UNIT A: Cell Biology

Chapter 2: The Molecules of Cells

Chapter 3: Cell Structure and Function

Chapter 4: DNA Structure and Gene Expression: Section 4.3

Chapter 5: Metabolism: Energy and Enzymes

Chapter 6: Cellular Respiration

Chapter 7: Photosynthesis

In this chapter you will learn about the expression of an organism’s genes, a complex series of events involving genetic and environmental factors.

UNIT A Chapter 4: DNA Structure and Gene Expression

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Chapter 4: DNA Structure and Gene Expression

How does DNA store information that leads to the development, structure, and metabolic activities of organisms?

How are genes expressed?

4.3 Gene ExpressionGene expression relies on different forms of the

nucleic acid RNA (ribonucleic acid). The most important are•messenger RNA (mRNA)•transfer RNA (tRNA)•ribosomal RNA (rRNA)

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Chapter 4: DNA Structure and Gene Expression

Processes of Gene Expression

Gene expression requires two processes.•Transcription: mRNA is synthesized from a DNA template (in eukaryotes it takes place in the nucleus)•Translation: Protein is synthesized from an mRNA template (in eukaryotes it takes place in the cytoplasm)

Genetic information lies in the sequence of DNA bases, which is transferred to the sequence of bases in mRNA. This is translated to an amino acid sequence of a protein, which determines the protein’s structure and function. Proteins determine the phenotype of an organism.

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Chapter 4: DNA Structure and Gene Expression

A gene is a segment of DNA that acts as a template for RNA. •Messenger RNA (mRNA) is formed by transcription•Transcription begins by RNA polymerase binding to a promoter on DNA•RNA polymerase catalyzes incorporation of RNA nucleotides that are complementary to the DNA •The mRNA formed has a sequence of bases that is complementary to the DNA strand that was the template for transcription

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Chapter 4: DNA Structure and Gene Expression

Transcription

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Chapter 4: DNA Structure and Gene Expression

Figure 4.6 Transcription of DNA to form mRNA. During transcription, complementary RNA is made from a DNA template. At the point of attachment of RNA polymerase, the DNA helix unwinds and unzips, and complementary RNA nucleotides are joined together. After RNA polymerase has passed by, the DNA strands rejoin, and the mRNA transcript is released.

Transcription

Messenger RNA• In eukaryotes, the primary mRNA is processed to mature RNA before it is exported from the nucleus to cytoplasm

• Introns of the mRNA are removed, and the remaining sections of DNA to be translated, called exons, are joined

• A cap is added to one end of the mRNA and a poly-A tail is added to the other end of the mRNA

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Chapter 4: DNA Structure and Gene Expression

Figure 4.7 mRNA processing.

TranslationTranslation is the second process in gene expression. It requires several enzymes and different types of RNA molecules that include mRNA, rRNA, and tRNA.

Translation relies on the genetic code to convert mRNA sequence to the amino acid sequence of a protein. The genetic code

•consists of triplets of RNA nucleotides called codons•has codons that code for amino acids•is degenerate and almost universal (suggesting it dates back to first organisms on Earth and a common evolutionary history)

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Chapter 4: DNA Structure and Gene Expression

The Genetic Code

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Chapter 4: DNA Structure and Gene Expression

Figure 4.8 Messenger RNA codons. Notice that in this chart, each of the codons (in boxes) is composed of three letters representing the first base, second base, and third base. For example, find the box where C for the first base and A for the second base intersect. You will see that U, C, A, or G can be the third base. The bases CAU and CAC are codons for histidine.

Transfer RNAThe transfer RNAs (tRNAs) bring amino acids for protein synthesis to the ribosomes.

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Chapter 4: DNA Structure and Gene Expression

Figure 4.9 Transfer RNA: amino acid carrier.

• Each tRNA is single-stranded and folds back on itself to form a boot-like shape

• One end has an amino acid

• The other end has an anticodon that is complementary to an mRNA codon

Transfer RNAAlthough there are 64 different codons, there are only 40 different tRNA molecules. This is because the third nucleotide in some mRNA codons can vary, which is called the wobble effect.

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Chapter 4: DNA Structure and Gene Expression

For translation, the anticodons of tRNA-amino acid complexes pair with mRNA codons.

If the mRNA codon is CGG, what is the tRNA anticodon and amino acid attached to it?

Overview of Gene Expression

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Chapter 4: DNA Structure and Gene Expression

Figure 4.10 Overview of gene expression.

During transcription, the base sequence in DNA is copied into a base sequence in mRNA.During translation, tRNAs bring amino acids to ribosomes in an order determined by the sequence of codons in the mRNA.

Ribosomes and Ribosomal RNA

Ribosomes are composed of many proteins and ribosomal RNA (rRNA) and consist of large and small subunits.•They have binding sites for mRNA and three tRNAs, and help pair tRNA anticodons with mRNA codons

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Chapter 4: DNA Structure and Gene Expression

Figure 4.11a and b Polyribosome structure and function. a. Structure of a ribosome. b. Internal view of a ribosome showing the tRNA binding sites.

a b

Ribosomes and Ribosomal RNA

As ribosomes move along the mRNA, new tRNAs enter and the amino acids bond together, forming a protein. •Several ribosomes translate an mRNA at one time, forming a polyribosome

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Chapter 4: DNA Structure and Gene Expression

Figure 4.11c and d Polyribosome structure and function.

Translation Requires Three Steps

During translation, codons on the mRNA base pair with anticodons on the tRNAs that are carrying specific amino acids. The order of the mRNA codons determines the sequence of amino acids in the protein being synthesized.Translation is an orderly process that must produce a protein of a particular sequence. It involves three main steps•initiation (requires energy)•elongation (requires energy)•termination (does not require energy)

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Chapter 4: DNA Structure and Gene Expression

InitiationInitiation involves•assembly of components (initiation factors, small and large ribosomes, and mRNA)•a start codon, AUG, on the mRNA•tRNA with the first amino acid binding in the P site of the ribosome

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Chapter 4: DNA Structure and Gene Expression

Figure 4.12 Initiation During initiation, participants in the translation process assemble as shown. The start codon, AUG, codes for the first amino acid, methionine.

Elongation

Elongation is the protein synthesis step, when a polypeptide increases in length one amino acid at a time. Four steps are repeated until a codon that signals to stop is reached.

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Chapter 4: DNA Structure and Gene Expression

Figure 4.13 Elongation.

TerminationTermination is the final step in protein synthesis.•A stop codon on the mRNA indicates where termination occurs•A release factor cleaves the new protein from the tRNA•Components for translation disassemble•The protein begins to fold into its proper shape

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Chapter 4: DNA Structure and Gene Expression

Figure 4.14 Termination During termination, the finished polypeptide is released, as are the mRNA and the last tRNA.

Review of Gene Expression

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Chapter 4: DNA Structure and Gene Expression

A gene is expressed when a protein it codes for is made. It involvesA. Transcription•DNA serves as a template for mRNA synthesis. Bases in mRNA are complementary to the DNA. Each codon in mRNA is a triplet.B. In eukaryotes, the mRNA is processed and transported to the cytoplasmC. Translation•tRNAs with amino acids pair with mRNA codons until the protein is synthesized

Review of Gene Expression

Figure 4.15 Review of gene expression. Messenger RNA is produced and processed in the nucleus during transcription, and protein synthesis occurs at the ribosomes (in cytoplasm and rough ER) during translation.

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Chapter 4: DNA Structure and Gene Expression

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Chapter 4: DNA Structure and Gene Expression

Check Your Progress

1. Explain the roles of mRNA, tRNA, and rRNA in gene expression.

2. Describe the movement of information from the nucleus to the formation of a functional protein.

3. Discuss why the genetic code is said to be degenerate.

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Chapter 4: DNA Structure and Gene Expression

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Chapter 4: DNA Structure and Gene Expression