1. Basic Principles of Transcription and Translation Chp 13: Protein Synthesis (p. 308)
Transcription is the synthesis of RNA under the direction of DNA
Transcription produces messenger RNA (mRNA)
Ribosomes are the sites of translation

2. The structure of a ribosome

3. Central Dogma – as close as we get to a law
Dogma - a definite, authoritative tenet
Once information has been transcribed into RNA and translated into protein, the information cannot flow back into DNA
Phenotype

4. LE 17-3-5 DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide
Prokaryotic cell
Nuclear
envelope
TRANSCRIPTION
DNA
Pre-mRNA
RNA PROCESSING
mRNA
Ribosome
TRANSLATION
Polypeptide
Eukaryotic cell

5. Transcription Why would the cell transcribe DNA into mRNA?
DNA is the genetic material, it must be protected from damage and located where it can be accessed when needed.
All the DNA in a cell is very large
mRNA (messenger RNA) is made to use as a working copy of a small part of the larger instructions. This helps to protect the DNA and make the cellular processes more efficient.

6. Transcription 3 Steps RNA polymerase binds to “start” codon
Promoter
Transcription unit 5 3 3¢ 5¢
DNA
RNA polymerase binds to “start” codon
RNA polymerase unwinds and separates the 2 DNA strands
RNA polymerase adds complementary nucleotides to make a RNA strand
Start point
RNA polymerase
Initiation 5¢ 3¢ 3¢ 5¢
RNA
tran-
script
Template strand
of DNA
Unwound
DNA
Elongation
Rewound
DNA 5¢ 3¢ 3¢ 3¢ 5¢ 5¢
RNA
transcript
Termination 5¢ 3¢ 3¢ 5¢ 5¢ 3¢
Completed RNA transcript

7. Direction of transcription (“downstream”) Template strand of DNA
LE 17-7
Elongation
Non-template
strand of DNA
RNA nucleotides
RNA
polymerase 3¢
3¢ end 5¢ 5¢
Direction of transcription
(“downstream”)
Template
strand of DNA
Newly made
RNA

8. Transcription
Transcription is the process of producing complementary mRNA to a DNA template
Conceptually it is similar to DNA replication
mRNA has Uracil instead of Thymine
We will talk much more about the process of translation (RNA → Protein)

9. The Genetic Code
How are the instructions for assembling amino acids into proteins encoded into DNA?
There are 20 amino acids, but there are only four nucleotide bases in DNA
So, clearly each base does not code for a single amino acid.
Even if combinations of two bases were used that would only provide for 16 amino acids
Sets of three bases (a codon) code for an amino acid

10. Codons: Triplets of Bases
The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words
These triplets are the smallest units of uniform length that can code for all the amino acids
Example: AGT at a particular position on a DNA strand results in the placement of the amino acid serine at the corresponding position of the polypeptide to be produced

11. During transcription, a DNA strand called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript
During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction
Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide

12. Gene 2 DNA molecule Gene 1 Gene 3 DNA strand (template) 3¢ 5¢
TRANSCRIPTION
mRNA 5¢ 3¢
Codon
TRANSLATION
Protein
Amino acid

13. All 64 codons were deciphered by the mid-1960s
Cracking the Code
All 64 codons were deciphered by the mid-1960s
The genetic code is redundant but not ambiguous
Redundant – an amino acid can be specified by more than one codon.
Not ambiguous - no codon specifies more than one amino acid
Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced

14. The genetic code Second mRNA base First mRNA base (5¢ end)
Third mRNA base (3¢ end)

15. Evolution of the Genetic Code
The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals
Genes can be transcribed and translated after being transplanted from one species to another

16. Translation is the RNA-directed synthesis of a polypeptide
We can use the table with the genetic code to translate mRNA but how does the cell do it?
A cell translates an mRNA message into protein with the help of transfer RNA (tRNA)
Molecules of tRNA are not identical:
Each carries a specific amino acid on one end
Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA

17. Amino acids Polypeptide tRNA with amino acid attached Ribosome tRNA
LE 17-13
Amino
acids
Polypeptide
tRNA with
amino acid
attached
Ribosome
tRNA
Anticodon 5¢
Codons 3¢
mRNA

18. The Structure and Function of Transfer RNA
A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long
Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a cloverleaf
Amino acid
attachment site
Hydrogen
bonds 3¢ 5¢
Two-dimensional structure
Anticodon
Symbol used in this book
Three-dimensional structure A C C

19. Accurate translation requires two steps:
First step: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase
Second step: a correct match between the tRNA anticodon and an mRNA codon

20. Amino acid Aminoacyl-tRNA synthetase (enzyme) Pyrophosphate Phosphates
LE 17-15
Amino acid
Aminoacyl-tRNA
synthetase (enzyme)
Pyrophosphate
Phosphates
tRNA
AMP
Aminoacyl tRNA
(an “activated
amino acid”)

21. Ribosomes
Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis
The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)

22. Computer model of functioning ribosome
LE 17-16a
Exit tunnel
Growing
polypeptide
tRNA
molecules
Large
subunit E P A
Small
subunit 5¢
mRNA 3¢
Computer model of functioning ribosome

23. A ribosome has three binding sites for tRNA:
The P site holds the tRNA that carries the growing polypeptide chain
The A site holds the tRNA that carries the next amino acid to be added to the chain
The E site is the exit site, where discharged tRNAs leave the ribosome