1. By Dr. Friday Nwalo Dept. Biology/Microbiology/Biotechnology
Bio 210 Molecular Biology
By Dr. Friday Nwalo Dept. Biology/Microbiology/Biotechnology

2. DNA RNA POLYPEPTIDE

3. Gene 1
Gene 3
DNA molecule
Gene 2
DNA strand
TRANSCRIPTION
RNA
Codon
TRANSLATION
Polypeptide
Amino acid

4. DNA is in the language of nucleotides
RNA is in the language of nucleotides
Polypeptides are in the language of amino acids
Converting DNA to RNA is the process of transcription.
Where does transcription take place?

5. Transcription
Process by which a molecule of RNA is synthesized that is complementary to a specific sequence of DNA
Occurs in the nucleus of eukaryotic cells & cytoplasm of prokaryotic cells.
Is regulated by operons (bacterial cells) or transcription factors (multicellular organisms).
Involves 3 stages: initiation, elongation & termination

6. 1. Initiation RNA polymerase attaches to a promoter on DNA strand.
Promotor is a short section of DNA that marks the start of a gene. Thus, each gene on a chromosome has its own promotor.
Helicase unzips a short section of DNA.
Free RNA nucleotides move in & H-bond to complementary bases on DNA template strand.

7. 2. Elongation 3. Termination
RNA polymerase links RNA nucleotides together in a 5’ to 3’ direction.
Growing RNA strand peels away from DNA template.
3. Termination
RNA polymerase detaches when it reaches a terminator.
Terminator is a short section of DNA that marks the end of a gene. Thus, each gene has its own terminator.
Completed RNA molecule is released from DNA template.

8. DNA template C A G T A A G C C RNA strand G U C A U U C G G
Virtually all organisms share the same genetic code!
Just like we have 26 letters in the alphabet to construct words, the alphabet of DNA has 4 letters to use to construct polypeptides.
DNA template C A G T A A G C C
RNA strand G U C A U U C G G
So how is the code used to construct polypeptides?

9. Three major types of RNA are transcribed.
mRNA (messenger RNA) - encodes genetic information from DNA & carries it into the cytoplasm. 5’ 3’
codon
Examples:
AUG codes for amino acid methionine.
GGA codes for amino acid glycine.
UGU codes for amino acid cysteine.
AAG codes for amino acid lysine.
CGA codes for amino acid arginine.
Each three consecutive mRNA bases forms a genetic code word (codon) that codes for a particular amino acid.

10. rRNA (ribosomal RNA) - associates with proteins to form ribosomes.
tRNA (transfer RNA) - transports specific amino acids to ribosome during protein synthesis (translation).
large subunit
small subunit

11. A U A U A U G C C C G C The language of amino acids is based on codons
1 codon = 3 mRNA nucleotides
1 codon = 1 amino acid
A U A U A U G C C C G C
How many codons are in this sequence of mRNA?

12. Using this chart, you can determine which amino acid the codon “codes” for!
Which amino acid is encoded in the codon CAC?

13. Find the second letter of the codon CAC
Find the first letter of the codon CAC
Find the third letter of the codon CAC

14. CAC codes for the amino acid histidine (his).

15. What does the mRNA codon UAC code for?
Tyr or tyrosine

16. Notice there is one start codon AUG. Transcription begins at that codon!

17. Notice there are three stop codons
Notice there are three stop codons. Transcription stops when these codons are encountered.

18. The Nature of Genes
The central dogma of molecular biology states that information flows in one direction:
DNA RNA protein
Transcription is the flow of information from DNA to RNA.
Translation is the flow of information from RNA to protein.

19. The Genetic Code
Deciphering the genetic code required determining how 4 nucleotides (A, T, G, C) could encode more than 20 amino acids.
Francis Crick and Sydney Brenner determined that the DNA is read in sets of 3 nucleotides for each amino acid.

20. The Genetic Code
codon: set of 3 nucleotides that specifies a particular amino acid
reading frame: the series of nucleotides read in sets of 3 (codon)
only 1 reading frame is correct for encoding the correct sequence of amino acids

21. The Genetic Code
Marshall Nirenberg identified the codons that specify each amino acid.
RNA molecules of only 1 nucleotide and of specific 3-base sequences were used to determine the amino acid encoded by each codon.
The amino acids encoded by all 64 possible codons were determined.

23. The Genetic Code
stop codons: 3 codons (UUA, UGA, UAG) in the genetic code used to terminate translation
start codon: the codon (AUG) used to signify the start of translation
The remainder of the code is degenerate meaning that some amino acids are specified by more than one codon.

24. Proteins are complex macromolecules
Protein Structure
Proteins are complex macromolecules
composed of 20 (?)
different amino acids.

25. Amino Acids Proteins are made of polypeptides.
A polypeptide is a long chain of amino acids.
Amino acids have a free amino group, a free carboxyl group, and a side group (R).

27. Peptide Bonds Amino acids are joined by peptide bonds.
The carboxyl group of one amino acid is covalently attached to the amino group of the next amino acid.

28. Protein Synthesis: Translation
The genetic information in mRNA molecules is translated into the amino acid sequences of polypeptides according to the specifications of the genetic code.

29. Properties of the Genetic Code
The genetic code is composed of nucleotide triplets.
The genetic code is nonoverlapping. (?)
The genetic code is comma-free. (?)
The genetic code is degenerate. (yes)
The genetic code is ordered. (5’ to 3’)
The genetic code contains start and stop codons. (yes)
The genetic code is nearly universal. YES :)

30. A Triplet Code*

31. The Genetic Code Initiation and termination Codons
Initiation codon: AUG
Termination codons: UAA, UAG, UGA
Degeneracy: partial and complete
Ordered
Nearly Universal (exceptions: mitochondria and some protozoa)

32. Key Points
Each of the 20 amino acids in proteins is specified by one or more nucleotide triplets in mRNA. (20 amino acids refers to what is attached to the tRNAs!)
Of the 64 possible triplets, given the four bases in mRNA, 61 specify amino acids and 3 signal chain termination. (have no tRNAs!)

33. Key Points
The code is nonoverlapping, with each nucleotide part of a single codon, degenerate, with most amino acids specified by two to four codons, and ordered, with similar amino acids specified by related codons.
The genetic code is nearly universal; with minor exceptions, the 64 triplets have the same meaning in all organisms. (this is funny)