From DNA to Protein

1
Chapter 14

• From DNA to Protein

2
Impacts, Issues Ricin and Your Ribosomes

• Ricin comes from the castor oil plant
• Inactivates ribosomes, the protein-building
  machinery of all cells

3
Ricin and Your Ribosomes
Fig. 14-1a, p.218
4
Ricin and Your Ribosomes
Fig. 14-1b, p.218
5
Marvelous Mussel Adhesive

• Mussel binds itself to rocks with threads coated
  with the protein bysuss
• Gene for bysuss has been put into yeast
• Yeast synthesize the protein based on the
  instructions in the mussel DNA

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
Three Classes of RNAs

• Messenger RNA
• Carries protein-building instruction
• Ribosomal RNA
• Major component of ribosomes
• Transfer RNA
• Delivers amino acids to ribosomes

8
A Nucleotide Subunit of RNA
uracil (base)
phosphate group
sugar (ribose)
Fig. 14-2, p. 220
9
phosphate group
base (uracil)
sugar (ribose)
Fig. 14-2a, p.220
10
base (thymine)
sugar (deoxyribose)
Fig. 14-2b, 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 is template
• RNA polymerase catalyzes nucleotide addition
• Product is a single strand of RNA

13
Promoter

• 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

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 unwinding
DNA template winding up
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
d At the end of the gene region, the last stretch
of the new transcript is unwound and released
from the DNA template. Shown below is a model for
a transcribed strand of RNA.
Fig. 14-3d, p.221
18
Transcript Modification
unit of transcription in a DNA strand
exon
intron
exon
exon
intron
transcription into pre-mRNA
poly-A tail
cap
snipped out
snipped out
mature mRNA transcript
Fig. 14-4, p.221
19
unit of transcription in a DNA strand
exon
intron
exon
exon
intron
5
3
Stepped Art
Fig. 14-4, p.221
20
Genetic Code

• Set of 64 base triplets
• Codons
• 61 specify amino acids
• 3 stop translation

Fig. 14-6, p.222
21
tRNA Structure
codon in mRNA
anticodon
amino-acid attachment site
amino acid
OH
Figure 14.7Page 223
22
tRNA Structure
codon in mRNA
anticodon in tRNA
amino acid
Fig. 14-7, p.223
23
Ribosomes
funnel
small ribosomal subunit
large ribosomal subunit
intact ribosome

Fig. 14-8, p.223
24
Three Stages of Translation

• Initiation
• Elongation
• Termination

25
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

26
Binding Sites
binding site for mRNA
A (second binding site for tRNA)
P (first binding site for tRNA)
27
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

28
Elongation
29
Termination

• Stop codon into place
• No tRNA with anticodon
• Release factors bind to the ribosome
• mRNA and polypeptide are released

mRNA
new polypeptide chain
30
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

31
Overview
Transcription
mRNA
rRNA
tRNA
Mature mRNA transcripts
ribosomal subunits
mature tRNA
Translation
32
elongation
binding site for mRNA
P (first binding site for tRNA)
A (second binding site for tRNA)
Amino Acid 1
Amino Acid 1
Amino Acid 2
c Initiation ends when a large and small
ribosomal subunit converge and bind together.
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
33
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
34
Gene Mutations

• Base-Pair Substitutions
• Insertions
• Deletions

35
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
36
Frameshift Mutations

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

37
Frameshift Mutation
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. 14-10, p.226
38
Transposons

• DNA segments that move spontaneously about the
  genome
• When they insert into a gene region, they usually
  inactivate that gene

39
Transposons

• Barbara McClintock
• Nonuniform coloration of kernels in strains of
  indian corn

Fig. 14-11, p.227
40
Mutation Rates

• 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

41
Mutagens

• Ionizing radiation (X rays)
• Nonionizing radiation (UV)
• Natural and synthetic chemicals

42
Ionizing Radiation
Fig. 14-12, p.227