24.5 nucleic acids > 1 copyright © pearson education, inc., or its affiliates. all rights...

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved..

24.5 Nucleic Acids 24.5 Nucleic Acids >>

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Chapter 24The Chemistry of Life

24.1 A Basis for Life24.2 Carbohydrates24.3 Amino Acids and Their Polymers24.4 Lipids

24.5 Nucleic Acids

24.6 Metabolism



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In this lesson, you will learn about molecules that are involved in the inheritance of traits from parents.

CHEMISTRY & YOUCHEMISTRY & YOU

Why do children often look similar to their parents?



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DNA and RNA

What are the functions of DNA and RNA?

DNA and RNADNA and RNA



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More than 100 years ago, a Swiss biochemist discovered a class of nitrogen-containing compounds in the nuclei of cells.


• The eventual understanding of the biological role of the compounds has led to a revolution in biochemistry.



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These nitrogen-containing compounds, called nucleic acids, are polymers that are found primarily in a cell’s nucleus.

• They are indispensable components of every living thing.

• Two kinds of nucleic acid are in cells—deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).




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DNA stores the information needed to make proteins and governs the reproduction and growth of cells and new organisms.

RNA has a key role in the transmission of the information stored in DNA and in the synthesis of protein.




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The monomers that make up the DNA and RNA polymers are called nucleotides.


• Nucleic acids are therefore polynucleotides.



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The monomers that make up the DNA and RNA polymers are called nucleotides.


• Nucleic acids are therefore polynucleotides.

• Each nucleotide consists of a phosphate group, a five-carbon sugar, and a nitrogen-containing unit called a nitrogen base.



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There are four different nitrogen bases in DNA—adenine, guanine, thymine, and cytosine.


• These four bases are abbreviated A, G, T, and C, respectively.

AdenineA

GuanineG

CytosineC

Thymine (in DNA)T



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Ribose, which has one more oxygen than deoxyribose, is the sugar found in the nucleotide monomers of RNA.

• The base thymine is never found in RNA.

• Instead, it is replaced by a fifth nitrogen base, called uracil, which is abbreviated U.




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Chemists studying nucleic acids discovered that the amount of adenine in DNA always equals the amount of thymine (A = T).


• Similarly, the amount of guanine always equals the amount of cytosine (G = C).



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James Watson and Francis Crick proposed that the structure of DNA consists of two polynucleotide chains wrapped into a spiral shape.

• This spiral is the famous double helix of DNA.




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Every double-ringed base on one strand must be paired with a single-ringed base on the opposing strand.

• The pairing of A with T and G with C not only provides the best possible fit; it also allows the maximum number of hydrogen bonds to form between the opposing bases.




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The pairing of A and T and of G and C makes for the most stable arrangement in the double helix.




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In DNA, why is A always paired with T and G with C?



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In DNA, why is A always paired with T and G with C?

These pairings make for the most stable arrangement of the double helix. They provide the best fit and allow for the maximum number of hydrogen bonds.



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The Genetic Code

How many bases of DNA are required to specify one amino acid in a peptide chain?

The Genetic CodeThe Genetic Code



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An organism contains many proteins that are characteristic of that particular organism.

• The proteins of earthworms are different from the proteins of pine trees, which are different from the proteins of humans.

• How do cells in a given kind of organism know which proteins to make?

• Cells use instructions contained in the organism’s DNA.




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A gene is a segment of DNA that carries the instructions for making one peptide chain.

• Thus, the products of genes are the peptides and proteins found in an organism.

• You can think of DNA as a reference manual that stores the instructions for building proteins.

• The instructions are written in a simple language that has 4 “letters”—the bases A, T, G, and C.




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Experimental data show that each “word” in a DNA manual is exactly three letters in length.

• Each three-letter base sequence, or triplet, codes for one of the 20 common amino acids.

• The code words are strung together in the DNA molecule to form genes, which specify the order of amino acids in peptides and proteins.




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Three bases of DNA arranged in a specific sequence are required to specify one amino acid in a peptide or protein chain.




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Some Three-Letter DNA Code Words for the Amino Acids

Second Letter in Code Word

A G T C

A

AAA PheAAG Phe

AAT LeuAAC Leu

AGA SerAGG Ser

AGT SerAGC Ser

ATA TyrATG Tyr

ATT EndATC End

ACA CysACG Cys

ACT EndACC Trp

AG

TC

G

GAA LeuGAG Leu

GAT LeuGAC Leu

GGA ProGGG Pro

GGT ProGGC Pro

GTA HisGTG His

GTT GlnGTC Gln

GCA ArgGCG Arg

GCT ArgGCC Arg

AG

TC

C

CAA ValCAG Val

CAT ValCAC Val

CGA AlaCGG Ala

CGT AlaCGC Ala

CTA AspCTG Asp

CTT GluCTC Glu

CCA GlyCCG Gly

CCT GlyCCC Gly

AG

TC

Fir

st L

ette

r in

Co

de

Wo

rdT

hird

Letter in

Co

de W

ord

Interpret DataInterpret Data



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Note that most amino acids are specified by more than one code word, but a code word never specifies more than one amino acid.


• One of the code words (TAC) signifies the initiation of a peptide.

• Three code words (ATT, ATC, and ACT) are reserved as end, or termination, code words.



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The translation of a base sequence of DNA in a gene into the amino acid sequence of a peptide begins with the initiation code word and runs continuously until a termination code word is reached.

• The termination code word signals a stop to the addition of amino acids in the production of the peptide, similarly to what a period does at the end of a sentence.




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Even with only four bases, the number of possible sequences of nucleotides in a DNA chain is enormous.


• The sequence of the nitrogen bases A, T, G, and C in the DNA of an organism constitutes the genetic plan, or blueprint, for that organism.

• The genetic plan is inherited from parents and passed to offspring.

• Differences in the number and sequence of the bases in DNA ultimately are responsible for the great diversity of living creatures found on Earth.



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Children often look similar to their parents because genes are responsible for the traits that are expressed, and parents pass their genes on to their children.




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What DNA code words specify the amino acid leucine (Leu)? What amino acids does the code word GAG specify?



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What DNA code words specify the amino acid leucine (Leu)? What amino acids does the code word GAG specify?

AAT, AAC, GAA, GAG, GAT, and GAC specify the amino acid leucine. GAG specifies leucine only. It never specifies any other amino acids.



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Gene Mutations

What are gene mutations?

Gene MutationsGene Mutations



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When a change occurs in a DNA code word, the result is a mutation in the DNA.


Substitutions, additions, or deletions of one or more nucleotides in the DNA molecule are called gene mutations.



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The effect of the deletion of a single base from a gene can be illustrated by the following analogy.

• Suppose a string of letters of the alphabet goes as follows:

PATTHEREDCAT




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PATTHEREDCAT

• The letters may not make sense at first glance. However, if you separate them into three-letter words, they form a perfectly sensible statement:

PAT THE RED CAT




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PATTHEREDCAT

• Now delete the first letter and again separate the string into three-letter segments:

ATT HER EDC AT

• This last sequence is nonsensical.

• Similarly, the deletion of a base in the DNA base sequence can turn the information into nonsense.




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Such mutations might result in the production of a faulty protein or of no protein at all.

• Diseases that result from gene mutations are called genetic disorders.

• Thousands of genetic disorders have been identified.




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Galactosemia is an example of a genetic disorder that affects about 1 in 55,000 newborn babies.

• Galactosemia results from a mutation in an enzyme called GALT (galactose-1-phosphate uridyl transferase).

• GALT is needed to break down the sugar galactose into glucose.

• Without normal GALT, galactose can build up in the body and cause kidney failure, an enlarged liver, cataracts, and brain damage.




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• The only treatment is to avoid foods containing galactose, like dairy and dried beans.


Persons with galactosemia cannot complete the breakdown of lactose.



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Not all gene mutations are harmful.

• Occasionally, a mutation can result in the synthesis of a protein that is more efficient than the version that previously existed.

• Such a mutation could thus be beneficial to the survival of the affected organism.




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Using the sequence PAT THE RED CAT as the unmutated sequence, what mutations have occurred in the following sequences?

ATT HER EDC AT :

PAT THR RED CAT:

PAT STH ERE DCA T:



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Using the sequence PAT THE RED CAT as the unmutated sequence, what mutations have occurred in the following sequences?

ATT HER EDC AT : deletion (of p from pat)

PAT THR RED CAT: substitution (of e from the)

PAT STH ERE DCA T: addition (of s after pat)



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DNA Technologies

What are two examples of DNA technologies used today?

DNA TechnologiesDNA Technologies



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The sequences are similar for members of the same family but are slightly different for almost every individual.

• Identical twins look similar because they have identical DNA.

DNA Typing




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DNA typing uses the variation in the DNA of individuals as a basis for creating DNA profiles to identify a person from samples of his or her hair, skin cells, or body fluid.


DNA Typing



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DNA typing uses the variation in the DNA of individuals as a basis for creating DNA profiles to identify a person from samples of his or her hair, skin cells, or body fluid.


DNA Typing

• Because DNA sequences, like fingerprints, are unique for each individual, DNA typing has also been called DNA fingerprinting.



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To construct a DNA profile, scientists first isolate the DNA in a sample.

• Only a tiny sample is needed.

• A sample can be anything that contains DNA, including teeth, fingernails, blood, hair, saliva, and skin cells.


DNA Typing



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Samples can be typed in several different ways, but the method used most commonly is short tandem repeat (STR) analysis.

• A short tandem repeat is a short segment of DNA that is repeated several times.

• For example, one region of the DNA used by the FBI contains repeats of the sequence AGAT.


DNA Typing



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The DNA profile can then be compared with a sample of DNA from a known individual.


DNA Typing



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The FBI has a technology system called the Combined DNA Indexing Systems (CODIS) that allows laboratories throughout the country to share and search DNA profiles.


DNA Typing

• The chances of two people (except for identical twins) having the same DNA profile for these 13 regions is 1 in 1 billion.



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Recombinant DNA technology consists of methods for cleaving a DNA chain, inserting a new piece of DNA into the gap created by the cleavage, and resealing the chain.

• The altered DNA formed by this method is known as recombinant DNA.


Recombinant DNA Technology



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In this experiment, DNA from one organism is inserted into the DNA of a different organism.





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Applications in Medicine

The first practical application of recombinant DNA technology was to insert the gene for making human insulin into bacteria.



• Insufficient insulin production results in diabetes.

• The symptoms of diabetes can often be controlled by insulin injections.



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In the past, human insulin was not available for this purpose. Pig insulin, which is quite similar, was used as a substitute.



• Today, diabetic patients use the human form of insulin produced by bacteria that have been altered by recombinant DNA technology.

• Use of this insulin removes the need for the potentially dangerous use of pig insulin.



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Other proteins produced by recombinant DNA technology are used as medicinal drugs.



• An enzyme called tissue plasminogen activator (TPA) is used to dissolve blood clots in patients who have suffered heart attacks.

• Interferon is thought to relieve or delay some of the debilitating effects of multiple sclerosis.



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Recombinant DNA technology is also being applied to the cure of genetic disorders in an experimental treatment known as gene therapy.





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Applications in Agriculture

New recombinant DNA techniques can make plants resistant to pests and weed killers and produce fruits and vegetables that are better suited for shipping and storage.



• The most common traits in genetically modified crops are herbicide resistance and insect resistance in corn, cotton, soybean, and canola.



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Crops have also been genetically modified for pharmaceutical purposes.



• These so-called pharma crops are genetically modified to produce drugs to treat or prevent diseases such as cancer or AIDS.



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Genetically altered organisms have many potential benefits, but some people have concerns about their safety.



• There is also concern that genetically modified crops could contaminate other crops if they are grown and processed in close proximity to them.



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Cloning

Ethical concerns were raised in 1997 when Scottish scientists announced the birth of a lamb named Dolly.



• In normal animal reproduction, an offspring is a genetic mixture of the characteristics of both parents.– Dolly was a clone—an offspring of a single individual.



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Cloning

Polly and Dolly had no fathers.





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Cloning

The birth of cloned animals has raised the question of whether humans might eventually be cloned.



• Many people are concerned about some of the possible outcomes of cloning identical individuals.

• These situations are one aspect of more general concerns about the uniqueness of life.



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Which of the following is not an example of DNA technology?

A. Blood typing

B. DNA typing

C. Cloning

D. Genetically modified crops



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Which of the following is not an example of DNA technology?

A. Blood typing

B. DNA typing

C. Cloning

D. Genetically modified crops



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DNA stores information needed to make proteins and governs the reproduction of cells. RNA transmits information stored in DNA during protein synthesis.

A sequence of three bases of DNA is required to specify one amino acid in a peptide.

Key ConceptsKey Concepts



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Gene mutations occur when one or more nucleotides in DNA are substituted, added, or deleted.

Examples of DNA technology include DNA typing, producing bacteria that make human proteins, genetically modifying foods and animals, and cloning.

Key ConceptsKey Concepts



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• nucleic acid: a polymer of ribonucleotides (RNA) or deoxyribonuclotides (DNA) found primarily in cell nuclei; nucleic acids play an important role in the transmission of hereditary characteristics, protein synthesis, and the control of cell activities

Glossary TermsGlossary Terms



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Glossary TermsGlossary Terms

• nucleotide: one of the monomers that make up DNA and RNA; it consists of a nitrogen-containing base (a purine or pyrimidine), a sugar (ribose or deoxyribose), and a phosphate group

• gene: a segment of DNA that codes for a single peptide chain



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• Nucleic acids are polymers of nucleotides.

• The nucleic acid DNA carries the instructions for a cell.

BIG IDEABIG IDEA

Chemistry as the Central Science



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END OF 24.5END OF 24.5

24.5 nucleic acids > 1 copyright © pearson education, inc., or its affiliates. all rights...

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