Grade 12 Life Sciences: DNA Code of Life and Meiosis Study Guide

Introduction and Overview to Code of Life and Meiosis

• Subject Context: This self-study guide was developed by the Department of Basic Education (DBE) to mitigate learning disruptions caused by the COVID-19 pandemic. It focuses on Grade $12$ Life Sciences content deemed critical for mastering the curriculum.
• Topic 1: DNA - The Code of Life
  • Time Allocation: $8$ hours ($2$ weeks).
  • Exam Weighting: $27$ marks ($18\%$ of Paper $2$).
• Topic 2: Meiosis
  • Time Allocation: $8$ hours ($2$ weeks).
  • Exam Weighting: $21$ marks ($14\%$ of Paper $2$).

Prior Knowledge: Cell Structure and Organelles

Understanding the location and function of cellular components is essential for topics on DNA and Meiosis:

• Nucleoplasm (Nuclear Sap):
  • Location: Inside the nucleus.
  • Composition: A liquid surrounding chromosomes and nucleoli containing enzymes and free nucleotides.
  • Function: Facilitates nuclear activities like DNA replication.
• Nuclear Membrane (Envelope):
  • Description: A thin, double-walled membrane containing nuclear pores.
  • Function: Regulates the movement of substances (e.g., mRNA, proteins) between the nucleus and cytoplasm via the nuclear pores.
• Nucleolus:
  • Description: Small, dense structure within the nucleus made of RNA and protein (no membrane).
  • Function: Produces ribosomes.
• Chromatin Network:
  • Description: Tangled, threadlike material in the nucleus of non-dividing cells.
  • Function: Forms chromosomes; humans have $46$ chromosomes in somatic cells and $23$ in gametes.
• Ribosome:
  • Location: Found on the endoplasmic reticulum or free-floating in the cytoplasm.
  • Composition: Made of RNA and protein.
  • Function: The site of protein synthesis.
• Cytoplasm:
  • Description: Fluid part (cytosol) of the cell containing organelles.
  • Function: The site of most metabolic reactions.
• Mitochondria: Responsible for cellular respiration and energy production.
• Centrosome: Found in animal cells (composed of two centrioles); critical for cell division by ensuring equal distribution of chromosomes.

DNA: The Code of Life Terminology

• Deoxyribonucleic Acid (DNA): A double helix molecule found in the nucleus that carries hereditary information.
  • Nuclear DNA: Makes up genes on chromosomes.
  • Mitochondrial DNA (mtDNA): Found specifically in mitochondria.
  • Chloroplast DNA: Found in the chloroplasts of plants.
• Ribonucleic Acid (RNA): A single-stranded molecule responsible for protein synthesis. Contains the base Uracil ($U$) instead of Thymine ($T$).
• Nucleotide: The monomer (building block) of nucleic acids. Consists of a pentose sugar, a phosphate ion, and a nitrogenous base.
• Nitrogenous Bases:
  • Purines: Adenine ($A$) and Guanine ($G$).
  • Pyrimidines: Cytosine ($C$), Thymine ($T$ - DNA only), and Uracil ($U$ - RNA only).
• Base Pairing Rules: $A$ always bonds with $T$ (or $U$) via hydrogen bonds; $G$ always bonds with $C$.
• Genes and Genomes:
  • Gene: A segment of DNA that carries information for a specific characteristic.
  • Genome: The complete set of all genes in an organism.

DNA Replication

Process Overview: Replication occurs during Interphase of the cell cycle within the nucleus. It involves forming two identical DNA molecules from an original template.

Step-by-Step Procedure:

1. The DNA double helix unwinds.
2. Weak hydrogen bonds between nitrogenous bases break, causing the strands to unzip.
3. Each original strand acts as a template for building a new complementary strand.
4. Free nucleotides from the nucleoplasm attach to the template strands following base-pairing rules ($A$ to $T$; $C$ to $G$).
5. Result: Two identical DNA molecules are produced. Each consists of one original "old" strand and one "newly synthesized" strand (semi-conservative replication).

Importance:

• Doubles the genetic material to ensure daughter cells receive a full set during cell division.
• Ensures genetic continuity across generations.

Errors in Replication: If incorrect bases are added, deleted, or substituted, a gene mutation occurs, potentially altering the structure of the resulting protein.

DNA Profiling

• Definition: A technique producing a unique pattern of bands/bars on X-ray film used for identification.
• Scientific Basis: Every individual (except identical twins) has a distinct DNA profile.
• Applications:
  • Forensic investigations (matching crime scene evidence like blood or hair to suspects).
  • Paternity and maternity testing.
  • Identifying relatives or bodies of deceased individuals.
  • Establishing tissue compatibility for organ transplants.
  • Diagnosing genetic disorders.
• Paternity Testing Logic:
  • A child inherits half their DNA from each parent.
  • Step 1: Compare child's DNA bands to the mother's.
  • Step 2: Remaining bands in the child must match the biological father's profile.

Protein Synthesis

Protein synthesis occurs in two main stages: Transcription and Translation.

Stage 1: Transcription (Nucleus):

1. A section of DNA unwinds and unzips (hydrogen bonds break).
2. One DNA strand acts as a template.
3. Free RNA nucleotides form a complementary strand of messenger RNA (mRNA).
4. The mRNA molecule carries the genetic code in the form of triplets called codons.
5. mRNA moves out of the nucleus through nuclear pores into the cytoplasm.

Stage 2: Translation (Cytoplasm/Ribosome):

1. mRNA attaches to a ribosome.
2. Transfer RNA (tRNA) molecules in the cytoplasm carry specific amino acids.
3. Each tRNA has an anticodon complementary to an mRNA codon.
4. The tRNA brings the correct amino acid to the ribosome based on the mRNA sequence.
5. Amino acids are linked by peptide bonds to form a polypeptide chain (protein).

Genetic Codes Summary:

• DNA Triplets: Sets of three bases on DNA.
• mRNA Codons: Sets of three bases on mRNA.
• tRNA Anticodons: Sets of three bases on tRNA.

Mutations and Protein Structure

• Gene Mutation: A change in the nitrogenous base sequence of DNA.
• Impact on Proteins:
  • No Effect: The mutation results in a codon that still codes for the same amino acid (redundancy of the genetic code).
  • Alteration: A different amino acid is coded for, changing the amino acid sequence and potentially the protein's function or structure.

Meiosis: The Process and Significance

Definition: A reduction division that halves the chromosome number from diploid ($2n$) to haploid ($n$) to produce gametes.

Comparison to Mitosis:

• Mitosis: Produces two genetically identical diploid cells ($2n \rightarrow 2n$); occurs in somatic cells.
• Meiosis: Produces four genetically different haploid cells ($2n \rightarrow n$); occurs in sex organs (gonads).

Phases of Meiosis I:

1. Prophase I: Homologous chromosomes pair up (forming a bivalent). Crossing over occurs at the chiasmata, where chromatid segments are exchanged, leading to genetic variation.
2. Metaphase I: Homologous pairs line up randomly at the equator (Random Arrangement).
3. Anaphase I: Homologous chromosomes are pulled to opposite poles (centromeres do not split).
4. Telophase I: Two haploid daughter cells are formed.

Phases of Meiosis II:

1. Prophase II: Spindles form in the two daughter cells.
2. Metaphase II: Individual chromosomes line up at the equator.
3. Anaphase II: Centromeres split, and individual chromatids (now daughter chromosomes) move to opposite poles.
4. Telophase II: Four haploid, genetically unique gametes are formed.

Abnormal Meiosis (Non-disjunction):

• Occurs when chromosomes or chromatids fail to separate correctly during Anaphase I or II.
• Results: Gametes with too many or too few chromosomes. If a gamete with an extra chromosome at position $21$ is fertilized, it results in Down Syndrome (Trisomy $21$), characterized by a total of $47$ chromosomes in somatic cells.

Summary of Key Chromosome Terms

• Haploid ($n$): One set of chromosomes ($23$ in humans).
• Diploid ($2n$): Two sets of chromosomes ($46$ in humans).
• Autosomes: Chromosome pairs $1$ to $22$ (non-sex characteristics).
• Gonosomes: The $23^{rd}$ pair ($XX$ for females, $XY$ for males).
• Homologous Chromosomes: Pairs of chromosomes identical in shape/size carrying genes for the same traits.
• Centromere: Structure holding two chromatids together.
• Chiasma: The point where crossing over occurs.