TRANSCRIPTION AHL Topic 7.3 IB Biology Miss Werba

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

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

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

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

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

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

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

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

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

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

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

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

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

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