1

Higher Biology

DNADeoxyribonucleic Acid

Structure and Replication

2

DNA and Genes

A determines an organisms characteristics.

Genes are found on . Chromosomes are found in the

of all cells. Chromosomes are threadlike structures

made from .

DNA nuclei

chromosomes

gene

gene

chromosomes

nuclei

DNA

3

DNA Structure

In 1953 the structure of the DNA molecule was explained for the first time by two scientists in Cambridge- James Watson and Francis Crick.

They showed that DNA: Consists of two strands Each strand is made of repeating units (like

monomers) called nucleotides The two strands are antiparallel (run in

opposite directions) The two strands wrap around each other to

form a double helix.

4

DNA Structure

DNA structure 1

DNA structure 2

 

5

Nucleotides

Nucleotide structure

Insert and label the “nucleotide diagram”, from your pack, with the following:

Nitrogenous or organic base Deoxyribose sugar Phosphate

These structures are held together by strong chemical bonds.

6

Nucleotides

In DNA there are 4 different organic bases.

They are: Adenine Thymine Cytosine Guanine

7

Nucleotides

In DNA the bases ALWAYS bond (or pair) together according to the following rule

Adenine Thymine

Cytosine Guanine

The bonds that hold the complementary bases together are weak hydrogen bonds.

Insert and complete the “Structure of DNA” diagram from your diagram pack.

8

Quick Questions

1. What shape is the DNA molecule?2. What type of bonding holds the two

DNA strands together?3. Name the three components of a

nucleotide.4. Name the organic base which pairs with

cytosine.5. Name the organic base which pairs with

thymine.

9

Quick Questions

6. What do the following symbols represent?PSATGC

phosphate

sugar

adenine

thymine

guanine

cytosine

10

DNA Replication

During cell division DNA must be replicated (or copied) so that each daughter cell carries an identical set of instructions to those in the parent cell.

DNA is a unique molecule because it can copy itself exactly.

The replication is described as semi-conservative because each of the 2 resulting DNA molecules is composed of one original (or conserved) and one new strand.

11

DNA Replication

Replication of DNA is a fast but complex process that requires:

A DNA template Several enzymes including DNA

polymerase (the enzyme which joins new nucleotides to the growing DNA strand)

A supply of the 4 types of DNA nucleotides A supply of ATP to provide energy

Watch the DNA replication animation.

12

DNA Replication

Briefly the stages in DNA replication are:

1. The DNA molecule unwinds.2. The DNA molecule unzips (as the weak

hydrogen bonds, between complementary bases, break).

3. Free nucleotides align themselves with their complementary nucleotide on the open chain.

13

DNA Replication

4. New weak hydrogen bonds form between complementary bases (remember A-T, G-C).

5. Adjacent new nucleotides are linked through the sugar and phosphate molecules by strong chemical bonds to form the “backbone” of the new strand.

6. The new molecule winds up into a double helix.

14

DNA Replication

15

DNA Replication

Insert and complete the “DNA replication” diagram from your pack.

Use 2 different colours to draw a diagram to illustrate what is meant by semi- conservative replication.

16

Practice Questions

From the Torrance textbook:1. TYK page 47 Q42. TYK page 48 Q1-43. AYK page 49 Q1-6.