters one nucleotide
     • can be:
       • neutral
         • 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
       • beneficial
         • change of an amino acid may alter shape of protein allowing it to function better than before
       • harmful
         • 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
     • Types of point mutations
       • Deletion
         • 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
         • usually produce a non-functioning protein
           • harmful to the organism