1. Chapter 14
From DNA to Protein

2. Objectives
1. Know how the structure and behavior of DNA determines the structure and behavior of the three forms of RNA during transcription.
2. Understand the role the three forms of RNA play in determining the primary structure of polypeptide chains during translation.
3. Understand the types of mutations and their role in genetic variation.

3. 14.0: Ricin and Your Ribosomes
Ricin – comes from the castor oil plant, and can be used as a bioweapon.
Ricin has deadly effects – it inactivates ribosomes, the protein-building machinery of all cells.
Protein synthesis stops, cells die, the individual dies, there is no antidote.

4. Life depends on enzymes
Enzymes are proteins. All proteins consist of polypeptide chains.
The chains are sequences of amino acids that correspond to genes – sequences of nucleotide bases in DNA.
Enzymes may selectively unwind certain regions of DNA to encode information to make proteins.

5. The path from genes to proteins
DNA RNA Protein
The two steps include:
Transcription
Translation

6. Steps from DNA to Proteins
Same two steps produce all proteins:
1) DNA is transcribed to form RNA
Occurs in the nucleus
RNA moves into cytoplasm
2) RNA is translated to form polypeptide chains, which fold to form proteins

7. 14.1 How is RNA Transcribed from DNA
The first step in protein synthesis, is transcription, a sequence of nucleotide bases is exposed in an unwound region of a DNA strand.
That sequence is the template upon which a single strand of RNA is assembled from adenine, cytosine, guanine, and uracil subunits.

8. Three Classes of RNAs Messenger RNA (mRNA) Ribosomal RNA (rRNA)
Carries protein-building instruction or code
Ribosomal RNA (rRNA)
Combines with proteins to form ribosomes
Transfer RNA (tRNA)
Delivers the correct amino acids to ribosomes

9. RNA molecule One phosphate group A five carbon sugar – ribose
Four bases:
Adenine
Cytosine
Guanine
Uracil – instead of Thymine

10. A Nucleotide Subunit of RNA
uracil (base)
phosphate group
sugar
(ribose)
Fig. 14-2, p. 220

11. Base Pairing during Transcription
DNA
base pairing during transcription
RNA
DNA
base pairing during DNA
replication
DNA
Fig. 14-2c, p.220

12. Transcription & DNA Replication
Like DNA replication
Nucleotides added in 5’ to 3’ direction
Unlike DNA replication
Only small stretch of DNA is template
RNA polymerase catalyzes nucleotide addition
Product is a single strand of RNA

13. Promoter
The promoter is a base sequence in the DNA that signals the start of a gene.
For transcription to occur, RNA polymerase must first bind to a promoter, and then move along to the end of the gene.

14. Promoter promoter region
RNA polymerase, the enzyme that catalyzes transcription
a RNA polymerase initiates transcription at a promoter region in DNA. It recognizes a base sequence located next to the promoter as a template. It will link the nucleotides adenine, cytosine, guanine, and uracil into a strand of RNA, in the order specified by DNA.
Fig. 14-3a, p.220

15. Gene Transcription DNA template at selected transcription site
newly forming RNA transcript
DNA template winding up
DNA template unwinding
b All through transcription, the DNA double helix becomes unwound in front of the RNA polymerase. Short lengths of the newly forming RNA strand briefly wind up with its DNA template strand. New stretches of RNA unwind from the template (and the two DNA strands wind up again).
Fig. 14-3b, p.220

16. Adding Nucleotides direction of transcription 3´ 5´ 5´ 3´
growing RNA transcript
c What happened at the assembly site? RNA polymerase catalyzed the assembly of ribonucleotides, one after another, into an RNA strand, using exposed bases on the DNA as a template. Many other proteins assist this process.
Fig. 14-3c, p.221

17. Transcript Modification of mRNA
unit of transcription in a DNA strand
exon
intron
exon
intron
exon
transcription into pre-mRNA
5’ end Cap
3’ end poly-A tail
snipped out non- coding introns
snipped out non-coding introns
mature mRNA transcript
Fig. 14-4, p.221

18. Modifications of mRNA In nucleus
5’ end is capped with guanine – “start” signal for translations. Cap will also help bind the mRNA to a ribosome
3’ end a “poly-A-tail” is added. 100 – 200 molecules of adenine is added
Introns (non-coding) are snipped out, exons (coding portions) are spliced together
Alternative splicing results in different proteins

19. 14.2 Genetic Code Set of 64 base triplets Codons
61 specify amino acids
3 stop translation
Fig. 14-6, p.222

20. The Genetic Code
Every three bases in mRNA (triplet) code of an amino acid.
Both DNA and its RNA transcript are linear sequences of nucleotides carrying the hereditary code.
The genetic code consists of 61 triplets that specify amino acids, AUG – “start” codon Methionine, and three “stops”

21. Genetic Code DNA mRNA mRNA codons amino acids threonine proline
glutamate
glutamate
lysine
Fig. 14-5, p.222

22. 14.3 The other RNA’s - tRNA codon in mRNA anticodon amino-acid
attachment site
amino
acid OH
Figure 14.7 Page 223

23. tRNA
Each kind of tRNA has an anticodon that is complementary to an mRNA codon
Each tRNA also carries one specific amino acid.
After the mRNA arrives in the cytoplasm an anticodon on a tRNA bonds to the codon on the mRNA, and thus a correct amino acid is brought into place.

24. Ribosomes A ribosome has two subunits
Each ribosome is composed of rRNA and structural proteins) that come together only during translation.
Polypeptide chains are built on ribosomes.

25. small ribosomal subunit large ribosomal subunit
Ribosomes
funnel
small ribosomal subunit +
large ribosomal subunit
intact ribosome
Fig. 14-8, p.223

26. 14.4 Three Stages of Translation
Initiation
Elongation
Termination

27. Initiation Initiator tRNA binds to small ribosomal subunit
Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon
Large ribosomal subunit joins complex

28. Binding Sites binding site for mRNA A (second binding site for tRNA)
P (first binding site for tRNA)

29. Elongation mRNA passes through ribosomal subunits
tRNAs deliver amino acids to the ribosomal binding site in the order specified by the mRNA
Peptide bonds form between the amino acids and the polypeptide chain grows

30. Elongation

31. Termination Stop codon into place No tRNA with anticodon
Release factors bind to the ribosome
mRNA and polypeptide are released into the cytoplasm.
mRNA
new
polypeptide
chain

32. What Happens to the New Polypeptides?
Some just enter the cytoplasm
Many enter the endoplasmic reticulum and move through the cytomembrane system where they are modified
Polysome- several ribosomes moving along mRNA at the same time.

33. Overview Transcription Translation mRNA rRNA tRNA
Mature mRNA transcripts
ribosomal subunits
mature tRNA
Translation

34. Fig. 14-9a-e, p.224 elongation binding site for mRNA
P (first binding site for tRNA)
A (second binding site for tRNA)
Amino
Acid 1
Amino
Acid 1
c Initiation ends when a large and small ribosomal subunit converge and bind together.
Amino
Acid 2
Amino
Acid 2
d The initiator tRNA binds to the ribosome.
e One of the rRNA molecules
b Initiation, the first stage of translating mRNA, will start when an initiator tRNA binds to a small ribosomal subunit.
initiation
a A mature mRNA transcript leaves the nucleus through a pore in the nuclear envelope.
Fig. 14-9a-e, p.224

35. Fig. 14-9f-k, p.224 g A third tRNA binds with the next codon
f The first tRNA is released
h Steps f and g are repeated
termination
i A STOP codon moves into the area where the chain is being built.
j The new polypeptide chain is released from the ribosome.
k The two ribosomal subunits now separate, also.
Fig. 14-9f-k, p.224

36. Base-Pair Substitutions
14.5 Gene Mutations
A gene mutation is a change in one to several bases in the nucleotide sequence of DNA, which can result in a change in the protein synthesized.
Base-Pair Substitutions
Insertions
Deletions

37. Base-Pair Substitution
a base substitution within the triplet (red)
original base triplet in a DNA strand
During replication, proofreading
enzymes make a substitution
possible outcomes: or
original, unmutated sequence
a gene mutation

38. Frameshift Mutations Insertion Deletion Both shift the reading frame
Extra base added into gene region
Deletion
Base removed from gene region
Both shift the reading frame
Result in many wrong amino acids

39. Frameshift Mutation Fig. 14-10, p.226 part of DNA template
mRNA transcribed from DNA
resulting amino acid sequence
THREONINE
PROLINE
GLUTAMATE
GLUTAMATE
LYSINE
base substitution in DNA
altered mRNA
altered amino acid sequence
THREONINE
PROLINE
VALINE
GLUTAMATE
LYSINE
deletion in DNA
altered mRNA
THREONINE
PROLINE
GLYCINE
ARGININE
altered amino acid sequence
Fig , p.226

40. Transposons DNA segments that move spontaneously about the genome
When they insert into a gene region, they usually inactivate that gene
See the non-uniform coloration of kernels in strains of Indian Corn

41. Mutation Rates
Mutations occur spontaneously while DNA is being replicated, but fortunately special enzymes correct most of the mistakes.
Each gene has a characteristic mutation rate
Average rate for eukaryotes is between 10-4 and 10-6 per gene per generation
Only mutations that arise in germ cells can be passed on to next generation

42. Mutagens Ionizing radiation (X rays) Nonionizing radiation (UV)
Natural and synthetic chemicals - alkylating agents – add methyl or ethyl groups to DNA making it more vulnerable to mutations. (smoking)

43. Mutations
If a mutation arises in a somatic cell, it will affect only that cell and will not be passed on to offspring.
If, however, the mutation arises in a gamete, it may be passed on and thus enter the evolutionary arena.
Either kind of mutation may prove to be harmful, beneficial, or neutral in its effects.