Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Define and describe DNA replication.
• Explain transcription and translation.
• Compare and contrast DNA and RNA.
• Describe the genetic code.
• Explain the significance of these processes.

Lesson summary

This week, we delve deeper into the fascinating world of DNA, the molecule that holds the genetic code for all living organisms. Understanding DNA is crucial for understanding inheritance, genetic diseases, evolution, and even forensic science, all of which have direct relevance to South African society. For example, understanding genetic predispositions to diseases prevalent in South Africa like diabetes or heart disease can empower individuals to make informed lifestyle choices.

Furthermore, DNA evidence plays a vital role in our justice system.

Lesson notes

DNA Replication 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), ensuring that each daughter cell receives a complete and accurate copy of the genetic information.

The Process: Initiation: Replication begins at specific sites on the DNA molecule called origins of replication. Enzymes called helicases unwind the DNA double helix, separating the two strands and creating a replication fork. Think of it like unzipping a jacket.

Elongation: The enzyme DNA polymerase adds complementary nucleotides to the existing strand, using the original strand as a template. DNA polymerase can only add nucleotides to the 3' end of the growing strand, meaning replication proceeds in a 5' to 3' direction. One strand, the leading strand, is synthesized continuously in the 5' to 3' direction. The other strand, the lagging strand, is synthesized discontinuously in short fragments called Okazaki fragments.

Priming: DNA polymerase needs a short RNA primer to start adding nucleotides. This primer is synthesized by an enzyme called primase.

Ligation: The Okazaki fragments on the lagging strand are joined together by an enzyme called DNA ligase. Ligase acts like glue, sealing the gaps between the fragments to create a continuous strand.

Key Enzymes: Helicase: Unwinds the DNA double helix.

Primase: Synthesizes RNA primers.

DNA Polymerase: Adds nucleotides to the growing DNA strand.

DNA Ligase: Joins Okazaki fragments.

Worked example

Imagine a short segment of DNA: 3'-TACGATT-5'. If this strand is used as a template, what will be the sequence of the newly synthesized DNA strand?

Solution: DNA polymerase adds nucleotides complementary to the template strand. Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C). Since DNA polymerase adds nucleotides in the 5' to 3' direction, the new strand will be: 5'-ATGCTAA-3'.

Why it's important: Accuracy is paramount during DNA replication. Errors can lead to mutations, which can have harmful consequences. DNA polymerase has a proofreading function that allows it to correct most errors.

Transcription

Transcription is the process by which the information encoded in DNA is copied into a complementary RNA molecule called messenger RNA (mRNA). It is the first step in gene expression.