BIOLOGY UNIT 1 K2

Structure and Replication of DNA

What do nucleotides consist of ?

• Phosphate

• Deoxyribose sugar

• base

What does DNA consist of ?

DNA consists of repeating units called nucleotides twisted into a double stranded helix

Where do the base and phosphate attach to a deoxyribose sugar in a nucleotide ?

Deoxyribose sugar in a nucleotide has a base attached to its first (1st) carbon and a phosphate attached to its fifth (5th) carbon

What does the base sequence of DNA form ?

The base sequence of DNA forms the genetic code

What are nucleotides joined by ?

Nucleotides are joined by their deoxyribose sugar and phosphates to form a stand with a sugar phosphate backbone

What are the base pairs ?

Adenine - Thymine 

Cytosine - Guanine 

What are base pairs of opposite strands held together by ?

Base pairs on opposite stands are held together by weak hydrogen bonds between complementary base pairs

What is the double stranded anti-parallel structure ?

The double stranded anti-parallel structure is the strands of DNA (2) whose backbones run in opposite directions and each strand has a deoxyribose sugar at 3’ end and a phosphate group at the 5’ end

What part of the nucleotide is at the 3’ end and what part is at the 5’ end ?

A deoxyribose sugar is at the 3’ end and. phosphate group is at the 5’ end

5’ ………………………………………… ………. ... 3’

3’ ………………………………………5’

What do the lines between the base pairs represent ?

Weak hydrogen bonds

What are primers ?

A primer is a short strand of nucleotides which bind to the 3’ end of the template DNA strand allowing DNA polymerase to add nucleotides

What is needed to start DNA replication and why ?

Primers are needed to start DNA replication as DNA polymerase can only add nucleotides in one direction - to the 3’ end of the new strand

Name boxes 1 , 2 and 3

1. DNA polymerase adding nucleotides continuously

2. primer

3. 3’ end of template strand

What are the requirements for DNA replication ?

DNA replication will take place if the nucleus contains

• DNA template

• Free DNA nucleotides

• Primers

• Enzymes (DNA polymerase and DNA ligase)

• Supply of ATP

What are the stages of DNA replication ?

1. The DNA molecule unwinds

2. Hydrogen bonds between base pairs break to form two template strands 

3. A primer binds to the 3’ end of the original template strand 

4. DNA polymerase enzyme adds DNA nucleotides using complementary base pairing, to the 3’ end of the new DNA strand 

5. This results in the leading strand replicating continuously 

6. On the lagging strand, nucleotides are added by DNA polymerase to form DNA fragments 

7. Ligase enzyme joins DNA fragments on the lagging strand to form a complete strand

Name boxes 1, 2 and 3

1. primer

2. DNA polymerase adding nucleotides

3. ligase joining DNA fragements

Which strand replicates continuously and which strand replicates in fragments

• The leading strand replicates continuously

• The lagging strand replicates in fragments

What is the Polymerase Chain Reaction (PCR)?

Polymerase Chain Reaction (PCR) is a laboratory technique that amplifies specific target sequences of DNA

What does PCR use and why ?

PCR uses primers which are complementary to specific target sequences at the two ends of the region of DNA to be amplified 

What are the stages of PCR ?

1. DNA is heated to between 92 and 98*C to separate the DNA strands ( breaking hydrogen bonds )

2. It is then cooled to between 50*C and 65*C to allow primers to bind to target sequences

3. It is then heated to between 70*C and 80*C for heat tolerant DNA polymerase to replicate the target region of DNA

4. Two identical copies of DNA after the first cycle . Repeated cycles of heating and cooling amplify the target region of DNA and allow billions of copies of the target sequence to be produced

What can PCR amplify DNA to help?

• solve crimes

• settle paternity suits

• diagnose genetic disorders

How many molecules are there after one cycle of PCR ?

Thera would be two molecules as the number continues to double with every cycle