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DNA__RNA__and_Protein_Synthesis Powered By Docstoc
					             BELLRINGER #3
1. What does the abbreviation DNA stand for?
2. Distinguish between purines and
3. What was the significance of Wilkins and
   Franklin’s X-ray diffraction photographs
   regarding DNA structure?
                  BELLRINGER #3
1. What does the abbreviation DNA stand for?
   DNA stands for deoxyribonucleic acid
2. Distinguish between purines and pyrimidines.
   Purines are nitrogenous bases made of two rings of carbon and
        nitrogen atoms.
   Pyrimidines are nitrogenous bases made of a single ring of carbon
        and nitrogen atoms.
3. What was the significance of Wilkins and Franklin’s X-ray
    diffraction photographs regarding DNA structure?
   Their photographs suggested that the DNA molecule resembled a
        tightly coiled helix and was composed of two or three chains
        of nucleotides.
DNA Replication, Transcription,
      and Translation
• Summarize the process of DNA
• Describe how errors are corrected during
  DNA replication.
• Compare the structure of RNA with that
  of DNA.
• Summarize the process of transcription.
              DNA Replication
• In order for a cell to create to identical copies
  of itself, it must make a replicate of its DNA.

• DNA replication is the process by which DNA is
  copied before a cell divides
                DNA Replication
•    Replication occurs in three major steps:
    1. Enzymes called helicases break apart the
       hydrogen bonds between nitrogen bases
    2. Enzymes called DNA polymerases attach free-
       floating nucleotides to each of the original
    3. Polymerases fall off, resulting in two identical
       new strands.
             DNA Replication
• DNA replication is called semi-conservative
  replication because each new molecule is
  composed of one original strand.
             DNA Replication
• DNA replication occurs in different directions
  on each strand

• Replication begins and occurs at many points
  on a DNA strand. This allows the DNA to be
  replicated more quickly.
      Errors in DNA Replication
• DNA polymerases “proofread” DNA to check
  for accuracy.

• If a mistake goes unchecked, a change in
  nucleotide sequence called a mutation occurs

• Mutations can disrupt normal function and
  can lead to cancer.
            Protein Synthesis
• Genes are segments on a chromosome that
  code for a hereditary character (hair color)

• Genes direct the synthesis of proteins that
  affect the phenotype

• Genes use RNA to direct the synthesis of
  various proteins.
             Protein Synthesis
• Transcription:
  – RNA is made from a DNA template

• Translation:
  – RNA directs the assembly of proteins at the

           DNA  RNA  Proteins
                 RNA Structure
•    RNA differs from DNA:
    1. RNA contains the sugar ribose

    2. RNA contains the base uracil instead of thymine

    3. RNA is single stranded instead of double
                  RNA Structure
• Types of RNA
  – Messenger RNA (mRNA) – carries the instructions
    (message) from the genes

  – Ribosomal RNA (rRNA) – part of the structure of a
    ribosome (synthesis occurs at the ribosomes

  – Transfer RNA (tRNA) – transfers amino acids to the
    ribosome to make a protein.
• Genetic instructions from DNA are written in

• Takes place in the nucleus
•   Three Steps:
    1. RNA polymerase binds to a promoter
      •   A promoter is just a nucleotide sequence on DNA that
          initiates transcription
    2. RNA polymerase adds free, complimentary
    3. RNA polymerase reaches a termination (stop)
       signal and releases the DNA and new RNA strand
• The RNA made from transcription can be:
  – mRNA
  – tRNA
  – rRNA
            The Genetic Code
• The subunits of proteins are amino acids.

• Amino acids are assembled based on the
  “instructions” encoded in the mRNA
            The Genetic Code
• Three adjacent nucleotides in mRNA code for
  a specific amino acid.

• Each of the 3 adjacent nucleotides is called a

• Codons may also signify when protein
  synthesis stops or starts
            The Genetic Code
• No codon codes for more than 1 amino acid

• The genetic code is nearly universal
            The Genetic Code
• Codons for you to know:

  – AUG (methionine): start codon, indicates where
    translation begins

  – UAA, UAG, UGA: stop codons
   Translation (Protein Synthesis)
• Protein Structure
  – Made from 1 or more polypeptides
     • Chains of amino acids joined by peptide bonds
     • Only 20 amino acids
  – Consists of hundreds to thousands of amino acids
    arranged in specific sequences
     • Sequence determines the shape and function of the
• Translation is the “decoding” of the
  instructions from mRNA

• By “reading” the mRNA strand, the specific
  amino acid sequences are joined together.
             Steps of Translation
1. Two ribosomal units, tRNA, and mRNA join
  •   Amino acids are attached to one end of a tRNA
  •   An anticodon is located on the othe end of the
      •   Anticodons are complimentary to codons
  •   A tRNA carrying methionine pairs with the
      codon AUG on the mRNA
           Steps of Translation
• Since the star codon is AUG, the first amino
  acids in most proteins is methionine

• The methionine may be removed later
           Steps of Translation
2. Another tRNA pairs with the next codon
  •   The two amino acids are joined by a peptide
  •   The first tRNA is released from the ribosome
  •   The ribosome moves a distance of 1 codon
  •   Another tRNA moves in to bring its amino acid
          Steps of Translation
3. Polypetide chain continues to grow on
   amino acid at a time

4. Ribosome reaches “stop” codon and the
   amino acid chain falls off

5. Components of translation come apart
• Several ribosomes may translate the same
  mRNA at the same time

• Translation occurs only after transcription is
          The Human Genome
• Scientists now know the complete gene
  sequence of humans

• There are about 3.2 billion base pairs in our 23

• Now we must work to learn what information
  each of the 30,000 genes encode

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