Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Define DNA and its nucleotide components.
• Explain the process of DNA replication.
• Describe the relationship between DNA, genes, and chromosomes.
• Understand the significance of DNA in heredity.

Lesson summary

DNA, deoxyribonucleic acid, is the very essence of life. It’s the instruction manual that determines everything from the colour of your eyes to your predisposition to certain diseases. Understanding DNA is crucial because it allows us to understand inheritance, genetic disorders, and even the possibilities of genetic engineering. In South Africa, where we face challenges like high rates of HIV/AIDS and genetic predispositions to certain cancers, understanding DNA is particularly important for developing effective treatments and preventative measures.

Lesson notes

2.1 DNA Structure: The Double Helix DNA is a polymer made of repeating units called nucleotides. Each nucleotide consists of three components: A deoxyribose sugar: This is a five-carbon sugar.

A phosphate group: This provides the backbone structure and gives DNA its acidic properties.

A nitrogenous base: There are four types of nitrogenous bases in DNA: Adenine (A) Guanine (G) Cytosine (C) Thymine (T) Adenine and Guanine are purines (double-ring structures), while Cytosine and Thymine are pyrimidines (single-ring structures). DNA exists as a double helix, resembling a twisted ladder. The sides of the ladder are formed by the sugar-phosphate backbone, and the rungs are formed by the paired nitrogenous bases.

Base pairing is specific: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This is known as complementary base pairing. These bases are held together by hydrogen bonds. A-T pairs have two hydrogen bonds, while G-C pairs have three, making G-C pairs stronger. The two strands of DNA are antiparallel, meaning they run in opposite directions (5' to 3' and 3' to 5'). The '5' and '3' refer to the carbon atoms on the deoxyribose sugar.

Example: If one strand of DNA has the sequence 5'-ATGCGTTA-3', the complementary strand would be 3'-TACGCAAT-5'. 2.2 DNA Replication: Copying the Code DNA replication is the process by which a DNA molecule is copied to produce two identical DNA molecules. This is essential for cell division (mitosis and meiosis) to ensure that each daughter cell receives a complete set of genetic instructions. DNA replication is a semi-conservative process, meaning that each new DNA molecule consists of one original (template) strand and one newly synthesized strand.

The Process of DNA Replication: Unwinding: The enzyme DNA helicase unwinds the double helix at specific locations called replication origins, forming a replication fork.

Primer Binding: An enzyme called primase synthesizes short RNA primers complementary to the template strand. These primers provide a starting point for DNA polymerase.

DNA Polymerase Action: DNA polymerase is the key enzyme that adds nucleotides to the 3' end of the existing primer, extending the new DNA strand. DNA polymerase can only add nucleotides in the 5' to 3' direction.

Leading and Lagging Strands: Because DNA polymerase can only add nucleotides in the 5' to 3' direction, replication occurs differently on the two strands.

Leading strand: Synthesized continuously in the 5' to 3' direction towards the replication fork. Only requires one primer.

Lagging strand: Synthesized discontinuously in short fragments called Okazaki fragments in the 5' to 3' direction away from the replication fork. Each Okazaki fragment requires a separate primer.

Primer Removal and Replacement: The RNA primers are removed by another DNA polymerase (DNA polymerase I in some organisms) and replaced with DNA nucleotides.

Joining Fragments: The enzyme DNA ligase joins the Okazaki fragments on the lagging strand to create a continuous strand.

Proofreading: DNA polymerase also has a proofreading function, correcting any errors that may occur during replication.