Gene Expression and Control

Chapter 7

Part 1

7.1 Impacts/IssuesRicin and Your Ribosomes

The ability to make proteins is critical to all life processes – ricin kills because it inactivates ribosomes that assemble proteins

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

3 phosphate groups

base (guanine)

sugar (ribose)

An RNA nucleotide: guanine (G), or guanosine triphosphate

Fig. 7-2b, p. 117

base (guanine)

3 phosphate groups

sugar (deoxyribose)

A DNA nucleotide: guanine (G), or deoxyguanosine triphosphate

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

RNA polymerase

gene 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

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

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)

Gene transcription details

Three Genes, Many RNA Polymerases

Many polymerases can transcribe a gene region at the same time

Pre-mRNA transcript processing

Transcription

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

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

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

anticodon

amino acid attachment site

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

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

Fig. 7-8, p. 122

Transcription

ribosome subunitsRNA

transporttRNA

5 Polysomes3 Convergence of RNAs

mRNA 4 Translation

polypeptide

1

2

Translation in Eukaryotes

Animation: Translation

The major differences between prokaryotic and eukaryotic protein synthesis

Overview of transcription and translation