Transcription and Translation by JI4c4Mm


									          Unit 4
Biological Information Flow
Information Flow
    REPLICATION - The Players
• Leading strand DNA
• Lagging strand DNA
• DNA Polymerase III
• Helicase
• RNA Primase and
  RNA Primers
• Okazaki Fragments
• Ligase
    REPLICATION (DNA Synthesis)

• DNA Helicase partially unwinds the
  double helix at an area known as the
  replication fork.
• As the two DNA strands separate and the
  bases are exposed, the enzyme DNA
  polymerase III moves into position at the
  point where synthesis will begin.
• Primase synthesizes RNA primers (why
  are there more on lagging strand?)
           REPLICATION cont.

• Leading strand is synthesized
  contiguously but lagging strand is
  synthesized discontiguously….WHY?
• After DNA is synthesized, another DNA
  Polymerase removes RNA primers and
  replaces them with DNA
• The gaps in sugar-phosphate backbone
  are then sealed with ligase resulting in 2
  exact copies of DNA! 
Replication cont.
           making mRNA from DNA
RNA = three different functions:
  • Ribosomal RNAs – (rRNA) makes us the
  ribosome. Each ribosome consists of about 60%
  ribosomal RNA and 40% protein.
  • Messenger RNAs – (mRNA) "record" information
  from DNA in the cell nucleus and carry it to the
  • Transfer RNAs- (tRNA) delivers amino acids one
  by one to protein chains growing at ribosomes
        TRANSCRIPTION cont.
•   Not all DNA is transcribed, only
    transcription units (genes).
•   Each transcription unit encodes an
    RNA transcript or family of transcripts.
•   These transcripts are
    –   processed
    –   transported to the cytoplasm
    –   translated (mRNA)

Note: mRNA is made in the NUCLEUS!
       Transcription Unit

(Left to right, 3'' to 5'' on copied [template] strand)
1. Carried out by an enzyme complex known as RNA
2. Faithfully copies one of two DNA strands
3. RNA polymerase binds to a special region (DNA
   sequence) called the promoter.
4. RNA polymerase unwinds about 1 turn of DNA (about
   10 base pairs) to produce a single stranded template.
   (hydrogen bonds of the DNA are broken by the
   unwinding process)
5. The first two nucleotides are hydrogen-bonded to DNA
   and are joined 5' to 3'
6. Continue to add new nucleotides to the 3' end.
               Transcription cont.
7. As synthesis proceeds, the RNA polymerase moves in
    the 3' to 5' direction on the copied strand.
8. When a specific terminator sequence is reached,
    transcription stops.
9. Enzymes and the single stranded RNA product are then
10. The orientation of the RNA is determined by the
    promoter, that is, by which strand contain the sequence
    that is recognized by RNA polymerase. This can be
    oriented in either direction.

NOTE: DNA is always read 3' to 5' so that the new RNA
  is made in the 5' to 3' direction.

Many RNA polymerase molecules may
process the same transcription unit at
           the same time.
   See animation of transcription
- Involves the transfer of biological
   information from the 4 nucleotide
   code of nucleic acids (A,C,G,T) into
   the 20 amino acid code of proteins

- Each 3 nucleotide coding unit that
   specifies an amino acid is called a
   codon. A codon is a group of three
   consecutive nucleotides required to
   specify a single amino acid
              (3 easy steps)
   - an mRNA, a ribosome, and the first tRNA
   molecule come together.
   - The tRNA containing MET will recognize the
   start signal and bind the “P” site of the
   ribosome complex.
          TRANSLATION cont.
  - The incoming tRNA binds the A site (the only
  tRNA able to attach is complementary to the
  codon of the A site on the mRNA.)
  - A peptide bond forms bewteen amino acids
  in A and P sites. (growing peptide now at A
  - The ribosome complex moves so the growing
  peptide is now in P site, tRNA from Met is in the
  Exit and released, a new tRNA containing
  another amino acid binds the A site.
        TRANSLATION cont.
 - A "stop" codon (UAA, UGA, or UAG) signals
 the end of the process.
 - An enzyme called the releasing factor binds
 the P site freeing the PROTEIN
 - The mRNA molecule is released from the
 ribosome complex which then falls apart
 - All mRNA messages are eventually degraded
 when the protein no longer needs to be made.

    Key Points to remember
• Transcription: DNA to RNA
  – DNA is read 3’ to 5’ while RNA is made 5’ to 3’
• Translation: making a polypeptide chain from
  – mRNA is read 5’ to 3’.

• Complementary base pairing. A with T, C with
  G. In RNA the T is replaced by Uracil (U).
       Practice Problem:
• Look at the following DNA template
  strand. Figure out the complementary
  mRNA sequence, the tRNA
  anticodons, and the appropriate
  amino acid sequence

 3’- A G C T T A C C G T G G - 5’

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