Chapter 4Sec 1 – What

does DNA Look Like?

DNA stands for…

Deoxyribonucleic acid

What does DNA do?

DNA stores and passes on genetic information from one generation to the next

1950sRosalind Franklin and Maurice Wilkins took x-ray

photographs of DNA

1953James Watson and Francis Crick determine that

DNA is a double helix

So saying that DNA is a double helix is the same as saying it’s like

a twisted ladder.

The Sides of the Ladder are made up of millions of alternating sugars

and phosphates.Sugar

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• The sugar is named deoxyribose.

• The two sides of the ladder are held together by rungs attached to the sugars.

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Rungs of ladder – each rung contains 2 bases •Bases contain nitrogen so we call them nitrogenous bases• The 2 bases of each rung are connected by a hydrogen bond•There are 4 bases to choose from:

AdenineThymineGuanineCytosine

Erwin Chargraff discovered that the amount of adenine always equals the amount of thymine, AND the amount of

guanine always equals the amount of cytosine

He concluded that a rung can either have A and T OR it can have C and G but no

other combinations are possible.

Complementary Base Pairing

Adenine always pairs with thymine. Guanine always pairs with cytosine. Thymine always pairs with adenine.Cytosine always pairs with guanine.

So, if you know ONE side of the DNA, you should be able to figure out the

other!

DNA – Master Molecule (15:20)

What is a nucleotide?A nucleotide is a small piece of DNA

which contains:•1 base•1 sugar

•1 phosphate

Lots of nucleotides link up together to make a big DNA molecule.

DNA Replication

• Process in which DNA makes an exact copy of itself

• Occurs during __________________________Interphase before Mitosis

DNA Replication is semi-conservative.

half

saved

1. When it is time to replicate, the DNA unzips (at the hydrogen bonds)

2. New complementary nucleotides move in to match BOTH halves of the DNA and form hydrogen bonds with the old nucleotides.

3. Product: 2 identical DNA molecules!

Steps of DNA Replication

Identical base sequences

Important:

The 2 new DNA’s are identical to each other AND identical to the original DNA because the bases are complementary.

DNA replication animation

Chapter 4Sec How DNA

Works

Protein Synthesis

DNA carries a code for each type of protein that

your cells need.

DNA is like the blueprints for a house.

How genes work

Genes carry the instructions for making proteins.Whenever a cell needs a particular protein, that gene

is triggered to make a protein.

There are 2 steps to making a protein:

1.Transcription – the gene for the protein that you want is copied in the nucleus so that the original DNA never has to leave the nucleus. The copy goes to the ribosome.

2.Translation – the copy of the gene is used by the ribosome to make a protein

Transcription is like taking the blueprints and copying a single section about one specific part of the house.

The copy of the gene leaves the nucleus and goes to the ribosome.

Translation

The second part of protein synthesis in which the amino acids that the

mRNA codes for are linked together in the ribosome.

Translation is like taking the single section of blueprints that you copied and using it to build one

portion of the house.

Translation: The ribosome reads the code on the copy of the gene and

assembles all of the amino acids

that the code calls for.

The amino acids link up together and make a protein.

Codon = 3 letters on mRNA

Anticodon = 3 complementary letters on tRNA (transfer RNA)

The anticodon on tRNA matches up with the codon on mRNA and brings with it the amino acid that the codon codes for!

Peptide Bond = bond that connects 2 amino acids

• As the tRNA’s bring amino acids to the ribosome, the amino acids connect to each other with peptide bonds.

• Many amino acids connected together makes a protein.

A U G G G C U U A A A G C A G U G C A C G U U

This is a molecule of messenger RNA.

It was made in the nucleus by transcription from a DNA molecule.

mRNA molecule

codon

A U G G G C U U A A A G C A G U G C A C G U U

A ribosome on the rough endoplasmic reticulum attaches to

the mRNA molecule.

ribosome

A U G G G C U U A A A G C A G U G C A C G U U

It brings an amino acid to the first three bases (codon) on the mRNA.

Amino acid

tRNA molecule

anticodon

U A C

A transfer RNA molecule arrives.

The three unpaired bases (anticodon) on the tRNA link up with the codon.

A U G G G C U U A A A G C A G U G C A C G U U

Another tRNA molecule comes into place, bringing a second amino acid.

U A C C C G

Its anticodon links up with the second codon on the mRNA.

A U G G G C U U A A A G C A G U G C A C G U U

A peptide bond forms between the two amino acids.

Peptide bond

C C G U A C

A U G G G C U U A A A G C A G U G C A C G U U

The first tRNA molecule releases its amino acid and moves off into the cytoplasm.

C C G U

A C

A U G G G C U U A A A G C A G U G C A C G U U C C G

The ribosome moves along the mRNA to the next codon.

A U G G G C U U A A A G C A G U G C A C G U U

Another tRNA molecule brings the next amino acid into place.

C C G

A A U

A U G G G C U U A A A G C A G U G C A C G U U

A peptide bond joins the second and third amino acids to form a polypeptide chain.

C C G C C G

A U G G G C U U A A A G C A G U G C A C G U U

The polypeptide chain gets longer.

G U C

A C G

The process continues.

This continues until a termination (stop) codon is reached.

The polypeptide is then complete.