(2) Structure and Replication of DNA (A) DNA Structure (C) DNA Replication (B) Chromosome Structure

Structure and Replication of DNA (a+b) Describe the structure of DNA in terms of nucleotides, backbone and complementary base pairing State how 2 DNA strands are held together Describe the antiparallel structure of the DNA strands State where genetic information is stored Define the term ‘genotype’ State what the genotype is determined by State what the molecule of inheritance is Describe the structure of a chromosome

1= Nucleotides 2= DNA double helix 3=Genes 4=Chromosomes 5= Nucleus of cell 6=You!

What is DNA & why is it important? Deoxyribonucleic acid All cells store their genetic information in the base sequence of DNA, which is found inside the nucleus. DNA is the molecule of inheritance and within the structure of DNA, there are many instructions to make specific proteins. The sequence of DNA determines the genotype The genotype is the term given to the genes that are present in an organisms. The genotype determines the order of amino acids The order of the amino acids determines the type of protein that is to be made.

Structure of DNA DNA is made up repeating units called nucleotides Each nucleotide contains: A phosphate molecule A deoxyribose sugar A base Sugar-Phosphate Bond

The Deoxyribose Sugar

Carbon 5 Carbon 1 Carbon 4 Carbon 2 Carbon 3

The SUGAR-PHOSPHATE Backbone The deoxyribose Sugar contains 5 carbons The base is attached to carbon 1 The phosphate is attached to carbon 5 The phosphate of the NEXT nucleotide is attached to carbon 3 by a SUGAR-PHOSPHATE BOND

DNA Bases In DNA, there are 4 different bases: Therefore, DNA has 4 different bases. Base

Complementary Base pairs What do you notice about the 2 strands? Complementary Base pairs ADENINE THYMINE CYTOSINE GUANINE WEAK HYDROGEN BOND

Anti-Parallel Strands DNA is double stranded with the two strands running in opposite directions. This is known as an antiparallel arrangement. 5’ 3’ 3’ 5’

Double Helix

Structure of DNA (Summary) DNA is made up repeating units called nucleotides Each nucleotide contains: A phosphate molecule A deoxyribose sugar A base There are 4 bases. Adenine pairs with Thymine and Guanine pairs with Cytosine. This is known as complementary base pairing. The two DNA strands are held together by weak hydrogen bonds (between each of the bases) The DNA strands have an antiparallel structure because the strands run in opposite directions to each other The DNA strand has 2 distinct ends: a phosphate group is at the 5’ end whereas the 3’ end is on the deoxyribose sugar DNA has a sugar–phosphate backbone The deoxyribose sugar of the 1st nucleotide is joined onto the phosphate molecule of the next nucleotide They are joined together by a strong sugar-phosphate bond

5’ C-G-T-T-T-T-A-A-G-C-C-C-T-G-A 3’ Problem Solve! What is the correct sequence of bases to the single DNA strand below? If a DNA molecule contained a total of 1000 bases and 400 were ADENINE bases, how many bases of CYTOSINE would it contain? 5’ C-G-T-T-T-T-A-A-G-C-C-C-T-G-A 3’ 3’ G-C-A-A-A-A-T-T-C-G-G-G-A-C-T 5’ 400 Adenine = 400 Thymine 1000-800 = 200 200/2 = 100 guanine & 100 cytosine

How was all this discovered? Your Task: Your task is to research one of the individuals or groups of scientists below whose work contributed to the discovery of DNA structure and produce a report on them: Griffiths Avery et al. Hershey and Chase Chargaff Franklin and Wilkins Watson and Crick WHAT TO INCLUDE: Who the scientist(s) is, where they worked and when The aims of the experiments What the experiments were, including a diagram What the conclusions were other interesting information that you have gathered that lends interest to the story. You could do this OR your could do the next slide

How was all this discovered? 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 here Hershey and Chase used viruses called phage to demonstrate that the molecule changing the bacteria was DNA – See animation here Erwin Chargaff worked out the base pair rule - See animation here Wilkins and Franklin used X ray crystallography to investigate the structure of DNA – See animation here Watson and Crick were the first to work the structure of the double helix – See animation here You could do this OR your could do the previous slide

What happens when the DNA strands have been formed? Video: zoom in on the structure of chromosomes and DNA Video: How DNA is packaged

DNA is tightly coiled up around 8 proteins These proteins are called Histones The DNA/protein ‘bundle’ is called a nucleosome Many nucleosomes make up a chromosome

Chromosome Structure Chromosomes consist of tightly coiled DNA and are packaged with associated proteins

Questions What is the name given to the molecule inside cells that controls inheritance of all features ? What feature of the molecule contains the code controlling inheritance of features? Draw a single nucleotide and label the three component parts Draw a second nucleotide and show how the two nucleotides are joined to form part of the backbone and label the bond. Explain the base pair rule What type of bond holds the bases between opposite strands together? What name is given to the final shape of DNA? The two strands of DNA run in opposite directions. What name is given to this arrangement? What special molecules are DNA packaged around? What structures contain the all of the final DNA/protein complexes? Could do show me boards or get pupils to write them in their jotters

Structure and Replication of DNA (a+b) Describe the structure of DNA in terms of nucleotides, backbone and complementary base pairing State how 2 DNA strands are held together Describe the antiparallel structure of the DNA strands State where genetic information is stored Define the term ‘genotype’ State what the genotype is determined by State what the molecule of inheritance is Describe the structure of a chromosome