Gene Expression and Control

Chapter 7

Part 1

7.1 Impacts/Issues

Ricin and Your Ribosomes

The ability to make proteins is critical to all life

processes – ricin kills because it inactivates

ribosomes that assemble proteins

Video: Ricin and your ribosomes

7.2 The Nature of Genetic Information

DNA carries all the genetic information needed

to build a new individual

• Genetic information consists of base sequences

• Genes are subunits of that sequence

Gene

• Part of a DNA base sequence

• Specifies structure of an RNA or protein product

From Gene to RNA to Protein

Gene expression involves transcription (DNA to

RNA), and translation (mRNA, or messenger

RNA, to protein)

Gene expression

• Process by which the information in a gene

becomes converted to an RNA or protein product

Transcription

A gene’s nucleotide base sequence encodes

instructions for building an RNA or protein

product

A cell transcribes the base sequence of a gene

into mRNA

mRNA carries a protein-building message

Transcription

Transcription

• Process by which an RNA is assembled from

nucleotides using the base sequence of a gene

as a template

Messenger RNA (mRNA)

• Type of RNA that has a protein-building message

Translation

Translation requires the participation of tRNA

(transfer RNA) and rRNA (ribosomal RNA)

Translation

• Process by which a polypeptide chain is

assembled from amino acids in the order

specified by an mRNA

RNA and DNA Nucleotides

Fig. 7-2a, p. 117

Fig. 7-2a, p. 117

3 phosphate groups

base

(guanine)

sugar

(ribose)

An RNA nucleotide: guanine (G),

or guanosine triphosphate

Fig. 7-2b, p. 117

Fig. 7-2b, p. 117

base

(guanine)

3 phosphate groups

sugar

(deoxyribose)

A DNA nucleotide: guanine (G), or

deoxyguanosine triphosphate

Animation: Uracil-thymine comparison

7.3 Transcription: DNA to RNA

Base-pairing rules in DNA replication also apply

to RNA synthesis in transcription, but RNA uses

uracil in place of thymine

The Process of Transcription

In transcription, RNA polymerase binds to a

promoter in the DNA near a gene

RNA polymerase

• Enzyme that carries out transcription

Promoter

• In DNA, a sequence to which RNA polymerase

binds

The Process of Transcription

Polymerase moves along the DNA, unwinding

the DNA so it can read the base sequence

RNA polymerase assembles a strand of RNA by

linking RNA nucleotides in the order determined

by the base sequence of the gene

The new mRNA is a copy of the gene from which

it was transcribed

Transcription: DNA to RNA

Fig. 7-3a, p. 118

Fig. 7-3a, p. 118

RNA

polymerasegene region

promoter sequence in DNA

1 RNA polymerase binds to a promoter in the DNA. The binding

positions the polymerase near a gene. In most cases, the base sequence

of the gene occurs on only one of the two DNA strands. Only the DNA

strand complementary to the gene sequence will be translated into RNA.

Fig. 7-3b, p. 118

Fig. 7-3b, p. 118

RNA

DNA winding up DNA unwinding

2 The polymerase begins to move along the DNA and unwind it. As it

does, it links RNA nucleotides into a strand of RNA in the order specified

by the base sequence of the DNA. The DNA winds up again after the

polymerase passes. The structure of the “opened” DNA at the

transcription site is called a transcription bubble, after its appearance.

Fig. 7-3c, p. 119

Fig. 7-3c, p. 119

direction of

transcription

3 Zooming in on the gene region, we can see that RNA polymerase covalently bonds successive nucleotides into an RNA strand. The base sequence of the new RNA strand is complementary to the base sequence of its DNA template strand, so it is an RNA copy of the gene. Figure It Out: After the guanine, what is the next nucleotide that will be added to this growing strand of RNA? Answer: Another guanine (G)

Animation: Gene transcription details

Three Genes, Many RNA Polymerases

Many polymerases can transcribe a gene region

at the same time

Fig. 7-4, p. 119

RNA transcripts DNA molecule

Animation: Pre-mRNA transcript

processing

Animation: Transcription

3D Animation: Transcription

7.4 RNA Players in Translation

Three types of RNA are involved in translation:

mRNA, rRNA, and tRNA

mRNA produced by transcription carries protein-

building information from DNA to the other two

types of RNA for translation

mRNA and the Genetic Code

The information in mRNA consists of sets of

three nucleotides (codons) that form “words”

spelled with the four bases A, C, G, and U

Codon

• In mRNA, a nucleotide base triplet that codes for

an amino acid or stop signal during translation

mRNA and the Genetic Code

Sixty-four codons, most of which specify amino

acids, constitute the genetic code

• 20 amino acids in proteins; most have more than

one codon

Genetic code

• Sixty-four mRNA codons; each specifies an

amino acid or a signal to start or stop translation

The Genetic Code

Animation: Genetic code

Translating mRNA to Amino Acids

p. 120

a gene

region in DNA

methionine

(met)

tyrosine

(tyr)

serine

(ser)

valine

(val)

tyrosine

(tyr)

amino acid

sequence

translation

mRNA

transcription

rRNA and tRNA – the Translators

Ribosomes and transfer RNAs (tRNA) interact to

translate an mRNA into a polypeptide

Ribosomes consist of two subunits of rRNA and

structural proteins

Ribosomal RNA (rRNA)

• A type of RNA that becomes part of ribosomes

Ribosomes

During translation, one large and one small

ribosomal subunit (rRNA) converge as a

ribosome on an mRNA

rRNA reads the mRNA and acts as an enzyme

to form peptide bonds between amino acids,

assembling them into a polypeptide chain

A Ribosome

Fig. 7-6a, p. 121

Fig. 7-6a, p. 121

tunnel

A large subunit of a ribosome

Fig. 7-6b, p. 121

Fig. 7-6b, p. 121

mRNA

B small subunit of a ribosome

Fig. 7-6 (right), p. 121

Fig. 7-6 (right), p. 121

an intact ribosome

tRNA

tRNAs deliver amino acids to ribosomes in the

order specified by mRNA

Transfer RNA (tRNA)

• Type of RNA that delivers amino acids to a

ribosome during translation

tRNA

Each tRNA has two attachment sites

• An anticodon that can base-pair with a codon

• A site that binds to the kind of amino acid

specified by the codon

Anticodon

• Set of three nucleotides in a tRNA

• Base-pairs with mRNA codon

tRNA for Tryptophan

Fig. 7-7a, p. 121

Fig. 7-7a, p. 121

anticodon

amino acid

attachment site

Fig. 7-7b, p. 121

Animation: Structure of a ribosome

Animation: Structure of a tRNA

7.5 Translating the Code: RNA to Protein

Translation, the second part of protein synthesis,

occurs in the cytoplasm of all cells

Translation is an energy-requiring process that

converts the protein-building information carried

by an mRNA into a polypeptide

Three Stages of Translation

Initiation

• mRNA joins with an initiator tRNA and two ribosomal subunits

Elongation

• Ribosome joins amino acids delivered by tRNAs in the order specified by mRNA codons

Termination

• Polymerase encounters a stop codon; mRNA and polypeptide are released; ribosome disassembles

Elongation

p. 122

p. 122

start

codon

(AUG)

initiator

tRNA

first amino

acid of polypeptide peptide bond

p. 123

p. 123

start

codon

(AUG)

initiator

tRNA

first amino

acid of polypeptide

p. 122-123

Stepped Art

peptide bond

Polysomes

In cells making a lot of protein, many ribosomes

may simultaneously translate the same mRNA

Polysome

• Cluster of ribosomes that are simultaneously

translating an mRNA

p. 123

mRNA

a polysome

Translation in Eukaryotes

Fig. 7-8, p. 122

Transcription

ribosome

subunitsRNA transport

tRNA

5 Polysomes3 Convergence of RNAs

mRNA 4 Translation

polypeptide

1

2

Animation: Translation

Animation: The major differences between

prokaryotic and eukaryotic protein synthesis

Animation: Overview of transcription and

translation