2. Structure and Replication of DNA. Higher Human Biology

Transcription

1 2. Structure and Replication of DNA Higher Human Biology

2 Learning Intention Describe the structure of DNA Explain the base pairing rule using adenine, thymine, cytosine and guanine

3 Today we are moving onto look at DNA Write down everything you remember about DNA! You have 5 minutes

4 DNA Structure DNA consists of units called nucleotides. Nucleotides are made of 3 parts:

5 Deoxyribose Sugar The deoxyribose consists of 5 carbon atoms:

6 Nucleotides There are 4 different types of nucleotide, depending on the base that they have:

7 Base Pairing Certain bases are complementary to each other - A only pairs with T and C only pairs with G.

8 Base sequence The base sequence of DNA forms the genetic code.

9 DNA Strand There is a strong chemical (covalent) bond between the phosphate and sugar and bases on opposite strands are held together by weak hydrogen bonds Strong chemical bond Weak hydrogen bond

10 DNA Molecule We call the structure of DNA a double helix. glow clip

11 Antiparallel Strands (running in opposite directions) A DNA strand s 3 end has a deoxyribose sugar and the 5 end has a phosphate. Nucleotides can only be added to the 3 end of the original template strand. The 2 DNA strands have their sugar-phosphate backbones running in opposite directions. 3" 5" explained

12 Anti-Parallel The double helix is described as having two anti-parallel chains of nucleotides because one side goes from 5 to 3 and the opposite side goes from 3 to 5.

13 Learning Intention Outline the stages of DNA replication, including the role of DNA polymerase and DNA ligase.

14 Lesson starter 1. What does DNA stand for? 2. What is DNA? 3. Where is it found specifically in the body? 4. How do we describe the structure of DNA? 5. What 3 things make up a nucleotide? 6. What does the D in DNA stand for? Why? 7. Name the 4 bases and state the rule! 8. What is a chromosome and name 3 parts 9. DNA codes for what?

15 LESSON STARTER 1. What is the difference between cell reproduction and cell differentiation? Cells reproduce themselves by repeated mitosis whilst remaining undifferentiated Cells differentiate into a diverse range of specialised cells when required to by the organism that possesses them. E.g. unspecialised specialised 2. What are somatic cells? Give an example Somatic cells - Somatic cells are the differentiated cells that form the different types of body tissue that exist (divide by mitosis). Muscle cells form muscle tissue 3. What are germline cells and what can they become? Germline cells - Germline cells include the sex cells or gametes and the cells that produce the gametes called gamete mother cells

16 Starter Draw and label a DNA nucleotide (including numbering the carbons). Copy the DNA strand below and draw its complementary strand: A A T G G C T A C G T A C T A C A

17 What s the Question? 1. The 5 end 2. The 3 end 3. Hydrogen bond 4. Protein

18 Give an account of cell differentiation under the following headings. (i) Stem cells (4) (ii) Somatic cells (4) (iii) Germline cells (2)

19 (b) DNA REPLICATION

20 Replication of DNA by DNA polymerase and primers Prior to cell division, DNA is replicated by a DNA polymerase. DNA polymerase needs primers to start replication. A primer is a short strand of nucleotides which binds to the 3' end of the template DNA strand allowing polymerase to add DNA nucleotides. Primer DNA Polymerase Nucleotides bound to 3 end Cell division

21 Step 1 DNA is unwound and hydrogen bonds between bases are broken to form two template strands.

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23 Step 2 Primer allows DNA Polymerase to add DNA nucleotides, using complementary base pairing (A-T, G-C), to the deoxyribose (3') end of the new DNA strand

24 Step 3 DNA polymerase can only add DNA nucleotides in one direction resulting in the leading strand being replicated continuously and the lagging strand replicated in fragments. DNA Polymerase an enzyme that adds nucleotides to the 3 end of a template strand and also binds the sugar and phosphate of the new strand

25 **DNA IS ALWAYS BUILT FROM THE 5 END 3 END ** NUCLEOTIDES CAN ONLY BE ADDED TO THE 3 END OF THE TEMPLATE STRAND tch?v=vnxfk_d6y80 m/watch?v=cdwjtlng Eyw Primer

26 5 This is the parent opening like a zip 3 UNZIP PING POINT HERE This can build continuously in the correct direction 5 3 TOWARDS the point where the unzipping point This 5 3 Can only be built 5 3 AWAY from the unzipping point so must be built in sections which are glued together

27 5 This is the parent opening like a zip 3 UNZIP PING POINT HERE This can build continuously in the correct direction 5 3 TOWARDS the point where the unzipping point This 5 3 Can only be built 5 3 AWAY from the unzipping point so must be

28 On the leading strand The enzyme DNA polymerase controls the sugarphosphate bonding between nucleotides on the new DNA strand. This will only work if it has two nucleotides to join. Therefore a primer is needed to initiate DNA replication. A primer is a short strand of complementary nucleotides formed at the 3 end of the parental DNA strand. DNA replication on this strand is continuous

29 On the lagging strand DNA polymerase can only add nucleotides to the free 3 end of a growing strand. Therefore the DNA on the parental strand that has the 5 end exposed is replicated in fragments starting at the 3 end of a primer. STEP 4 As the replicated strand is discontinuous, the enzyme LIGASE joins the fragments together

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31 A T C G T A G C C G A T A C T G C A T G A C G T

32 A T A T C G C G T A T A G C G C C G A T T C G A C A T G

33 A T A T C G C G T A T A G C G C C G C G A T A T

34 5. Sugarphosphate bonds form 6. Daughter DNA molecules form 4. Weak hydrogen bonds form between bases 3.. Free nucleotides from nucleoplasm find complementary bases 2. Hydrogen bonds break and DNA unzips 1. Parent DNA unwinds Warning this gives the stages but not the correct orientation for both strands as it is too difficult to do just now.

35 Copied DNA 2 Daughter DNA strands Original DNA

36 IMPORTANT!!!! Please remember This process can only take place if the nucleus contains: DNA (acts as template for new molecule) Supply of DNA nucleotides Appropriate enzymes DNA Polymerase & Ligase Supply of ATP Primer

37 Video Summary Summary (includes structure of RNA) Video Skip from 5:40 to 9:10

38 Lesson starter 1. List 3 things required for DNA Replication 2. What happens after DNA has unzipped? 3. What is a primer? 4. What is the role of DNA polymerase?