Nucleic Acids and Protein Synthesis: DNA , RNA, and How Its Done

DNA Structure

• DNA Structure: • Deoxyribonucleic acid– Organic compound—nucleic acid• Only two nucleic acids (RNA and DNA)

– Polymer—large molecule made of repeating subunits

Building DNA Building Blocks

Nucleotide

Complimentarity

• Pyrimidines bond to purines– Adeninethymine (only)– thymineadenine– guaninecytosine (only)– cytosineguanine

• specific bonding called complimentary base pairing• creates a double helix

DNA Double Helix

DNA is Made of Two Long Chains of Nucleotides Joined by Hydrogen Bonds

G and C are complementary as are A and T

A Nucleotide

DNA Code

• Order of nitrogen bases determines the DNA code

• A—T• G—C• A—T• T—A• G—C• G—C

Complementary Base Pairing Allows Each Strand of DNA to Serve as a Template for DNA Replication

DNA is a perfect illustration of function following form (structure dictates function).

DNA Replication

DNA Replication: The process

• DNA Replication• DNA replication: the process of copying DNA

in a cell– The Replication Process• DNA strands unwind and separate– Separation point called the replication fork– The strands are separated by enzymes called DNA

helicases

DNA Replication

DNA Replication: The Process (cont.)

Accuracy and Repair

• Very accurate process• One error in every 10,000 nucleotides• One change of one nucleotide can cause big problems

—called mutation• Special enzymes check the DNA for accuracy and

correct the mistakes which keeps the error rate to 1/1,000,000,000 nucleotides• Some errors caused by damage to DNA from sunlight

or chemicals- called mutagens

Chapter 10

Click below to watch the Visual Concept.

Visual Concept

Comparing DNA and RNA

Section 4 Protein Synthesis

RNA: Ribonucleic Acid

• RNA Structure:– Single stranded– Ribose sugar in the

nucleotide– Does NOT have thymine,

but instead has the nitrogen base Uracil

– So: G—C and A—U (not A—T; that is only DNA)

Types of RNA– Messenger RNA (mRNA):

• Straight chain of RNA nucleotides• Copied from the DNA template• Carries information for protein synthesis to the cytoplasm

– Transfer RNA (tRNA): • Bent, “hairpin” shaped molecule• Assembles amino acids during protein synthesis

– Ribosomal RNA (rRNA): • Globular form of RNA • Makes up the ribosomes• Site of protein synthesis

DNA Transcriptions

– the process by which genetic information is copied from DNA to RNA

– mRNA serves as the messenger of the DNA code to the cell

– mRNA is made from the DNA template and sent from the nucleus to the ribosomes with the instructions for making a protein

RNA Transcription

Steps to transcription

• 1. Begins with the enzyme RNA polymerase • RNA polymerase binds to a special area of

the DNA called the promoter– The promoter marks the beginning of the gene– Only the gene involved in the specific protein

being made are transcribed into mRNA– mRNA’s are short sequences copied from parts of

the DNA (called transcripts)

Steps to Transcription

2. The DNA separates (unzips) at the gene• one of the strands will be transcribed—the

template strand

RNA Transcription

3. RNA polymerase attaches to the 1st nucleotide in the gene sequence and begins adding the complimentary base pairs to form the RNA molecule

• the order of the nucleotides in the DNA determines the sequence of the mRNA– complimentary base pairing occurs the same as

in DNA EXCEPT• A—U (the A on the DNA bonds to Uracil, not thymine;

but still G—C

RNA Transcription

4. Transcription continues until RNA polymerase reaches a termination signal

• Marks the end of the gene • 5. Piece of mRNA is released from the DNA

and the DNA “zips” back up

RNA Transcription

• 6. Piece of mRNA is moved through the nuclear pores to the rough ER and finally the ribosomes

– tRNA and rRNA are made in the same manner

Protein Synthesis

• The formation of proteins using the information coded on DNA and carried out of the nucleus by the mRNA

Protein Structure

– Made up of amino acids linked together into chains called polypeptides• 20 different amino acids occur in nature• for a protein to function properly it must be made

correctly• the structure is determined by the order, or sequence,

of amino acids in the polypeptides

– DNA holds the code that determines the order of amino acids, and so, the function of the protein

Genetic Code

• Codons: 3-letter words– Three sequential bases on the mRNA

• • Ex. • mRNA A U C G U G C A C

– Each codon codes for one specific amino acid.• Ex. AUC-- → Isoleucine• Ex. GUG-- → Valine• Ex. CAC-- → ______________histidine

Protein Synthesis

What amino acids are coded for in the following codons?

C G U G G U C A U

↓ ↓ ↓ Arginine Glycine Histidine What message do you get from codons UAA

and UAG?

Translation

• Translation: reading the DNA to put together the amino acids– Occurs in the cytoplasm at the ribosomes

Steps to Translation

mRNA moves out of the nucleus to the ribosome

• tRNA molecules transport amino acids to the mRNA.

• Each tRNA has a specific amino acid• Amino acids are free floating in the

cytoplasm• 20 different kinds of tRNA • anticodon: loop end of tRNA • has sequence of bases • complimentary to mRNA codon

Steps to Translation (cont)

• tRNA bonds to codon of mRNA with the anticodon and the amino acids are lined up in the correct order.

• Amino acids correctly arranged form peptide bonds and are released as a polypeptide when assembly is completed. Several polypeptides may be needed for one protein.

Translation

Mutation• Gene Mutation: a change in the DNA at a point

in the gene• • Can affect phenotype by changing the sequence

of DNA and, therefore, the resulting proteins.• • Ex. Codon CGA is replaced by CCA. The amino

acid in that chain would switch from arginine to proline. That would change the function of the protein.

Mutation: Point Mutations

• Types of gene mutation• Point mutation: one base on the DNA

changed.– Substitution: one base is substituted for another.

• Ex. • CCU AAA UUU GGG GGC • Becomes:• CCU UAA UUU GGG GGC

Mutation

Frame Shift: Addition

• Frame Shift: – Addition: a base is inserted into the sequence

• Ex. • CCU AAA UUU GGG GGC• Becomes• CCU AAA AUU UGG GGG C…..

Frame Shift: Deletion

• Deletion: a base is deleted from the sequence

• CCU AAA UUU GGG GGC• Becomes• CCU AAU UUG GGG GC…..

Mutation

Some mutation is good, too much is bad.

The effects of mutation are usually bad or neutral - only sometimes are mutations beneficial.

Cells employ elaborate mechanisms to prevent mutation – but the mechanisms aren’t perfect.

Mutations are the root cause of cancer (bad).

Mutations are the only way to introduce novel alleles into a species (good for evolution).

So, just like Goldilocks – not to hot, not too cold, just right – the optimal rate of new mutation is a balancing act.

Mutations• All of the above can occur randomly or be caused by:

• Mutagens: environmental factors that cause mutations.

• Sunlight (ultraviolet radiation skin cancer)• Chemicals (asbestos, cigarette smoke)• Viruses• Radiation• Radon• Carcinogens (PCB’s)

DNA Damage is Often the Root Cause of Mutation

DNA is chemically altered (i.e. damaged) spontaneously and by chemicals and radiation.

Mutation as Villain

Cancerous growths that result from loss of a protein that polices DNA for errors.

Cancer Incidence Increases Sharply with Age

The increase is due at least in part to the age-related accumulation of multiple mutations in single cells.