Lesson Notes By Weeks and Term v5 - Grade 12
DNA: code of life – Week 1 focus
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DNA: code of life – Week 1 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Life Sciences
Class: Grade 12
Term: 1st Term
Week: 1
Theme: General lesson support
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DNA, Deoxyribonucleic Acid, is the molecule that carries the genetic instructions for all living organisms and many viruses. It's the blueprint of life! Understanding DNA is crucial because it underpins everything from inherited diseases that disproportionately affect certain South African populations (like familial hypercholesterolemia in Afrikaner communities) to forensic science used in solving crimes within our country. It also forms the basis of biotechnology, which could revolutionize our agricultural sector, ensuring food security for all South Africans.
2.1 What is DNA? DNA (deoxyribonucleic acid) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and many viruses. It acts like a "biological instruction manual". 2.2 The Structure of DNA: A Double Helix DNA has a double helix structure, resembling a twisted ladder. Imagine a spiral staircase - that’s what DNA looks like.
The Sides of the Ladder (Backbone): The sides of the "ladder" are made of alternating deoxyribose sugar and phosphate groups. These are held together by strong covalent bonds. This sugar-phosphate backbone is identical for every DNA molecule. The Rungs of the Ladder (Nitrogenous Bases): The "rungs" are formed by pairs of nitrogenous bases.
There are four types of bases: Adenine (A) Thymine (T) Guanine (G)
Cytosine (C)
Base Pairing Rules: The bases pair up in a specific way: Adenine (A) always pairs with Thymine (T). Guanine (G) always pairs with Cytosine (C). These base pairs are held together by weak hydrogen bonds. Crucially, A always pairs with T, and G always pairs with
C. This is complementary base pairing. 2.3 Nucleotides: The Building Blocks of DNA Each repeating unit of DNA is called a nucleotide.
A nucleotide consists of: A deoxyribose sugar molecule: A five-carbon sugar.
A phosphate group: This provides the negative charge to DN
A. A nitrogenous base: One of the four bases (A, T, G, or C). Think of nucleotides as Lego bricks; each one is slightly different depending on the base it carries, but they all fit together in a specific way to form the long DNA molecule. 2.4 DNA Replication DNA replication is the process by which a DNA molecule makes a copy of itself. This is essential for cell division because each new cell needs a complete set of genetic instructions. The basic steps of DNA replication are as follows: Unwinding: The double helix unwinds with the help of an enzyme called helicase. This breaks the hydrogen bonds between the base pairs, separating the two strands.
Primer Binding: Before DNA polymerase can start replicating, a primer binds to the start of the section of DNA that needs to be copied.
Base Pairing: An enzyme called DNA polymerase moves along each strand, adding complementary nucleotides to form a new strand. For example, if the original strand has an Adenine (A), DNA polymerase will add a Thymine (T) to the new strand.
Proofreading: DNA polymerase also acts as a proofreader, checking for errors and correcting them. This is crucial to minimize mutations.
Joining: Eventually the separated original strand will join to its newly constructed pair. The result is two identical DNA molecules, each consisting of one original strand and one newly synthesized strand. This is called semi-conservative replication.