IB Biology Review Chapter 17: DNA Transcription and

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IB Biology Review

• Chapter 17 DNA Transcription and Translation

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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 body
• The blueprint for the protein is copied from
  DNA

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Transcription Vocab.

• Template Strand/Anti-sense Strand/Non-Coding
  Strand. The DNA strand that is a template for
  mRNA complementary base pairing during
  transcription
• Non-template Strand/Sense Strand/Coding Strand
  The DNA strand that is complementary to the
  template DNA strand during transcription 
• 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 molecule)
• 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 protein
  translation because they code for specific amino
  acids that form proteins

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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 RNAs directions

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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 ribosomes
• 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 polypeptide
• Insertions and Deletions. These types of
  mutations lead to frameshift mutations and always
  produce proteins that are non-functional

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What is the difference between DNA and RNA
nucleotides?

• DNA nucleotides are
• Adenine
• Thymine
• Guanine
• Cytosine
• RNA nucleotides are
• Adeneine
• Uracil
• Guanine
• Cytosine

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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

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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

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The Three Steps of Transcription

• 1. Initiation
• RNA polymerase binds to helix
• RNA polymerase starts to add RNA nucleotides
• 2. Elongation
• RNA polymerase continues to add nucleotides
• 3. Termination
• Transcription finishes
• RNA polymerase reaches terminator region
• mRNA breaks off
• RNA polymerase detaches from helix
• Helix rewinds

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What are the steps of translation?

• 1. mRNA binds to a ribosome
• Small sub-unit first
• Then large sub-unit
• mRNA translated in whatdirection?
• 5 to 3, like always
• 2. The first transfer RNA (tRNA) binds its
  anticodon to the start codon of the mRNA
• The start codon is always AUG

tRNA
Anticodon
mRNA
Codon
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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

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Codons and Anticodons Sample Problem

• The template strand of DNA is
• TAC-TTG-CTA-CAG-ATT
• What will the mRNA (codon) sequence be?
• AUG-AAC-GAU-GUC-UAA
• What will the tRNA (anticodon) sequence be?
• UAC-UUG-CUA-CAG-AUU
• tRNA sequence is the same as the template strand,
  only with U in place of T

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HL Only tRNA Activating Enzymes

• each amino acid has a specific tRNA-activating
  enzyme
• called aminoacyl-tRNA synthetase
• this enzyme binds the specific/correct amino acid
  to the correct tRNA
• uses ATP for energy

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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

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HL Only DNA Sense and Antisense Strands

• DNA sense strand / coding strand has the same
  base sequence as is as
• mRNA
• Except uracil replaces thymine
• DNA antisense strand / template strand has the
  same base sequence as
• Strand of DNA that is being transcribed (copied)

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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

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Genetic Code Table

• Be familiar with the genetic code table
• Be able to use this to translate a segment of
  mRNA into a protein

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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
• Start/Met.
• Serine
• Alanine
• Leucine
• Serine
• Tyrosine

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HL Only Free and Bound Ribosomes

• Free ribosomes
• floating in the cytoplasm
• produce proteins that stay inside the cell
  (intracellular proteins)
• Bound ribosomes
• on the rough endoplasmic reticulum
• produce proteins that go outside the cell
  (extracellular proteins)

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mRNA Processing

• mRNA must be processed to remove what?
• Introns
• What are introns?
• Non-coding segments of DNA
• Why would introns have to be taken out?
• You wouldnt want a protein made from them
  because they dont do anything!

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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)
IB Exam Question

• A. 30
• B. 40
• C. 360
• D. 480
• Correct answer C
• Each amino acid has three nucleotides
• 120 x 3 360

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2. Distinguish between the structure of DNA and
RNA.
(3 marks)
IB Exam Question

• RNA
• ribose
• (normally) single stranded
• uracil
• no double helix
• DNA
• deoxyribose
• double stranded
• thymine
• double helix

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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)
IB Exam Question

• 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

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4. Explain the process of translation.

(15 marks)
IB Exam Question

• 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 its 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

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5. Compare DNA transcription with translation.(5
marks)
IB Exam Question

• 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
  transcription

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6. What is removed to form mature eukaryotic
mRNA?
(1 mark)
IB Exam Question

• A. RNA primers
• B. Exons
• C. RNA polymerases
• D. Introns
• Correct answer D

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

• This means that each gene codes for one unique
  polypeptide

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8. Describe the consequence of a base
substitution mutation with regards to sickle cell
anemia. (Total 7 marks)

• A mutation is a change in the DNA sequence
• This mistake often occurs during DNA
  replication
• This changes the mRNA during transcription
• Which, in turn, can change the amino acid
  sequence
• A substitution mutation leads to a change to one
  codon
• In sickle cell anemia, glutamic acid is changed
  to valine / GAG to GTG
• This changes the shape of hemoglobin
• The resulting hemoglobin cannot carry oxygen as
  well

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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