CfE Higher Human Unit 1 - Section 2 The Structure of DNA and its Replication

Learning Intentions By the end of this lesson you should know: The genotype of a cell is determined by the sequence of bases in its DNA. DNA is the molecule of inheritance and can direct its own replication. Structure of a DNA nucleotide (deoxyribose sugar, phosphate and base). Nucleotides bond to form a sugar–phosphate backbone. Base pairs (adenine, thymine and guanine, cytosine,) are held by weak hydrogen bonds forming a double helix. The DNA double helix has an antiparallel structure with deoxyribose and phosphate at 3' and 5' ends of each strand. Chromosomes consist of DNA tightly coiled and packaged with associated proteins. You should also have looked at experimental work carried out by famous biologists

DNA Do you remember what DNA stands for? Where is it found? What does it code for? How do the bases pair?

DNA Each molecule of DNA consists of 2 strands, where each strand is made up of repeating units called NUCLEOTIDES. The nucleotide is made of : Deoxyribose sugar Phosphate Organic Base

A Deoxyribose Sugar 2C2C 1C1C 3C3C 5C5C 4C4C This carbon skeleton diagram of the sugar shows the carbons and allow you to get an idea of which way is ‘up’. This is called the orientation

Nucleotides There are 4 different types of nucleotides. This is because there are 4 different bases: Guanine Cytosine Thymine Adenine

Complementary base pairing Each base can only join with one other base. 4 bases: AdenineThymine GuanineCytosine This is always the case in DNA – G-C, T-A pairs

GUANINE ADENINE STRONG CHEMICAL BOND links the nucleotides between the phosphate and sugar. They are not easily broken and join nucleotides together into a permanent strand Sugar-phosphate Bond

GUANINE ADENINETHYMINE CYTOSINE Weak hydrogen bond The 2 strands are arranged in a twisted coil called a “DOUBLE HELIX”. It is often likened to a spiral ladder, where the sugar-phosphate backbone is the railing and the base are the steps

GUANINE ADENINETHYMINE CYTOSINE Notice how the two strands run anti-parallel to each other 3’ 5’ 3’

Once both strands of the DNA are assembled and the double helix formed it is carefully wound around protein units and packaged into chromosomes. It is important that they are tightly coiled so that the strands don’t tangle. How DNA is packaged using proteins DNA wrapped around 8 special proteins Interesting facts you don’t need to know for HH The proteins are known as histone proteins DNA/protein ‘bundles’ are called nucleosomes Each nucleus in a cell contains 1 meter of DNA carefully packaged so that it can be copied at any time

DNA/protein ‘bundles’

Now try the following: 1)What is the name given to the molecule inside cells that controls inheritance of all features ? 2) What feature of the molecule contains the code controlling inheritance of features? 3) Draw a single nucleotide and label the three component parts 4) Draw a second nucleotide and show how the two nucleotides are joined to form part of the backbone and label the bond. 5) Explain the base pair rule DNA Genes A-T and C-G

6) What type of bond holds the bases between opposite strands together? 7) What name is given to the final shape of DNA? 8) The two strands of DNA run in opposite directions. What name is given to this arrangement? 9)DNA is packaged by winding it around special proteins – What are these proteins called? 10) What structures contain the final DNA/protein complex – What are these called? ( hint – a human cell has 46 of these inside each nucleus Weak hydrogen bonds Double Helix Antiparallel histone proteins Chromosomes

Research Homework The discovery of DNA structure involved some key stages and scientists Avery and Griffiths used bacterial transformation experiments to change one form of bacteria to another using the isolated DNA ( they were not sure what the mystery molecule was that had this effect ) – See animation herehere Hershey and Chase used viruses called phage to demonstrate that the molecule changing the bacteria was DNA – See animation herehere Erwin Chargaff worked out the base pair rule - See animation herehere Wilkins and Franklin used X ray crystallography to investigate the structure of DNA – See animation herehere Watson and Crick were the first to work the structure of the double helix – See animation herehere Task - You should also have looked at experimental work carried out by one famous biologist – Write a paragraph giving details of the biologist concerned and their experimental work