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Gene Expression and Control
Chapter 7
Part 1
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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
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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
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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
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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
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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
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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
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RNA and DNA Nucleotides
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Fig. 7-2a, p. 117
3 phosphate groups
base (guanine)
sugar (ribose)
An RNA nucleotide: guanine (G), or guanosine triphosphate
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Fig. 7-2b, p. 117
base (guanine)
3 phosphate groups
sugar (deoxyribose)
A DNA nucleotide: guanine (G), or deoxyguanosine triphosphate
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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
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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
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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
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Transcription: DNA to RNA
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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.
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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.
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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)
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Gene transcription details
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Three Genes, Many RNA Polymerases
Many polymerases can transcribe a gene region at the same time
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Pre-mRNA transcript processing
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Transcription
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Transcription
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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
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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
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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
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The Genetic Code
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Animation: Genetic code
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Translating mRNA to Amino Acids
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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
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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
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A Ribosome
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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
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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
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tRNA for Tryptophan
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Fig. 7-7a, p. 121
anticodon
amino acid attachment site
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Structure of a tRNA
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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
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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
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Elongation
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start codon (AUG)initiator tRNA
first amino acid of polypeptide
p. 122-123
Stepped Art
peptide bond
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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
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Fig. 7-8, p. 122
Transcription
ribosome subunitsRNA
transporttRNA
5 Polysomes3 Convergence of RNAs
mRNA 4 Translation
polypeptide
1
2
Translation in Eukaryotes
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Animation: Translation
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The major differences between prokaryotic and eukaryotic protein synthesis
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Overview of transcription and translation