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Nucleic Acids

RNA and Transcription

The Genetic Code

Protein Synthesis

Mutations

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Types of RNA

• Messenger RNA (mRNA)Carries genetic information for protein synthesis from DNA in nucleus to the ribosomes

• Transfer RNA (tRNA)Small molecules of 20 types that recognize and transfer amino acids for protein synthesis

• Ribosomal RNA (rRNA)Makes up 2/3 of ribosomes (1/3 protein) where protein synthesis takes place

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Transcription

• Genetic information contained

in nucleus• One strand of DNA is copied• Complementary bases build mRNA

DNA(1 strand) mRNA• In mRNA uracil (U) complements A (DNA)• New mRNA moves out of nucleus to

ribosomes in cytoplasm

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Genetic Code

• Needed to build a particular protein• The sequence of amino acids is coded by

the mRNA• Each triplet of bases along mRNA codes

for an amino acid• The triplet are called codons• Codons are known for all 20 amino acids• Some codons signal the “start” and “end”

of a polypeptide chain

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Codons and Amino Acids

Suppose that a section of a mRNA has the following series of bases.

CCU–AGC –GGA –CUU

Use a codon reference to determine the order of amino acids

CCU = Proline AGC = Serine GGA = Glycine CUU = Leucine

The mRNA codes for the peptide chain of Pro –Ser –Gly –Leu

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Learning Check G1

What is the order of amino acids coded for by a section of RNA with the base sequence GCC –GUA –GAC ?

GGC = Glycine GAC = Aspartic acid

CUC = Leucine GUA =Valine

GCC = Alanine CGC = Arginine

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Solution G1

GGC = Glycine GAC = Aspartic acid

CUC = Leucine GUA =Valine

GCC = Alanine CGC = Arginine

GCC –GUA –GAC

Ala – Val – Asp

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Activation of tRNA

Each tRNA binds to its specific amino acid

GGG

Pro

GGG

Pro

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Anticodons on tRNA

• A three-base sequence on each tRNA• Complements a triplet on mRNA

anticodon

codon on mRNA

GGG

CCC

Pro

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Initiation and Elongation

• mRNA attaches to a ribosome• tRNA with anticodon UAC binds to first codon

(AUG) to initiate synthesis• The second codon picks up a tRNA with the

proper anticodon. • A peptide bond forms between the amino

acids at the first and second codons.• The first tRNA detaches and the ribosome

shifts to the next codon on the mRNA

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Peptide Formation

Peptide starts to form

UAC AGA AGA GAG

• • • • • • • • • • • •

AUG UCU CUC UCU CUC UUU

Met Ser

Met

Ser Leu

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Termination

• Protein grows as tRNAs bring amino acids to the codons on the mRNA

• When all amino acids for a protein are linked the next codon is “stop”

• There is no tRNA for the “stop” codon• Protein synthesis ends• Protein released from ribosome

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Learning Check G2

Match the following processes in protein synthesis with the statements:

(1) Activation (2) Initiation

(3) Elongation (4) Termination

A. Ribosome moves along mRNA to add new amino acids to a growing peptide chain

B. Completed peptide chain released

C. A tRNA attaches to its specific amino acid

D. tRNA binds to the AUG codon of the mRNA on the ribosome

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Solution G2

Match the following processes in protein synthesis with the statements:

(1) Activation (2) Initiation

(3) Elongation (4) Termination

A. (3) Ribosome moves along mRNA to add new amino acids to a growing peptide chain

B. (4) Completed peptide chain released

C. (1) A tRNA attaches to its specific amino acid

D. (2) tRNA binds to the AUG codon of the mRNA on the ribosome

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Learning Check G3

The following section of DNA is used to build a mRNA for a protein.

GAA-CCC-TTT

A. What is the corresponding base sequence

on mRNA?

B. What are the anticodons for the tRNAs?

C. What is the amino acid order in the

peptide?

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Solution G3

GAA-CCC-TTTA. What is the corresponding base sequence

on mRNA? CUU-GGG-AAAB. What are the anticodons for the tRNAs?

GAA CCC UUUC. What is the amino acid order in the

peptide?Leu-Gly-Lys

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Mutations

• Caused by mutagens such as radiation and chemicals

• Alter the nucleotide sequence of DNA• Produce an incorrect series of codons in the

mRNA from the altered DNA• A different codon binds with a different tRNA• The protein has one or more wrong amino

acids in the peptide sequence• Can result in a defective protein or enzyme

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Examples of Genetic Diseases

Galactosemia

Cystic fibrosis

Downs syndrome

Muscular dystrophy

Huntington’s disease

Sickle-cell anemia

Hemophilia

Tay-Sachs disease