biology 12 - dna mutations and expression

UNIT A: Cell Biology

Chapter 2: The Molecules of Cells

Chapter 3: Cell Structure and Function

Chapter 4: DNA Structure and Gene Expression: Sections 4.4, 4.5

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?

4.4 Gene Mutations and Cancer

A gene mutation is a permanent change in DNA sequence.• Germ-line mutations occur in sex cells and can be passed

on to future generations• Somatic mutations occur in body cells and are not passed

on to future generations

Both types of mutations may lead to the development of cancer.

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Causes of Mutations

• Errors in replication are mistakes made while DNA is copied. These are rare (1 mistake per billion nucleotide pairs)

• Mutagens are environmental factors, such as radiation, X rays, and some chemicals, that cause mutations

• Transposons are DNA sequences that move within and between chromosomes. Transposons can “jump” into another gene, causing a change in gene expression

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Figure 4.16 Transposon.

Effect of Mutations on Protein Activity

Gene mutations can have a range of possible effects on protein activity, from no effect to complete inactivity or even lack of production at all.

A point mutation is a single nucleotide change. It can cause•no change in amino acid sequence•a change in amino acid sequence that produces a protein that does not function properly •introduction of a stop codon, which shortens the protein

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Point Mutations

Figure 4.17 Point mutations in hemoglobin. The effect of a point mutation can vary. a. Starting at the top: Normal sequence of bases in hemoglobin; next, the base change has no effect; next, due to base change, DNA now codes for valine instead of glutamic acid, and the result is that normal red blood cells (b) become sickle-shaped (c); next, base change will cause DNA to code for termination and the protein will be incomplete.

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Nonfunctional Proteins

Frameshift mutations involve one or more nucleotides being added or deleted. This can cause a change in codons that are translated and production of a nonfunctional protein. •If the codons made a sentence, an example would be

THE CAT ATE THE RAT; deleting the C, becomes

THE ATA TET HER AT •Just as the meaning of the sentence is scrambled, a nonfunctional protein can have a dramatic effect on a phenotype

Many reactions in cells occur in a series called a pathway. If one protein (enzyme) is nonfunctional, it can affect the entire pathway of reactions.

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Mutations Can Cause Cancer

The development of cancer involves a series of accumulating mutations, which depend on the type of cancer. Most cancers follow a common progression. •They begin as a benign growth of abnormal cells•They can become a malignant tumour and spread to other areas

Figure 4.18 Progression of cancer.

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Characteristics of Cancer Cells

The primary characteristics of cancer cells:•Cancer cells are genetically unstable. Tumour cells have multiple mutations and can have chromosomal changes.•Cancer cells do not correctly regulate the cell cycle. The rate of division and number of cells increases.•Cancer cells escape the signals for cell death. Normal cell signals for programmed cell death do not occur.•Cancer cells can survive and proliferate elsewhere in the body. Invasion of new tissues can occur (metastasis), which includes new blood vessel formation (angiogenesis).

UNIT A Section 4.4

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UNIT A Section 4.4

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Check Your Progress

1. Explain how gene mutations occur.

2. Distinguish between a point mutation and a frameshift mutation.

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4.5 DNA Cloning

Genetic engineering involves altering the genome, or genetic material, of an organism. This often involves gene cloning, which is the production of copies of a gene. Gene cloning is done to

•study what biological functions a gene is associated with

•produce large quantities of protein

•produce transgenic organisms

•help cure human diseases

UNIT A Section 4.5

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Recombinant DNA Technology

Gene cloning involves introducing a gene into a vector (often a plasmid) to produce recombinant DNA (rDNA).•A restriction enzyme cleaves the vector and the gene, which combine by base pairing between the “sticky ends”

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Many restriction enzymes leave overhangs of nucleotides when they cut DNA, which are called “sticky ends” because they can easily base pair with other overhangs.

Recombinant DNA Technology

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• DNA ligase enzyme seals the gene and vector DNAs.

The rDNA is added to an organism such as bacteria, which makes many copies of the gene.

Gene Cloning

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Figure 4.19 Cloning a human gene. Human DNA and bacterial plasmid DNA are cleaved by a specific type of restriction enzyme. For example, human DNA containing the insulin gene is spliced into a plasmid by the enzyme DNA ligase. Gene cloning is achieved aftera bacterium takes up the plasmid. If the gene functions normally as expected, the product (for example, insulin) may also be retrieved.

The Polymerase Chain Reaction

The polymerase chain reaction (PCR) is a way of making billions of copies of a segment of DNA in a test tube. PCR involves three steps that are repeated many times in cycles.

1.Denaturation: The DNA is heated to 95oC, and it becomes single-stranded.

2.Annealing: The temperature is lowered to 50 60− oC, and primers are added that base pair to the DNA to be copied.

3.Extension: At 72oC, DNA polymerase used for PCR adds nucleotides to the ends of the primers. Eventually both DNA strands are copied and new double-stranded DNA forms.

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The Polymerase Chain Reaction

PCR is a chain reaction because the DNA is repeatedly copied. The amount of DNA doubles with each cycle.

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Figure 4.20 Polymerase chain reaction (PCR).

DNA AnalysisPCR has numerous applications, which includes identification of people based on their DNA fingerprint.•Short tandem repeat (STR) profiling identifies individuals according to how many repeats of a DNA sequence he or she has at a particular STR locus.

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Figure 4.21 The use of STR profiling to establish paternity.

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Check Your Progress

1. Summarize the two required steps for producing recombinant DNA.

2. Explain why STRs may be used for identification.

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biology 12 - dna mutations and expression

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