DNA

Structure and replication of DNA - syllabus content

Structure of DNA — nucleotides contain deoxyribose sugar, phosphate and base. DNA has a sugar–phosphate backbone, complementary base pairing — adenine with thymine and guanine with cytosine.

The two DNA strands are held together by hydrogen bonds and have an antiparallel structure, with deoxyribose and phosphate at 3' and 5' ends of each strand.

Chromosomes consist of tightly coiled DNA and are packaged with associated proteins.

Structure of DNA – what you should know

Structure of DNA

• Composed of nucleotides• Nucleotides contain deoxyribose sugar, phosphate and base. • DNA has a sugar–phosphate backbone,• Complementary base pairing — adenine with thymine and guanine with

cytosine. • The two DNA strands are held together by hydrogen bonds and have an

antiparallel structure, with deoxyribose and phosphate at 3' and 5' ends of each strand.

Chromosomes consist of tightly coiled DNA and are packaged with associated proteins.

DNADNA is a nucleic acid

Like other nucleic acids it is composed of smaller molecules called nucleotides

A DNA nucleotide molecule has 3 parts

• deoxyribose sugar

• phosphate

• a base

Deoxyribose sugar

phosphate

base

Diagram representing a DNA nucleotide

DNA bases

A DNA nucleotide can have one of 4 possible bases:

• Adenine (A)

• Thymine (T)

• Guanine (G)

• Cytosine (C)

Therefore there are 4 possible DNA nucleotides each with a different base

G A T C

DNA nucleotides are linked by covalent bonds between sugar and phosphate to form DNA strands

Position of sugar tophosphate bonds linking nucleotides to form a DNAstrand

A DNA molecule has two strands of nucleotidesThe two strands are antiparallel (run in opposite directions)Phosphate is found at the 5’ end of a strand and deoxyribose at the 3’ endThe two strands held together by hydrogen bonds between complementary basesAdenine bonds to thymineGuanine bonds to cytosine

The two strands are coiled into a double helix

Protein

DNA

Arrangement of DNA in Chromosomes

Chromosomes are made from tightly coiled DNA.The DNA is wrapped around molecules of protein.

DNA replication – syllabus content

Replication of DNA by DNA polymerase and primer.

DNA is unwound and unzipped to form two template strands.

DNA polymerase needs a primer to start replication and can only add complementary DNA nucleotides to the deoxyribose (3') end of a DNA strand.

This results in one strand being replicated continuously and the other strand replicated in fragments which are joined together by ligase.

DNA replication – what you should know

DNA molecule is unwound and hydrogen bonds break to allow the strands to separate DNA polymerase is the enzyme that joins nucleotides to form a DNA strandPrimer is a short section of joined nucleotidesPrimers bind to the separated DNA template strandsDNA polymerase can only begin replication by adding nucleotides to these short nucleotide strandsDNA polymerase can only add nucleotides to the 3’ end of a strandThis results in one new DNA strand, the leading strand, being formed continuously The other strand, the lagging strand, is formed in short fragments which are then joined together by the enzyme DNA ligase.

DNA replication Unwinding and unzipping the parent DNA molecule The DNA molecule is unwound by an enzyme.Another enzyme breaks the hydrogen bonds between base pairs and the two strands separate.The bases of each strand are now exposed at a Y shaped replication fork - the exposed bases act as templates for making new complementary DNA strands

Need for primers The enzyme DNA polymerase joins nucleotides to make a new DNA strand.This enzyme can only add nucleotides to a pre-existing nucleotide chain – so for DNA polymerase to join nucleotides a primer must be present. A primer consists of a short sequence of nucleotides.

Leading and lagging strands Since the two strands of the DNA molecule are antiparallel, one of the separated strands ends with a deoxyribose 3’ end (the complementary strand made from this is called the leading strand), the other strand ends with a 5’ end (the complementary strand made from this is the lagging strand)

5’ 3’

3’ 5’DNA copied here is the leading strand

DNA copied here is the lagging strand

DNA polymerase

Nucleotides are joined together to make a DNA strand by the enzyme DNA polymerase.DNA polymerase can only add nucleotides to the 3’ end of a DNA strand.

Formation of the leading DNA strand

3’

5’ 3’

5’

A primer forms at the 3’ end of the parental DNA strand.

Individual nucleotides for the new DNA strand form hydrogen bonds with their complementary bases on the DNA template strand.

This leading strand is formed continuously.

Individual DNA nucleotides bind by hydrogen bonds between bases to DNA template strand

DNA polymerase joins a nucleotide to the primer and adds the other nucleotides to the growing DNA strand

5’ end of primer

3’ end of primer

Primer binds to 3’ end of parent DNA strand

Hydrogen bonds

Formation of the lagging DNA strand Since DNA polymerase can only add nucleotides to the 3’ end of a strand, primers bind at various points to the DNA templateThe DNA template strand that has the 5’ end is replicated in fragments, each starting at the 3’ end of a primer.  When completed these fragments are joined together by an enzyme called ligase.‘the strand formed in this way is called the lagging strand.

The strand formed in this way is called the lagging strand.

Formation of the lagging strand

Since DNA polymerase can only add nucleotides to the 3’ end of a strand, primers bind at various points to the DNA template

The DNA template strand that has the 5’ end is replicated in fragments, each starting at the 3’ end of each primer.

When completed these fragments are joined together by an enzyme called ligase.

When DNA replication occurs

DNA replication occurs before a cell divides.It results in a copy being made of the genes so that each daughter cell receives a complete copy of the genetic information.

1 chromatid 2 identical

chromatids

result

DNA replication results in the chromatids being copied so that each chromosome has two chromatids when the cell is about to divide