Senior IB Bio Review by HC120617161223


									       IB Biology Review
Chapter 17: DNA Transcription and Translation
         What is Transcription?

The synthesis of mRNA from a DNA template
When the DNA nucleotides are copied into segments
of RNA
Transcription occurs when a protein is needed by the

The “blueprint” for the protein is copied from DNA
                Transcription Vocab.
• Template Strand/Anti-sense Strand/Non-Coding Strand. The DNA
  strand that is a template for mRNA complementary base pairing during
• Non-template Strand/Sense Strand/Coding Strand: The DNA strand
  that is complementary to the template DNA strand during
• Promoter Region: The region of DNA (a specific sequence of
  nucleotides) to which RNA polymerase binds to start transcription. (it
  signals the RNA polymerase where to begin translation along the DNA
• Terminator Region: The region of DNA that signals RNA polymerase
  to end transcription
• RNA processing: Removing the introns in pre-mRNA to produce
  mature mRNA that is ready for translation
• DNA Triplet: Three (3) DNA nucleotides such as ATA or GAC
• mRNA Codon: Three mRNA nucleotides such as UAU or CUG that
  were transcribed from the DNA triplet. Codons form the basis of
          What is Translation?

The synthesis of a polypeptide from the genetic
information on a mRNA molecule when it couples
with a ribosome. It occurs in 4 steps
When the RNA segment tells the ribosome which
amino acids to use to build a protein
Translation turns the DNA code into a protein

RNA is the “blueprint” for the protein
The ribosome follows the RNA’s directions
                         Translation Vocab.
•   tRNA Anticodon: The molecule that transfers/transports specific amino acids to the ribosome
    where polypeptides
•   Polypeptides: These are the precursors of proteins and are the product of Translation. They
    contain all the amino acids in the proper sequence, but the amino acid chain has not yet
    folded up into its final shape. After folding, carbohydrates are covalently bonded to the folded
    proteins in the Rough ER . Then, the molecule become a full-fledged protein and is sent to
    the Golgi Apparatus.
•   Golgi Apparatus: This is where the proteins produced in the Rough ER are sorted and
    shipped mostly as secretory proteins used outside the cell such as insulin.
•   Ribosomes: This organelle is the site of protein synthesis
•   Bound ribosomes: Ribosomes attached to the walls of the rough ER. Most extra-cellular
    proteins which will be exported from the cell by exocytosis are synthesized on these
•   Free ribsomes: Ribosomes that are floating free in the cytoplasm. Most proteins used within
    the cell (such as the transport proteins in cell membrane) are synthesized on these ribosomes
•   Polyribosomes: An aggregation of several ribosomes attached to one mRNA molecule. (see
    Figure 17.22). Polyribosomes speed up the translation of a protein.
•   Point Mutations: A change in only one base pair of DNA in a gene
•   Substitutions: The replacement of one nucleotide and its complementary nucleotide with
    another pair of nucleotides. Sometimes these types of mutations are silent mutations as when
    GAA mutates to GAG (they both code for glutamic acid. Others, such as the Sickle Cell
    Anemia Case Study described below, cause a change in one of the amino acids in a
      What is the difference between
       DNA and RNA nucleotides?
DNA nucleotides are

RNA nucleotides are
What are the steps of transcription?
1. RNA polymerase unwinds DNA helix
2. Transcription starts at promoter region

3. RNA polymerase starts
adding free RNA nucleosides
to template strand of DNA
Transcription goes in a
5’ to 3’ direction
What are the steps of transcription? Cont.

 4. RNA polymerase continues to add nucleosides

 Nucleosides lose two phosphates as they bind to each other
 Become nucleotides
 5. RNA polymerase reaches terminator region
 6. RNA polymerase detaches from template strand
 7. RNA strand, now called mRNA, is released from DNA
 8. DNA double helix rewinds
     The Three Steps of Transcription
1. Initiation
  RNA polymerase binds to helix
  RNA polymerase starts to add RNA
2. Elongation
  RNA polymerase continues to add
3. Termination
  Transcription finishes
  RNA polymerase reaches
  terminator region
  mRNA breaks off
  RNA polymerase detaches from
  Helix rewinds
  What are the steps of translation?
1. mRNA binds to a ribosome
  Small sub-unit first
  Then large sub-unit        Anticodon

mRNA translated in what
  5’ to 3’, like always

2. The first transfer RNA
(tRNA) binds its anticodon               Codon
to the start codon of the mRNA
The start codon is always AUG
        Codons and Anticodons
Codon is complementary to the DNA strand
Anticodon has the same sequence of nucleotides as
DNA (except thymine is uracil)

64 different codons code for 20 different amino acids
That means there are 64 anticodons that are attached
to 20 different amino acids
Codons and Anticodons Sample Problem

 The template strand of DNA is

 What will the mRNA (codon) sequence be?

 What will the tRNA (anticodon) sequence be?

 tRNA sequence is the same as the template strand,
 only with U in place of T
 HL Only: tRNA Activating Enzymes

each amino acid has a
specific tRNA-activating
  called aminoacyl-tRNA
this enzyme binds the
specific/correct amino acid
to the correct tRNA
  uses ATP for energy
   HL Only: Ribosome Binding Sites

 P site holds the tRNA
carrying the growing
polypeptide chain
A site holds the tRNA
carrying the next
amino acid to be
added to the chain
Discharged tRNAs
leave the ribosome
from the E site
HL Only: DNA Sense and Antisense Strands

DNA sense strand / coding strand has the same base
sequence as is as
  Except uracil replaces thymine

DNA antisense strand / template strand has the same
base sequence as
  Strand of DNA that is being transcribed (copied)
What are the steps of translation? cont.
3. Second tRNA binds to
ribosome and mRNA
Each tRNA brings a specific
amino acid with it
4. The large subunit moves
the mRNA strand down after
the second tRNA binds
5. Amino acid on the first
tRNA is bound to the second
tRNA by a peptide bond
Process continues until
ribosome reaches stop codon
6. Then polypeptide (protein)
is released
           Genetic Code Table
Be familiar with the genetic code table
Be able to use this to translate a segment of mRNA
into a protein
   Using the Genetic Code Table
The letters refer to the mRNA sequence
What is the amino acid sequence if the mRNA
sequence is     AUG-UCU-GCC-UUA-UCC-UAC
Sequence is
HL Only: Free and Bound Ribosomes

Free ribosomes
  floating in the cytoplasm
  produce proteins that stay inside the cell (intracellular

Bound ribosomes
  on the rough endoplasmic reticulum
  produce proteins that go outside the cell (extracellular
               mRNA Processing

  mRNA must be “processed” to remove what?

What are introns?
    Non-coding segments of DNA

Why would introns have to be taken out?
    You wouldn’t want a protein made from them because
    they don’t do anything!
IB Exam Question
1. A certain gene codes for a polypeptide that is 120 amino acids
long. Approximately how many nucleotides long is the mRNA
that codes for this polypeptide likely to be?           (1 mark)

            A.     30
            B.     40
            C.     360
            D.     480

        Correct answer: C
        Each amino acid has three nucleotides
          120 x 3 = 360
IB Exam Question
  2. Distinguish between the structure of DNA
  and RNA.                              (3 marks)
      (normally) single stranded
      no double helix

      double stranded
      double helix
IB Exam Question
3. Explain the process of DNA Transcription including the role of the
promoter region, RNA polymerase, and the terminator and 5’ to 3’
direction (7)

• Transcription is the synthesis of mRNA from a DNA template
• It occurs in the 5’ to 3’ direction
• It occurs in 3 stages: Initiation, Elongation, and Terminations
• In Inititiation, RNA polymerase binds to the promoter region
• RNA polymerase unwinds the DNA and begins adding
  complementary RNA nucleotides to the template strand of DNA
• In Elongation, the mRNA molecule lengthens
• In termination, the RNA polymerase reaches the terminator
  region and releases the mRNA molecule
IB Exam Question
          4. Explain the process of translation.
                                                                        (15 marks)
•   Translation consists of initiation, elongation, translocation and termination;
•   the binding of a ribosome to mRNA is called initiation;
•   the small ribosomal sub-unit attaches to mRNA, followed by the large subunit;
•   next, the first tRNA binds it’s anticodon to the start codon (AUG) of mRNA
•   This occurs at the A site of the large ribosomal subunit
•   each tRNA has an anticodon and the corresponding amino acid attached;
•   this is accomplished by a tRNA activating enzyme which links the correct amino acid
    to each tRNA;
•   next, the first tRNA moves to the P site on the ribosome and a second tRNA binds to
    A site
•   This is called translocation, the tRNA moves from the A site to the P Site after it
    delivers its “payload” amino acid
•   The amino acid on first tRNA is bonded to amino acid on second tRNA;
•   this forms a peptide bond between the amino acids
•   the mRNA is translated in the 5' to 3' direction;
•   and the polypeptide lengthens (called elongation)
•   this process continues until the ribosome reaches a stop codon (called termination)
•   at this point, the polypeptide is released;
IB Exam Question
5. Compare DNA transcription with translation.
                                          (5 marks)

• both in 5' to 3' direction;
• both require ATP;
• DNA is transcribed and mRNA is translated;
• transcription produces RNA and translation
  produces polypeptides;
• transcription occurs in the nucleus (of
  eukaryotes) and translation occurs in the
  cytoplasm at the rough ER;
• tRNA is needed for translation but not
IB Exam Question
 6. What is removed to form mature eukaryotic
  mRNA?                               (1 mark)

   A.      RNA primers
   B.      Exons
   C.      RNA polymerases
   D.      Introns

      Correct answer: D
IB Exam Question
  7. Discuss the relationship between one gene
  and one polypeptide.                  (1 mark)

• This means that each gene codes for one unique
 8. Describe the consequence of a base substitution
     mutation with regards to sickle cell anemia.
                                             (Total 7
• A mutation is a change in the DNA sequence;
• This “mistake” often occurs during DNA
• This changes the mRNA during transcription;
• Which, in turn, can change the amino acid
• A substitution mutation leads to a change to one
• In sickle cell anemia, glutamic acid is changed to
  valine / GAG to GTG;
• This changes the shape of hemoglobin
9. Describe the Genetic Code.

(6 marks)
• composed of mRNA base triplets;
• These are called codons;
• each codes for the addition of an amino acid to a
  growing polypeptide chain;
• the genetic code is degenerate;
• this means that more than one codon can code for
  a particular amino acid;
• In other words, there are 64 different codons that
  code for 20 amino acids
• the genetic code is universal;
• this means it is the same in almost all organisms;

To top