1. Transcription
Ms. Day AP Biology

2. The Flow of Genetic Information
DNA
information in a specific sequence (order) of nucleotides along 2 DNA strands
Leads to specific traits by controlling the synthesis of proteins
Gene expression includes two stages
Transcription: DNA  RNA
“transcribe” = to copy into another form
Translation: RNA  polypeptide
“translate” = to change into another language

4. What are the characteristics of RNA?
Ribose Nucleic Acid
Single stranded
Pentose sugar = ribose
Backbone= alternating ribose sugar/phosphates held together by PHOSPHODIESTER BONDS
Made of RNA nucleotides
Contains bases:
(A) Adenine
(G)Guanine
(C) Cytosine
(U) Uracil (replaces Thymine)

6. Brings message from DNA to ribosome to make protein
Type of RNA
Function Job
Picture
mRNA
(messenger RNA)
Brings message from DNA to ribosome to make protein
tRNA
(transfer RNA)
Transfers/moves amino acids to ribosomes
rRNA
(ribosomal RNA)
Makes up ribosomes along with proteins
snRNA
(small nuclear RNA)
Helps makes splicesomes  modifies mRNA
**ONLY in eukaryotes

7. Basic Principles of Transcription and Translation
Makes RNA from DNA
Produces messenger RNA (mRNA)
Translation (happens later)
actual synthesis of a polypeptide
mRNA  polypeptide  protein
Occurs in ribosomes

8. In prokaryotes, transcription and translation occur together
Prokaryotic cell. In a cell lacking a nucleus, mRNA produced by transcription is immediately translated without additional processing.
TRANSLATION
TRANSCRIPTION
DNA
mRNA
Ribosome
Polypeptide

9. “Transcript” is a fancy word for “message”
In eukaryotes, pre mRNA transcripts are modified (changed) before becoming true “mature” mRNA
Eukaryotic cell. The nucleus provides a
Separate compartment for transcription.
The original RNA transcript, called
pre-mRNA, is processed in various
ways before leaving the nucleus as mRNA.
(b)
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
mRNA
DNA
Pre-mRNA
Polypeptide
Ribosome
Nuclear
envelope
“Transcript” is a fancy word for “message”

10. Transcription: DNA  mRNA
RNA polymerase = enzyme used
breaks H bonds btw bases and hooks together RNA nucleotides
Follows RNA base pairing rules
A = U (T on DNA = A in RNA)
C = G

11. Transcription makes a coded mRNA message…
mRNA is created by RNA polymerase
mRNA contains codons
3 letter mRNA “word” = codon
1 codon = 1 amino acid in protein
There are 4 different RNA “letters” that can be used in codons
A, U, C, and G

12. Codons: Triplets of Bases
mRNA message is coded as a sequence of nonoverlapping codons
FOUND ONLY ON mRNA
Codons must be read in the correct order
Always read in 5’  3’ direction
**Need to use the Genetic Code table to then convert the codon to amino acids

13. THE
GENETIC CODE

14. During transcription, a gene determines the sequence of bases along length of mRNA.
Figure 17.4
DNA
molecule
Gene 1
Gene 2
Gene 3
DNA strand
(template)
TRANSCRIPTION
mRNA
Protein
TRANSLATION
Amino acid A C G T U
Trp
Phe
Gly
Ser
Codon 3 5

15. Making an mRNA Transcript
Initiation
DNA strands unwind
RNA polymerase initiates mRNA synthesis at start point on templates called promoters
RNA polymerase binds to promoter
Elongation
RNA polymerase makes mRNA in 5  3 direction
In wake of transcription, DNA strands re-form a double helix.
Termination
mRNA transcript is released at terminator signal
RNA polymerase detaches from the DNA

17. Promoters and Transcription
Promoters (on DNA)
RNA polymerase binds here to make mRNA (BOTH prokaryotes & eukaryotes)
“TATA box” = start signal on DNA promoter
Determines which strand is used as template  only 1 side is used at a time!

18. Transcription Animation

19. Elongation of the RNA Strand
RNA polymerase moves along DNA
continues to untwist helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA nucleotides

20. Termination of Transcription
Different in prokaryotes and eukaryotes
Prokaryotes
RNA polymerase falls off DNA when it hits “termination signal”  transcription ends & transcript is released

21. Termination of Transcription (con’t)
Eukaryotes
RNA polymerase transcribes until it hits a polyadenylation signal (TTATTT)
Makes “AAUAAA” on mRNA
Then it falls off mRNA!!!

22. Pre-mRNA Modification: pre-mRNA  mature mRNA
ONLY in eukaryotic cells
Occurs AFTER 1st mRNA made  called pre-mRNA
Pre-mRNA  made into mRNA
2 things have to happen…
NOT IN PROKARYOTES

23. 1. Alteration of mRNA Ends
Each end of pre-mRNA is modified
5 end gets a modified nucleotide cap
The 3 end gets a poly-A tail
GTP cap added to the 5 end
LOTS of adenine nucleotides (~200)
added to the 3 end
Protein-coding
segment
Polyadenylation
signal
Poly-A tail
3 UTR
Stop codon
Start codon
5 Cap
5 UTR
AAUAAA
AAA…AAA
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide G P 5 3
Figure 17.9

24. Function of 3’ tail (Poly A cap):
The function of 5’ cap (GTP) is:
helps attach to the ribosome
prevent mRNA degredation
Function of 3’ tail (Poly A cap):
helps export of mRNA from nucleus

25. 2. Split Genes and RNA Splicing
RNA splicing and RNA Modification
Removes introns and joins exons
Introns = non-coding regions (stay “in”side nucleus)
Exons = coding regions that EXIT nucleus
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
5 Cap
Exon
Intron 5 3
Poly-A tail
Introns cut out and
exons spliced together
Coding
segment 1
146
3 UTR
5 UTR
Intron
Pre-mRNA
Exon
Exon
Mature
mRNA
Figure 17.10

26. Called small nuclear RNA + proteins (ribonucleoproteins)
RNA splicing is carried out by spliceosomes
RNA transcript (pre-mRNA)
Exon 1
Intron
Exon 2
Other proteins
Protein
snRNA
snRNPs
Spliceosome
components
Cut-out
intron
Mature mRNA 5 1 2 3
Called small nuclear RNA + proteins (ribonucleoproteins)

27. RNA Splicing Animation

28. The Functional and Evolutionary Importance of Introns
Presence of introns allows for alternative RNA splicing
REASON HUMANS HAVE SO FEW GENES??
MORE TERRAIN FOR CROSSING OVER
Exon Shuffling
Proteins often have shapes consisting of discrete functional regions called domains
In many cases…
Different exons code for the different domains in a protein

29. Gene Transcription RNA processing Translation Polypeptide
DNA
Exon 1
Intron
Exon 2
Exon 3
Transcription
RNA processing
Translation
Domain 3
Domain 1
Domain 2
Polypeptide

30. RNA Splicing Animations #2