1. From Genes to Protein
Chapter 17

2. The Flow of Genetic Information
The information content of DNA is in the form of specific nucleotides along the DNA strands
Review: nucleotides-sugar, phosphate, nitrogenous base
DNA inherited by organisms leads to specific traits by dictating the synthesis of proteins
Protein synthesis occurs in two stages: transcription and translation

3. The Central Dogma
DNA mRNA Proteins
Transcription
Translation

4. The Genetic Code of Life
Genes Specify Enzymes
Beadle and Tatum:
Experiments on the fungus Neurospora crassa (bread mold)
Proposed that each gene specifies the synthesis of one enzyme
One gene-one enzyme hypothesis
Function of a gene is to dictate the production of a specific enzyme

5. Transcription RNA is a polymer of RNA nucleotides Types of RNA
RNA nucleotides contain the sugar ribose instead of deoxyribose
RNA nucleotides are of four types: uracil (U), adenine (A), cytosine (C), and guanine(G)
Uracil (U) replaces thymine (T) of DNA
Types of RNA
Messenger (mRNA) - Takes a message from DNA in the nucleus to ribosomes in the cytoplasm
Ribosomal (rRNA) - Makes up ribosomes, which read the message in mRNA
Transfer (tRNA) - Transfers the appropriate amino acid to the ribosomes for protein synthesis

6. Transcription Transcription is the process by which DNA makes RNA
Occurs in the nucleus
Contains 3 stages: initiation, elongation and termination

7. Transcription

8. Stage 1: Initiation
RNA polymerase recognizes and binds to DNA at th promoter region
Transcription factors (proteins) that bind with RNA polymerase
Recognizes TATA box

9. Stage 2: Elongation
RNA polymerase advances 3’5’ down template strand making pre-RNA in the 5’3’ direction
Antisense vs sense strands
RNA polymerase untwist the double helix and continues to add nucleotides following the base pairing rules
Transcription unit

10. Stage 3: Termination Final stage of transcription
RNA polymerase recognizes the termination sequence- AAUAAAA

11. RNA Processing
Pre-mRNA is modified before leaving the eukaryotic nucleus.
Modifications to the ends of the primary transcript:
Cap on the 5′ end
The cap is a modified guanine (G) nucleotide
Helps a ribosome determine where to attach when translation begins
Poly-A tail of adenines on the 3′ end
Facilitates the transport of mRNA out of the nucleus
Inhibits degradation of mRNA by hydrolytic enzymes
Helps ribosome attach to the RNA

12. RNA Processing: Final Step
Introns (intervening sequences)- Noncoding regions of the mRNA
Exons- expressed sequence of the mRNA
Introns are removed by snRNPs (small nuclear ribonucleoprotein) and splicesomes

13. Messenger RNA Processing in Eukaryotes
exon
exon
exon
DNA
intron
intron
transcription
exon
exon
exon
pre-mRNA 5
intron
intron 3
exon
exon
exon 5 3
cap
intron
intron
poly-A tail
spliceosome
exon
exon
exon 5 3
cap
poly-A tail
pre-mRNA
splicing
intron RNA
mRNA 5 3 13
cap
poly-A tail

14. Cracking the Code
Genetic information is encoded as a sequence of 3 nitrogenous bases called a codon
3 bases = 1 amino acid
20 total amino acids
Codons must be read in the correct reading frame

15. Genetic Code The genetic code is nearly universal
Shared by most organisms
64 codons
61 code for AA
AUG-start
UAA-stop
UGA-stop
UAG-stop
Wobble

16. Translation
Translation-process by which the codons of an mRNA are changed into an amino acid sequence
Occurs in the cytoplasm at the ribosome (rRNA)
Contains 3 stages: initiation, elongation and termination
tRNA carries amino acids to the rRNA (ribosome)

17. tRNA tRNA molecules come in 64 different kinds
All are very similar except that
One end bears a specific triplet (of the 64 possible) called the anticodon
The other end binds with a specific amino acid type
All tRNA molecules with a specific anticodon will always bind with the same amino acid

18. Structure of a tRNA Molecule
amino
acid
leucine 3’ 5’
hydrogen
bonding
amino acid end
anticodon
anticodon end G A A C A G U C C U U C C U C
mRNA 5’ 3’
codon a. b.

19. Ribosomes Ribosomes Ribosomal RNA (rRNA):
Produced from a DNA template in the nucleolus of a nucleus
Combined with proteins into large and small ribosomal subunits
A completed ribosome has three binding sites to facilitate pairing between tRNA and mRNA
The E (for exit) site
The P (for peptide) site, and
The A (for amino acid) site

20. Stage 1: Initiation
Initiation begins when mRNA becomes attached to a subunit of the ribosome
First codon is always AUG (methionine-met)

21. Stage 2: Elongation
Continues as tRNA brings amino acids to the ribosome and polypeptide chain is formed

22. Stage 3: Termination
Translation is stopped when a ribosome reaches one of the 3 termination or stop codons.
Release factor breaks the bond between the tRNA and the last amino acid
mRNA is broken down

23. Summary of Protein Synthesis in Eukaryotes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
TRANSCRIPTION
TRANSLATION
1. DNA in nucleus serves
as a template for mRNA. DN A
3. mRNA moves into
cytoplasm and
becomes associated
with ribosomes.
2. mRNA is processed
before leaving the nucleus.
large and small
ribosomal subunits 5
mRNA
introns
pre-mRN A 3
mRN A
amino
acids
4. tRNAs with
anticodons carry
amino acids
to mRNA.
nuclear pore
peptide
ribosome
tRNA U A C U A C 5 A U G 3
anticodon
codon
5. During initiation, anticodon-codon
complementary base pairing begins
as the ribosomal subunits come
together at a start codon.
8. During termination, a
ribosome reaches a stop
codon; mRNA and
ribosomal subunits
disband. C C C 5 C C C U G G U U U G G G A C C A A A G U A 3
6. During elongation, polypeptide
synthesis takes place one
amino acid at a time.
7. Ribosome attaches to rough
ER. Polypeptide enters lumen,
where it folds and is
modified.