Nucleotides, DNA and Chromosomes Syllabus link: DNA is an information carrying molecule. It’s sequences of bases determines the structure of proteins, including enzymes.

By the end of the lesson you should know: The double helix structure of DNA, enabling it to act as a stable information carrying molecule in terms of: The components of DNA nucleotides: deoxyribose, phosphate and the bases adenine (A), thymine (T), cytosine (C) and guanine (G). 2 sugar-phosphate backbones held together by hydrogen bonds between base pairs. Specific base paring.

Introduction DNA molecules are huge. Each DNA molecule consists of 2 strands twisted into a double helix. Each strand is made from millions of subunits called nucleotides. So each strand is a polynucleotide. Don’t get muddled between a molecule and a strand of DNA. A molecule of DNA is made up of 2 strands of DNA.

Each DNA strand or polynucleotide is made up of many individual nucleotide. A nucleotide is made up of: A phosphate group. A 5 carbon sugar molecule. An organic nitrogenous base. The 3 sub-units are joined by covalent bonds = nucleotide molecule.

5 C sugar In DNA the 5 carbon sugar is called Deoxyribose. In RNA the 5 carbon sugar is called Ribose.

Phosphate The phosphate attaches onto the carbon in position 5.

Organic, nitrogenous bases.  For each 1 nucleotide, there is 1 organic, nitrogenous base, attached to the carbon in position 1.  In DNA there are 4 possible bases adenine (A), thymine (T), guanine (G) and cytosine (C).  In RNA 3 of the bases are the same G, C and A. But T is replaced by Uracil (U).

A single nucleotide The separate components of the nucleotide join via condensation reactions and form covalent bonds.

DNA and RNA In eukaryotic organism almost all the DNA is in the nucleus, where acts as information store. It does not float free but is associated with proteins called histones and forms chromosomes. Where is the rest of the DNA found? RNA is needed to read and translate the information carried by the DNA, in order to produce proteins. RNA is single stranded and is found in the nucleus and cytoplasm of the eukaryotic cell. Prokaryotic cells use DNA & RNA to produce their proteins too.

Student activity In group of 2 use the cards provided to produce a single nucleotide found in DNA and a single nucleotide found in RNA. Then individually in your workbook complete the table of comparisons between DNA and RNA, (structurally and functionally).

Comparison between DNA and RNA DNARNA Contains the genetic information in the form of genes. Contains the genetic code to produce proteins. RNA is needed to read and translate the information carried by the DNA, in order to produce proteins. 2 strands, as a double helix.A single chain Each of the 2 polynucleotides/nucleic acids is made up of individual nucleotides. The polynucleotide/nucleic acid is made up of individual nucleotides. 5C sugar - Deoxyribose5C sugar - Ribose Contain phosphateContains phosphate Organic bases – Guanine, cytosine, adenine and thymine. Organic bases – Guanine, cytosine, adenine and uracil. Found in nucleus, mitochondria and chloroplasts of a eukaryotic cell. Found in nucleus and cytoplasm of a eukaryotic cell. Found in a prokaryotic cell.

Joining nucleotides together forms nucleic acids. Individual nucleotides join via condensation reactions to form nucleic acids or polynucleotides. 2 mononucleotides can join to form a dinucleotide. Many can linked together to form a polynucleotide. Phosphate group of the nucleotide below bonds to the hydroxyl group, on carbon 3 of the sugar, of the nucleotide above. The bond linking 2 nucleotides is a covalent phosphodiester bond. This forms a “sugar phosphate backbone”. Organic bases project outside. It is sequence of these bases that forms coded information in nucleic acids.

Formation of the sugar phosphate backbone

Student activity Combine your nucleotide with the 2 groups next to you, to create a sugar phosphate backbone. Use the pipe cleaners as your bonds.

Organic bases are either Purines or Pyrimidines. Purines are double ring structure. Include Adenine and Guanine. Are longer molecules. Pyrimidines are single ring structures. Include Thymine, Uracil and Cytosine.

Too much nucleic acid causes gout. If you have too many purines then uric acid by the liver. Excess uric acid causes crystals to be deposited in the joints at extremities, like toes, (as is insoluble at low temps). Joints are painful and swollen = gout.

Questions 1. How many molecules of water will be produced when a nucleotide is made from its’ separate component parts? 2. Explain why nucleic acids are named after the sugar they contain. 3. In a nucleic acid polymer, which component groups are at the ends of the molecule? Answers 1. Two 2. One type of polymer contains ribose and the other deoxyribose – this makes it easy to distinguish the 2 types of polymers. 3. Phosphate at one end and a 5C sugar at the other.

Structure of DNA Made up of 2 strands of polynucleotides. One strand goes in one direction and the other in the opposite direction (= anti-parallel). In this way they can join via their organic bases. Joined by hydrogen bonds. Weak bonds but many, so strong structure. Bases pair up in a specific way. A purine always pairs with a pyrimidine. This is known as complementary base pairing. Adenine pairs with Thymine. Cytosine pairs with Guanine. Hydrogen bonds hold the bases together. A-T = 2 hydrogen bonds & G-C = 3 hydrogen bonds

The Watson-Crick double helix of DNA

Questions 4. Define the term “antiparallel” 5. A DNA strand has the base sequence ATTAGGCTAT. Write down the complementary strand sequence. 6. A DNA molecule is 20% thymine (T). What percentage of each of the other types of bases would it contain? Answers 4. Parallel but with chains running in opposite directions. 5. TAATCCGATA 6. 20% A, 30% C, 30% G.

The double helix The 2 nucleotide strands are bonded together by their bases, (via hydrogen bonds). The two strands then twist around to form a double helix. This DNA holds all the genetic information for that organism, including all the codes for producing all the organisms' necessary proteins. A set of 3 bases codes for 1 amino acid. (See later notes on protein synthesis.)

Student activity Join the nucleic acid chains together as a class to produce a (straight molecule) of DN. Use the pip cleaners to represent your hydrogen bonds. Remember to make sure you have included the correct number of bonds.

DNA Past paper questions Answers:

1(a)A = Phosphate; Y = Pentose / Deoxyribose;2 b)(i) ; 1 (ii)DNA is double stranded; Pairing of bases / A pairs with T / C pairs with G; (I reference to bases being same as or equal to each other) Evidence of calculation of octopus figures; max 2 2.(a)nucleotide; 1 (b)(i)21.4, 21.4; 28.6;2 (ii) amounts of A and T /C and G/complementary bases different; therefore no base-pairing; 2 max

3.(a)Phosphate; Deoxyribose;2 (b)4;1 (c)(i)14;1 (ii)36;1 (d)Different proteins; Different genes; Different (DNA) base sequences; 2 max