A level Biology taster session DNA and DNA

A level Biology taster session – DNA and DNA replication Previously: You have learnt about the structure of DNA at GCSE (unit 3). You will build on this at A level and learn about how DNA is replicated. Today’s objectives: � To understand the structure of a nucleotide and the structure of DNA. � To understand how DNA is replicated using the semi-conservative method. Big Question: What is DNA and how is it replicated? Next lesson: Usually, you would go on to learn about the experiments carried out by two scientists called Meselson and Stahl to prove the semi-conservative method of replication. DNA can only be copied in one direction. Explain why the enzyme DNA polymerase can only work in one direction.

What can you remember from GCSE? Use the diagram to prompt your memory. Remember to use as many keywords as possible. • • • The shape of DNA is called a double helix. It is made of two sugar-phosphate backbones DNA contains complementary bases Adenine pairs with Thymine Cytosine pairs with Guanine The bases are held together by weak hydrogen bonds. At A level, you also need to know that: • The two backbones run anti-parallel (opposite directions) • A and T pair with 2 hydrogen bonds • C and G pair with 3 hydrogen bonds • The sugar-phosphate backbone is joined by a phosphodiester bond. • DNA has a 5’(prime) and 3’ end

What does a nucleotide look like? A DNA nucleotide contains • Deoxyribose sugar • A phosphate group. • A base (A, T, C or G) RNA is another type of genetic material. It is single stranded and much shorter than DNA. An RNA nucleotide contains • Ribose sugar • A phosphate group. • A base – in RNA, thymine (T) is replaced by uracil (U)

Task – Produce a table to summarise the differences between DNA and RNA. DNA RNA Shape Double helix, held by H bonds Single stranded Pentose sugar Bases Deoxyribose Ribose Size Adenine, thymine, cytosine, guanine Adenine, uracil, cytosine, guanine Long Relatively short

How does DNA replicate? • DNA copies itself before cell division (mitosis) so each new cell contains a full amount of DNA. • The method used is called semi-conservative replication because when new DNA is made, one strand is brand new and one strand is from the original parent DNA. What is happening in each diagram? DNA replication video

• Each original strand acts as a template for a new strand. • Free nucleotides are attracted to and pair with complementary exposed bases. • A pairs with T and C pairs with G • The enzyme helicase breaks the hydrogen bonds between the bases on the two polynucleotide DNA strands. • The helix unwinds to form two separate strands.

• A condensation reactions (water is removed) join nucleotides of the new strands together. • This is catalysed by the enzyme DNA polymerase. • Hydrogen bonds form between the bases of the original and new strands. • An enzyme called DNA ligase joins the nucleotides along the back bone. • Each new molecule of DNA contains one strand from the original DNA and one new strand. DNA can only be copied in one direction. Explain why the enzyme DNA polymerase can only work in one direction.

Quick quiz 1. The diagram shows a DNA nucleotide. Name parts A, B and C. 2. Name the 4 possible bases in DNA. 3. One base is replaced in RNA. Which base is replaced and what is it replaced with? 4. Name the sugar in RNA. 5. The diagram shows a molecule of DNA. Show the two strands that would be produced from this molecule if it is replicated by semi-conservative replication.

Quick quiz – How did you do? 1. The diagram shows a DNA nucleotide. Name parts A, B and C. A = phosphate, B = deoxyribose, C = base (nitrogen containing) 2. Name the 4 possible bases in DNA. Adenine, Thymine, Cytosine, Guanine. 3. One base is replaced in RNA. Which base is replaced and what is it replaced with? Thymine is replaced by Uracil. 4. Name the sugar in RNA. Ribose

Quick quiz – How did you do? 5. The diagram shows a molecule of DNA. Show the two strands that would be produced from this molecule if it is replicated by semi-conservative replication. In semi-conservative replication of DNA, each of the new molecules of DNA contains a strand of the original molecule (shown in black) and a new strand (shown in grey).

What do A level exam questions look like?

8 Phosphodiester bond

(Unwinding DNA and) breaking hydrogen bonds / bonds between chains / bases / strands. Accept H bonds Accept hydrolyses for breaks joins (adjacent) nucleotides OR forms phosphodiester bond / sugar-phosphate backbone; Reject forms hydrogen bonds (between nucleotides / bases)

Deoxyribose 18 32 If you know A = 18% and A pairs with T, there must be the same % of T. A&T make up 36% (18+18) so the rest is C and G. 100 – 36 = 64% 64 / 2 = 32%

DNA polymerase

Make sure you use both figures! 1. (Figure 1 shows) DNA has antiparallel strands / described; You need to use knowledge of enzymes from GCSE 2. (Figure 1 shows) shape of the nucleotides is different / nucleotides aligned differently; 3. Enzymes have active sites with specific shape; 4. Only substrates with complementary shape / only the 3’ end can bind with active site of enzyme / active site of DNA polymerase.

Stepping up to A level http: //www. swanwickhallschool. org/wp-content/uploads/2020/04/Biology-2020. pdf