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

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 Messenger RNA (mRNA) 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

An RNA nucleotide: guanine (G), or guanosine triphosphate base (guanine) 3 phosphate groups sugar (ribose) Figure 7.2 Comparison between (A) an RNA nucleotide and (B) a DNA nucleotide. An RNA nucleotide: guanine (G), or guanosine triphosphate Fig. 7-2a, p. 117

A DNA nucleotide: guanine (G), or deoxyguanosine triphosphate base (guanine) 3 phosphate groups sugar (deoxyribose) Figure 7.2 Comparison between (A) an RNA nucleotide and (B) a DNA nucleotide. A DNA nucleotide: guanine (G), or deoxyguanosine triphosphate Fig. 7-2b, p. 117

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

promoter sequence in DNA RNA polymerase gene region promoter sequence in DNA Figure 7.3: Animated! Transcription assembles a strand of RNA from nucleotides using a gene region in DNA as a template. 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. 7-3a, p. 118

RNA DNA winding up DNA unwinding Figure 7.3: Animated! Transcription assembles a strand of RNA from nucleotides using a gene region in DNA as a template. 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. 7-3b, p. 118

direction of transcription Figure 7.3: Animated! Transcription assembles a strand of RNA from nucleotides using a gene region in DNA as a template. 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) Fig. 7-3c, p. 119

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 Anticodon 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

amino acid attachment site anticodon amino acid attachment site Figure 7.7 tRNA. (A) Models of the tRNA that carries the amino acid tryptophan. Each tRNA’s anticodon is complementary to an mRNA codon. Each also carries the amino acid specified by that codon. (B) During translation, tRNAs dock at an intact ribosome. Here, three tRNAs are docked at the small ribosomal subunit (the large subunit is not shown, for clarity). The anticodons of the tRNAs line up with complementary codons in an mRNA (shown in red). Fig. 7-7a, p. 121

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 peptide bond Stepped Art p. 122-123

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

Translation in Eukaryotes Transcription 1 2 ribosome subunits RNA transport tRNA 5 Polysomes 3 Convergence of RNAs mRNA 4 Translation Figure 7.8: Animated! Translation in eukaryotes. 1 In eukaryotic cells, RNA is transcribed in the nucleus. 2 Finished RNA moves into the cytoplasm through nuclear pores. 3 Ribosomal subunits and tRNA converge on an mRNA. 4 A polypeptide chain forms as the ribosome moves along the mRNA, linking amino acids together in the order dictated by the mRNA codons. 5 Many ribosomes can translate an mRNA at the same time. polypeptide Fig. 7-8, p. 122

Animation: Translation

The major differences between prokaryotic and eukaryotic protein synthesis

Overview of transcription and translation