Cellular Control F215

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  • Cellular Control
    • The Genetic Code
      • Genes are lengths of DNA that code for the structure of polypeptides.
      • Polypeptides may be proteins or code for part of a protein.
        • The proteins include structural proteins and enzymes.
      • A sequence of organic bases in DNA, the bases are read in triplets
        • Each triplet codes for a specific amino acid
        • The number of triplets determines the number of amino acids in the polypeptide
    • Protein Synthesis
      • 1st Transcription
        • 1. Transcription is the conversion of the genetic code to a sequence of nucelotidesin messenger RNA (mRNA)
          • 2. The DNA molecule is too large to leave the nucleus so smaller mRNA is made.
            • 3. The DNA molecule is unwound and split by the action of enzyme RNA polymerase.
              • 4. RNA nucleotides form a molecule complimentary to the template strand of the DNA molecule. The rules of base pairing are followed.
      • 2nd Translation
        • 1. The mRNA attaches itself to a ribosome and transfer RNA (tRNA) molecules carry amino acids to the ribosome.
          • 2. A tRNA molecule, with an anticodon that's complimentary to the first codon on the mRNA, attaches itself to the mRNA by complimentary base pairing.
            • 3. A second tRNA molecule attaches itself to the next codon on the mRNA in the same way.
              • 4. The two amino acids attached to the tRNA molecules are joined by a peptide bond. The first tRNA molecule moves away leaving the amino acid behind.
                • 5. A third tRNA molecule binds to the next codon on the mRNA. Its amino acid binds to the first two and the second tRNA molecule moves away.
                  • 6. The process continues producing a chain of linked amino acids until a stop codon is reached. The polypeptide chain then moves away.
    • Mutations
      • Any change to the base (nucleotide) sequence of the DNA is called a mutation
      • Substitution - one base is swapped for another one
      • Deletion - one base is removed
        • Cause a frameshift
          • Insertion - one base is added
      • Insertion - one base is added
      • Duplication - one or more bases are repeated
      • Inversion - a sequence of bases is reversed
      • Neutral Mutations
        • The mutation is in a non-coding region of DNA
        • Silent mutation - although the base triplet has changed, it still codes for the same amino acid
      • Beneficial Mutations
        • A mutation may alter a polypeptide such that it works more effectively.
    • The Lac Operon
      • A functional unit of genes in the genome of prokaryotic cells found in the bacterium E. coli.
      • Contains two genes which code for the structure of proteins and a genetic control mechanism to enable genes to be switched on and off.
      • The bacterium is not able to digest the sugar lactose, this is because it does not make the enzyme B-galactosidase. When lactose is available the gene is switched on and can produce B-glacatosidase.
        • Lactose Absent
          • 1. The regulator gene is expressed and the repressor protein is synthesised. It has two binding sites, one that binds to lactose and one that binds to the operator region.
            • 2. The repressor protein binds to the operator region. This covers where RNA polymerase normally attaches.
              • 3. RNA polymerase can't bind to the promoter region so the structural genes can't be transcribed into mRNA.
                • 4. Without mRNA these genes can't be translated and the enzymes B-galactosidase  and lactose permease can't be synthesised.
        • Lactose Present
          • 1. Lactose binds to the repressor protein.
            • 2. The repressor protein can no longer bind to the operator region, but RNA polymerase can bind to the promoter region and start transcription.
              • 3. Structural genes Z and Y are now expressed into the lac enzymes, B-galactosidase and lactose permease.
                • 4. The E. coli bacteria can use the lactose permease enzyme to take up lactose into their cells. The lactose can then be converted to glucose and galactose using the B-galactosidase enzyme. The sugars can then be used for respiration.
    • Homeobox Sequences
      • Control the body plan of an organism
        • This is achieved by controlling the differentiation of cells and parts of the body through switching genes on and off at appropriate times during development.
        • As homeobox genes are activated, they activate structural genes in a carefully coordinated sequence to ensure that features develop in the correct way.
      • A homeobox gene contains a sequence of 180 base pairs known as a homeobox sequence.
        • This sequence codes for a sequence of 60 amino acids, which is found in the polypeptide produced.
      • Homeobox genes are activated in a particular order that matches the order in which they are expressed from head to tail.
        • Arranged into groups called hox clusters.
          • The fruit fly (Drosophila) has two clusters.
            • Cluster A controls the development of the head and thorax.
            • Cluster B controls development of the thorax and abdomen.
      • Homeobox genes are similar in all plants, animals and fungi, this is because they have the same role in each case.
        • They code for transcription factors that need to bind to DNA.
    • Apoptosis
      • A series of carefully controlled biochemical events that leads to orderly cell death.
      • 1. Enzymes break down the cytoskeleton
        • 2. The cell shrinks, the organelles are packed together
          • 3. The cell surface membrane breaks up to form vesicles, containing the cell contents.
            • 4. The vesicles are taken up by phagocytes and digested
      • Important part of development
        • Separates fingers and toes

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