DNA Replication and Protein Synthesis

I. DNA – Deoxyribonucleic Acid

Genetic Code – Way that cells store info (in nucleus) to be passed to the next generation.

II. DNA Structure

• DNA is a macromolecule made up of nucleotides.

Each nucleotide molecule has 3 subunits:Phosphate Group5-carbon sugarNitrogen base

Four Possible Nitrogen Bases:

1. Adenine

2. Guanine

3. Thymine

4. Cytosine

A & G: purines T & C: pyrimidines

Nucleotides are joined to form DNA molecules.

NOTE: The sugars and phosphates make up the backbone of DNA. (“siderails” of a ladder)

The Discovery of DNA

http://www.youtube.com/watch?v=sf0YXnAFBs8&feature=autoplay&list=PL4900A106005340D0&lf=results_video&playnext=2

Rosalind Franklin (1952)

Took an X-ray of the DNA molecule that led to the discovery of its shape.

Watson & Crick (early 1950’s) = Construct the DNA Double Helix Model.

Chargaff

Discovered “Base Pairing Rules.”

Adenine always pairs with Thymine (2 H bonds)

Cytosine and Guanine always pair (3 H bonds)

ALWAYS pyrimidine + purine

III. DNA Replication

The 2 strands of DNA are complementary.(fit together like puzzle pieces)

Each strand can serve as a pattern, to put together the sequence of bases on the other half.

Why must a cell duplicate its DNA???

DNA replication occurs before the cell divides so that each cell has

a complete copy of DNA.

1ST: DNA helicase unzips and unwinds the 2 strands by breaking H-bonds.

*Where DNA separates, it makes a Y-shape (replication fork)

2nd: DNA polymerases start at replicaton forks, adding new nucleotides and bases to the exposed strands

DNA polymerases also “proofread” the bases to make sure they are paired correctly.

(Usually only 1 mutation per 1 billion bases occurs.)

3rd: 2 identical molecules of DNA are produced. Each is made up of:

ONE ORIGINAL STRAND

ONE NEW STRAND

Original DNA Strand:

A-T-T-C-G-C-T-A-G-T-A

Complementary strand:

___________________________

IV. Introduction to RNA

Gene expression: turning genes into specific traits, done by RNA.

1st step: copy part of the DNA sequence into RNA (Ribonucleic Acid). TRANSCRIPTION

2nd: information in RNA is used to make a specific protein TRANSLATION

Structure of RNA RNA molecules are chains of

nucleotides. RNA vs. DNA

1. Sugar in RNA is ribose, DNA is deoxyribose.

2. It is single-stranded.3. 4 Nitrogen bases are:

AdenineGuanineCytosineUracil replaces Thymine

NOTE! All base-pair rules are followed:

C = G

A = U, since T is gone

3 Types of RNA

• Messenger RNA (mRNA) – made when DNA is transcribed into RNA

• Transfer RNA (tRNA) – reads the mRNA during translation, translates it into amino acids.

• Ribosomal RNA (rRNA) – RNA found in ribosomes

Transcription: RNA Synthesis

A. Copies DNA into mRNA

B. The RNA molecule will be COMPLEMENTARY to the DNA molecule, NOT identical –b/c it is in “RNA” language.

C. Carried out by RNA polymerase

Steps of Trans.• RNA polymerase binds to “promoter

” sequence (starting point on DNA)

• RNA poly unwinds & separates DNA.

• Complementary RNA bases are added to DNA bases, makes single mRNA strand

• mRNA carries genetic info from DNA out to the ribosomes, where proteins are made

Transcription Practice!

DNA = A-T-C-T-G-T-T-A

RNA = ?

The DNA code is determined by how the 4 nitrogen bases are arranged.

Why does DNA code for proteins and NOT other molecules??• Because proteins control the

biochemical pathways in cells.

II. Nature of the Genetic CodeGenes are “directions” that

help make proteins (made up of a.a.)

Each combination of 3 nucleotides on a strand of mRNA is called a CODON, or 3-letter code word.

Ex. C-U-G, A-C-U are both codons.

Each codon specifies an amino acid that is to be placed in the protein chain.

See page 307 in your book.

Translation – Process of using the info in mRNA to put together amino acids and make proteins.- Occurs in a series of steps, involves, 3 kinds of RNA, and results in a polypeptide.

In Translation:1. Ribosome, mRNA and

tRNA which is carrying the amino acid, methionine, bind together.

2. New tRNA arrives and binds to next codon on mRNA.

3. First tRNA detaches and leaves its amino acid,

4. Process ends when the “stop” codon is reached.

5. Amino acid chain is released and ribosome complex falls apart.

NOTE!!! tRNA knows which amino acids to bring b/c its anticodon will be complementary to the mRNA codon.