Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Describe the process of DNA replication and its semi-conservative nature.
• Explain the process of transcription, forming mRNA from a DNA template.
• Describe the process of translation and protein synthesis.
• Explain the genetic code and the relationship between DNA, mRNA, and amino acids.
• Distinguish between a gene, allele, and chromosome.

Lesson summary

DNA, the deoxyribonucleic acid, is the fundamental blueprint of life. Understanding DNA is crucial not just for biology, but also for addressing many challenges facing South Africa today. From understanding inherited diseases prevalent in specific communities, to using DNA technologies in forensics and agriculture for economic development, a solid grasp of DNA's structure and function is essential. This week, we will delve deeper into the DNA code, focusing on DNA replication, transcription, and translation - the processes that allow DNA to dictate the characteristics of an organism.

Lesson notes

2.1 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 and the inheritance of genetic information. The process is semi-conservative, meaning that each new DNA molecule consists of one original strand and one newly synthesized strand.

Steps of DNA Replication: Initiation: Replication begins at specific sites called origins of replication. Enzymes called helicases unwind the DNA double helix, creating a replication fork.

Elongation: The enzyme DNA polymerase adds nucleotides to the 3' end of a pre-existing strand (either a primer or another DNA fragment). This means DNA is always synthesized in the 5' to 3' direction. The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments called Okazaki fragments.

Lagging Strand Synthesis: On the lagging strand, an enzyme called primase synthesizes short RNA primers. DNA polymerase then adds nucleotides to these primers, creating Okazaki fragments.

Joining: Once the Okazaki fragments are synthesized, another DNA polymerase removes the RNA primers and replaces them with DNA nucleotides. The enzyme DNA ligase then joins the Okazaki fragments together to form a continuous strand.

Termination: Replication continues until the entire DNA molecule has been copied.

Important Enzymes: Helicase: Unwinds the DNA double helix.

DNA polymerase: Adds nucleotides to the growing DNA strand.

Primase: Synthesizes RNA primers.

Ligase: Joins DNA fragments together.

Worked example

Imagine a short sequence of DNA: 5'-ATGC-3'. Show how this sequence is replicated, assuming that primers have already been added.

Solution:

The original strand is used as a template.

DNA polymerase will add the complementary nucleotides to the 3' end of the primer.

The new strand will have the sequence 3'-TACG-5'. The direction of synthesis is always 5' to 3' on the new strand.

Why is it important that replication is semi-conservative? Semi-conservative replication ensures high fidelity in the duplication of genetic information. By using one original strand as a template, the new strand is highly likely to be an exact copy of the original. This reduces the chance of mutations being introduced during replication.

2.2 Transcription

Transcription is the process by which the genetic information encoded in DNA is copied into a complementary RNA molecule, specifically messenger RNA (mRNA). This mRNA then carries the genetic code from the nucleus to the ribosomes in the cytoplasm, where protein synthesis takes place.