Transcript chapter14

From DNA to Protein
Chapter 14
14.1 DNA, RNA, and Gene Expression
 What is genetic information and how does a cell
use it?
The Nature of Genetic Information
 Each strand of DNA consists of a chain of four
kinds of nucleotides: A, T, G and C
 The sequence of the four bases in the strand is
the genetic information
Converting a Gene to an RNA
 Transcription
• Enzymes use the nucleotide sequence of a gene
to synthesize a complementary strand of RNA
 DNA is transcribed to RNA
• Most RNA is single stranded
• RNA uses uracil in place of thymine
• RNA uses ribose in place of deoxyribose
adenine A
HC
NH 2
N CC N
N C N CH
guanine G
O
N CC
NH
HC
NC C
N NH
2
DNA
deoxyribonucleic acid
RNA
ribonucleic acid
nucleotide
base
sugar–
phosphate
backbone
cytosine C
NH 2
HC C N
HC
thymine T
N
N CC
N
HC
N C
CH
N
guanine G
O
N CC
NH
HC
NC C
N NH 2
NH 2
HC C N
HC N C O
base pair
CH 3 C C NH
HC N C O
Nucleotide
bases of DNA
NH 2
cytosine C
C O
O
adenine A
uracil U
O
HC C NH
HC N C O
DNA has one function: It
permanently stores a cell’s
genetic information, which
is passed to offspring.
RNAs have various
functions. Some serve
as disposable copies of
DNA’s genetic message;
others are catalytic.
Nucleotide
bases of RNA
Fig. 14-3, p. 217
RNA in Protein Synthesis
 Messenger RNA (mRNA)
• Contains information transcribed from DNA
 Ribosomal RNA (rRNA)
• Main component of ribosomes, where polypeptide
chains are built
 Transfer RNA (tRNA)
• Delivers amino acids to ribosomes
Converting mRNA to Protein
 Translation
• The information carried by mRNA is decoded
into a sequence of amino acids, resulting in a
polypeptide chain that folds into a protein
 mRNA is translated to protein
• rRNA and tRNA translate the sequence of base
triplets in mRNA into a sequence of amino acids
Gene Expression
 A cell’s DNA sequence (genes) contains all the
information needed to make the molecules of life
 Gene expression
• A multistep process including transcription and
translation, by which genetic information encoded
by a gene is converted into a structural or
functional part of a cell or body
14.2 Transcription: DNA to RNA
 RNA polymerase assembles RNA by linking
RNA nucleotides into a chain
 A new RNA strand is complementary in
sequence to the DNA strand from which it was
transcribed
DNA Replication and Transcription
 DNA replication and transcription both
synthesize new molecules by base-pairing
 In transcription, a strand of mRNA is assembled
on a DNA template using RNA nucleotides
• Uracil (U) nucleotides pair with A nucleotides
• RNA polymerase adds nucleotides to the
transcript
Base-Pairing in
DNA Synthesis and Transcription
The Process of Transcription
 RNA polymerase and regulatory proteins attach
to a promoter
 RNA polymerase moves over the gene in a 5' to
3' direction, unwinds the DNA helix, reads the
base sequence, and joins free RNA nucleotides
into a complementary strand of mRNA
Transcription
Fig. 14-5b, p. 219
Animation: Gene transcription details
14.3 RNA and the Genetic Code
 Base triplets in an mRNA are words in a proteinbuilding message
 Two other classes of RNA (rRNA and tRNA)
translate those words into a polypeptide chain
Post-Transcriptional Modifications
 In eukaryotes, RNA is modified before it leaves
the nucleus as a mature mRNA
 Introns
• Nucleotide sequences that are removed from a
new RNA
 Exons
• Sequences that stay in the RNA
Alternative Splicing
 After splicing, transcripts are finished with a
modified guanine “cap” at the 5' end and a polyA tail at the 3' end
Animation: Pre-mRNA transcript
processing
mRNA – The Messenger
 Codon
• A sequence of three mRNA nucleotides that
codes for a specific amino acid
Genetic Information
 From DNA to mRNA to amino acid sequence
Genetic Code
 Genetic code
• Consists of 64 mRNA codons (triplets)
• Some amino acids can be coded by more than
one codon
 Some codons signal the start or end of a gene
• AUG (methionine) is a start codon
• UAA, UAG, and UGA are stop codons
Codons of the Genetic Code
rRNA and tRNA – The Translators
 tRNAs deliver amino acids to ribosomes
• tRNA has an anticodon complementary to an
mRNA codon, and a binding site for the amino
acid specified by that codon
 Ribosomes, which link amino acids into
polypeptide chains, consist of two subunits of
rRNA and proteins
tRNA
14.4 Translation: RNA to Protein
 Translation converts genetic information carried
by an mRNA into a new polypeptide chain
Translation
 Translation occurs in the cytoplasm of cells
 Translation occurs in three stages
• Initiation
• Elongation
• Termination
Initiation
 An initiation complex is formed
• A small ribosomal subunit binds to mRNA
• The anticodon of initiator tRNA base-pairs with
the start codon (AUG) of mRNA
• A large ribosomal subunit joins the small
ribosomal subunit
Elongation
 The ribosome assembles a polypeptide chain as
it moves along the mRNA
• Initiator tRNA carries methionine, the first amino
acid of the chain
• The ribosome joins each amino acid to the
polypeptide chain with a peptide bond
Termination
 When the ribosome encounters a stop codon,
polypeptide synthesis ends
• Release factors bind to the ribosome
Elongation
C An initiator tRNA
carries the amino acid
methionine, so the first
amino acid of the new
polypeptide chain will be
methionine. A second
tRNA binds the second
codon of the mRNA (here,
that codon is GUG, so the
tRNA that binds carries
the amino acid valine).
A peptide bond
forms between
the first two
amino acids
(here, methionine
and valine).
Fig. 14-12c, p. 223
D The first tRNA is
released and the
ribosome moves to the
next codon in the mRNA.
A third tRNA binds to the
third codon of the mRNA
(here, that codon is UUA,
so the tRNA carries the
amino acid leucine).
A peptide bond
forms between the
second and third
amino acids
(here, valine
and leucine).
Fig. 14-12d, p. 223
E The second tRNA
is released and the
ribosome moves to the
next codon. A fourth
tRNA binds the fourth
mRNA codon (here, that
codon is GGG, so the
tRNA carries the amino
acid glycine).
A peptide bond
forms between the
third and fourth
amino acids (here,
leucine and
glycine).
Fig. 14-12e, p. 223
Termination
F Steps d and e are repeated over and
over until the ribosome encounters a stop
codon in the mRNA. The mRNA transcript
and the new polypeptide chain are
released from the ribosome. The two
ribosomal subunits separate from each
other. Translation is now complete. Either
the chain will join the pool of proteins in
the cytoplasm or it will enter rough ER of
the endomembrane system (Section 4.9).
Fig. 14-12f, p. 223
14.5 Mutated Genes
and Their Protein Products
 If the nucleotide sequence of a gene changes, it
may result in an altered gene product, with
harmful effects
 Mutations
• Small-scale changes in the nucleotide sequence
of a cell’s DNA that alter the genetic code
Common Mutations
 Base-pair-substitution
• May result in a premature stop codon or a
different amino acid in a protein product
• Example: sickle-cell anemia
 Deletion or insertion
• Can cause the reading frame of mRNA codons to
shift, changing the genetic message
• Example: Huntington’s disease
Common Mutations
Animation: Base-pair substitution
What Causes Mutations?
 Transposable elements
• Segments of DNA that can insert themselves
anywhere in a chromosomes
 Spontaneous mutations
• Uncorrected errors in DNA replication
 Harmful environmental agents
• Ionizing radiation, UV radiation, chemicals
Mutations Caused by Radiation
 Ionizing radiation damages chromosomes,
nonionizing (UV) radiation forms thymine dimers
Inherited Mutations
 Mutations in somatic cells of sexually
reproducing species are not inherited
 Mutations in a germ cell or gamete may be
inherited
Animation: Protein synthesis summary
Animation: Structure of a ribosome
Animation: Structure of a tRNA
Animation: Transcription
Animation: Uracil-thymine comparison