The Fourth Macromolecule!!!

Objectives:1. Describe the structure and function of DNA and RNA

2. Explain how DNA replicates itself3. Explain the purpose of DNA replication

There are two types of nucleic acid

1. DNA

2. RNA

1. DNA: deoxyribonucleic acid

• The genetic information of a cell

• Located in the nucleus

• Works with RNA to regulate the order of amino acids in proteins

• Made of 4 nucleotides: adenine, thymine, cytosine, guanine

• In DNA, the central sugar is a deoxyribose

Nucleotides join together through covalent bonds …

… And join together with another chain of nucleotides through hydrogen bonds

• DNA consists of two chains of nucleotides joined together in the middle by hydrogen bonds between the nitrogenous bases.

• Adenine pairs with Thymine A ====== T hydrogen bonds)

• Cytosine pairs with Guanine C ====== G hydrogen bonds)

• these pairs are called “complementary base pairs” meaning that they have shapes that fit together and bond

Question

• Give the complementary base pairing for the following sequence…

AGT TAA CCG

We know that to function as a genetic material, DNA has to be able to do three things:

1) replicate (duplicate) itself so that it can be transmitted to future generations

2) store information

3) undergo mutations (changes) that provide genetic variability (to account for the variety in living things)

• Before a cell divides, DNA must replicate or make an exact copy of itself.

• Each of the two new cells that will be produced after cell division occurs, will receive one complete set of DNA.

• Each strand serves as a template for the building of a new strand

• Each original strand produces a new strand of DNA beside itself by complementary base pairing

• The result is two new DNA double helices, identical to each other and to the original DNA molecule (two copies)

• DNA is making an exact copy of itself

Figure 10.6

Parental (old)DNA molecule

Daughter(new) strand

DaughterDNA molecule(double helices)

• DNA replication begins at multiple sites along the DNA called origins of replication: 1. DNA unwinds and unzips. Each strand will now

serve as a template (blueprint or mold)  2. Free nucleotides present in the nucleus are fitted

into place beside each original strand following the complementary base pairing rule

3. These new nucleotides are joined together into a strand by an enzyme called DNA polymerase. 

4. When the process is completed the result is two identical DNA molecules being formed.

Figure 10.8

Origin ofreplication

Origin ofreplication

Origin ofreplication

Parental strand

Daughter strand

Bubble

Two identical daughter DNA molecules

http://www.dnatube.com/video/365/DNA-Replication

• Mistakes in base pairing rarely occur during DNA replication

• Normally a DNA repair enzyme checks each base pair and repairs any mistakes, by removing the wrong base and inserting the correct one

• A mutation is an error in DNA that persists causing a permanent change in the sequence of bases

• A mutation can cause a change in the protein produced and ultimately a change in the trait controlled by that gene.

• This may be harmless or serious (disease causing)

• Mutations can also be caused by exposure to toxic chemicals, ionizing radiation, ultraviolet light (sun exposure) or viruses.

• If the sequence of DNA bases on one strand of the double helix is CGT-AAT-CGC-TTA, what will be the complementary sequence on the other side of the double helix?

Your Task

• Draw a diagram illustrating the process of DNA replication.

• Include in your diagram:– 1. The correct structure of DNA (labeled)– 2. The steps involved in DNA replication

(labeled)– 3. Show how the free nucleotides in the

nucleus pair up with their complementary base on each template strand of DNA

Plenary…

1. What are the three major groups of chemicals in DNA?

2. If DNA is only composed of three types of chemicals, why are living things so different from each other?

3. What are the base pairs in DNA?

But…what’s so important about DNA?

• In a cell all functions are carried out by proteins: control of cell reproduction, production of energy, production of hormones, production of digestive enzymes, etc.

• DNA contains genes: the blueprints for those proteins

• Each gene is an assembly guide (think IKEA) on how to put the protein together