1. TRANSCRIPTION
AHL Topic 7.3
IB Biology
Miss Werba

2. AHL TOPIC 7 – NUCLEIC ACIDS AND PROTEINS
7.1 DNA STRUCTURE
7.2
DNA REPLICATION
7.3
TRANSCRIPTION
7.4
TRANSLATION
7.5 PROTEINS
7.6 ENZYMES
J WERBA – IB BIOLOGY 2

3. THINGS TO COVER Direction of transcription
Sense and antisense strands of DNA
Prokaryotic transcription:
promoter region, RNA polymerase, NTPs & the terminator
Eukaryotic transcription
J WERBA – IB BIOLOGY 3

4. DIRECTION OF TRANSCRIPTION Command term = STATE
7.3.1
DIRECTION OF TRANSCRIPTION Command term = STATE
Transcription is carried out in a 5’→ 3’ direction.
This means the 5’ end of the RNA strand
The 5’ end of the free RNA nucleotide is added to the 3’ end of the RNA molecule that is already synthesized.
Addition of nucleotides is catalysed by RNA polymerase.
J WERBA – IB BIOLOGY 4

5. SENSE and ANTISENSE STRANDS Command term = DISTINGUISH
7.3.2
SENSE and ANTISENSE STRANDS Command term = DISTINGUISH
Sense strand:
the coding strand of DNA - ie. it contains the genes
not transcribed!
Antisense strand:
also known as the template strand
complementary to the sense strand
transcribed into mRNA
Why?
By transcribing the antisense strand, the base sequence of the mRNA will be the same as the coding DNA (except U replace T)
J WERBA – IB BIOLOGY 5

6. PROKARYOTIC TRANSCRIPTION Command term = EXPLAIN
7.3.3
PROKARYOTIC TRANSCRIPTION Command term = EXPLAIN
Transcription in prokaryotes involves a number of elements:
A promoter region
Attachment of ribonucleoside triphosphates
RNA polymerase
A terminator region
J WERBA – IB BIOLOGY
Ref: IB Biology, Oxford Study Courses 6

7. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
1) The promoter region
The promoter region is found on the sense strand
Specific sequence of DNA bases that acts as a binding point for RNA polymerase
RNA polymerase unwinds small sections of DNA
J WERBA – IB BIOLOGY 7

8. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
2) Ribonucleoside triphosphates & RNA polymerase
RNA polymerase adds ribonucleoside triphosphates to the DNA antisense strand using complementary base pairing
NTPs are added to the 3’ end of the RNA strand.
This forms messenger RNA (mRNA)
J WERBA – IB BIOLOGY 8

9. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
3) The terminator region
The terminator region is also found on the sense strand
Specific sequence of DNA bases that acts as an end point for transcription
RNA polymerase breaks free and the mRNA strand is released
J WERBA – IB BIOLOGY 9

11. EUKARYOTIC TRANSCRIPTION Command term = STATE
7.3.4
EUKARYOTIC TRANSCRIPTION Command term = STATE
Eukaryotic RNA needs the removal of introns to form mature mRNA.
Eukaryotic pre-mRNA contains exons and introns
Remember: exons stay in, introns move out
Introns are transcribed but not translated
Introns need to be removed before translation using splicing enzymes
This will form mature mRNA, ready for translation.
J WERBA – IB BIOLOGY 11

12. Sample questions
Q1. Where does the RNA polymerase bind during the process of transcription?
The polysome
The operator
The promoter
The initiator
J WERBA – IB BIOLOGY 12

13. Sample questions
Q2. The sequence of nucleotides in a section of RNA is: GCCAUACGAUCG
What is the base sequence of the DNA sense strand?
CGGUAUGCUAGC
GCCATACGATCG
CGGTATGCTAGC
GCCAUACGAUCG
J WERBA – IB BIOLOGY 13

14. Sample questions
Q3. Explain the process of transcription leading to the formation of mRNA. [8]
J WERBA – IB BIOLOGY 14

15. Sample questions
A1. C
A2. B
J WERBA – IB BIOLOGY 15

16. Sample questions RNA polymerase; (polymerase number is not required)
binds to a promoter on the DNA;
unwinding the DNA strands;
binding nucleoside triphosphates;
to the antisense strand of DNA;
as it moves along in a 5′→3′ direction;
using complementary pairing/A-U and C-G;
losing two phosphates to gain the required energy;
until a terminator signal is reached (in prokaryotes);
RNA detaches from the template and DNA rewinds;
RNA polymerase detaches from the DNA;
many RNA polymerases can follow each other;
introns have to be removed in eukaryotes to form mature mRNA;
8 max
J WERBA – IB BIOLOGY 16