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

Fig. 14-3, p. 217

DNA RNAadenine A

NH 2deoxyribonucleic acid ribonucleic acid

adenine A

NH 2

N C N CC N nucleotide base HC

C NHC

N N NC CH N C CH

sugar–phosphate backbone

guanine G

Oguanine G

ON C N CC NH C NH

HC HCN N NH 2 N NH 2

C C N C C

cytosine CNH 2

cytosine CNH 2

C CHC N HC N

HC C O HC C ON N

thymine TO base pair uracil U O

C CCH 3 C NH HC NH

HC C O HC C ON NDNA 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 DNA

Nucleotide bases of RNA

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 protein-building 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 poly-A 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

Fig. 14-12c, p. 223

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. 14-12d, 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. 14-12e, 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. 14-12f, 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).

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