1. DNADNA

2. 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.

3. 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.

4. DNA DNA 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

5. 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

6. DNA nucleotides are linked by covalent bonds between sugar and phosphate to form DNA strands Position of sugar to phosphate bonds linking nucleotides to form a DNA strand A DNA molecule has two strands of nucleotides The two strands are antiparallel (run in opposite directions) Phosphate is found at the 5’ end of a strand and deoxyribose at the 3’ end The two strands held together by hydrogen bonds between complementary bases Adenine bonds to thymine Guanine bonds to cytosine

7. The two strands are coiled into a double helix

8. Protein DNA Arrangement of DNA in Chromosomes Chromosomes are made from tightly coiled DNA. The DNA is wrapped around molecules of protein.

9. 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.

10. 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 strand Primer is a short section of joined nucleotides Primers bind to the separated DNA template strands DNA polymerase can only begin replication by adding nucleotides to these short nucleotide strands DNA polymerase can only add nucleotides to the 3’ end of a strand This 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.

11. 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.

12. 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.

13. 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

14. 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 template The 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.

15. 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.

16. 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 chromatid2 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