Topic 3.3: DNA Structure

Assessment Statements 3.3.1: Outline DNA nucleotide structure in terms of sugar (deoxyribose), base, and phosphate 3.3.2: State the names of the four bases of DNA 3.3.3: Outline how DNA nucleotides are linked together by covalent bonds into a single strand 3.3.4: Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds 3.3.5: Draw and label a simple diagram of the molecular structure of DNA

Nucleotides are the building blocks of DNA DNA (deoxyribonucleic acid)—long molecule – Subcomponents: Nucleotides – Each nucleotide of DNA is composed of a phosphate group, a sugar (deoxyribose), and a nitrogenous base

Nucleotides are the building blocks of DNA The four possible nitrogenous bases of DNA: – Adenine, thymine, cytosine, and guanine

Each strand of DNA is composed of nucleotides covalently linked DNA described shape of a double helix Each of the nucleotides in a single strand are covalently bonded together

Hydrogen bonds help form the double helix Imagine a twisted ladder – Two sides of the ladder are made up of the phosphate and deoxyribose sugars – The rungs of the ladder are made up of the nitrogenous bases (2)

Complementary Base Pairing Adenine pairs with Thymine – Held together with two hydrogen bonds Cytosine pairs with Guanine – Held together by three hydrogen bonds

Topic 3.4: DNA Replication

DNA replication involves ‘unzipping’ Cell must to prepare for cell division by doubling the DNA content of the cell – Process all DNA Replication Among the variety of molecules present in the nucleoplasm are two types that are important for DNA replication – Enzymes needed for replication (Helicase and DNA polymerase) – Free Nucelotides (nucleoside triphosphates)

DNA replication involves ‘unzipping’ One of the first events of DNA replication is the separation of the double helix into two single strands – Remember the double helix is held together by the hydrogen bonds between the complementary base pairs The enzyme that initiates this separation into two single strands is called helicase Helicase begins at a point in or at the end of a DNA molecule and moves one complementary base pair at a time, breaking the hydrogen bonds so the double-stranded DNA molecule becomes two separate strands

DNA replication involves ‘unzipping’ The unpaired nucleotides on each of these single strands can not be used as a template to help create two double-stranded DNA molecule identical to the original

Formation of two complementary strands In the environment of the nucleoplasm, there are many free- floating nucleotides. – These nucleotides are available to form complementary pairs with the single-stranded nucleotides of the unzipped molecule – The free nucleotides come and join the complementary pairs on the single stranded molecule. Then the DNA Polymerase catalyzes the formation of the covalent bond between the two new nucleotides

Formation of two complementary strands After the DNA polymerase forms the covalent bond another nucleotide comes and joins the next complementary base pair. – Both separated strands act as a template for the formation of another new strand The leading strand is going in one direction The lagging strand is going in the other direction

Significance of complementary base pairing The pattern of DNA replication ensures that two identical copies of DNA are produced from one This also means that no DNA molecules is every new Every DNA molecule after replication consists of a strand that was ‘old’ now paired with a strand that is ‘new’ – Described as a semi conservative process because half of a pre-existing DNA molecule is always conserved (saved)

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