What you'll learn

• How organisms reproduce sexually and asexually, and the pros and cons of each.
• How meiotic cell division produces genetically unique gametes.
• The molecular structure of DNA, genes, and the genome, and how to extract DNA from cells.
• How cells use the DNA code to synthesise proteins during transcription and translation.

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1. Sexual vs. Asexual Reproduction

All living organisms must reproduce to pass on their genetic information to the next generation. However, species achieve this using two very different biological strategies.

Because asexual reproduction does not involve the fusion of sex cells, there is no mixing of genetic material. Mitosis is the primary cellular division process used to create these offspring.

During sexual reproduction, genetic information from two parents is combined, introducing significant variation into the offspring's genetic makeup.

Advantages and Disadvantages (Separate Biology Only)

If you are studying Separate (Triple) Biology, you must understand the ecological trade-offs of both reproductive strategies.

Asexual Reproduction:

• Advantages:
  • No mate required: The organism does not need to waste time or metabolic energy searching for a partner. This is a huge benefit for isolated individuals.
  • Rapid reproductive cycle: Large numbers of offspring can be produced very quickly when environmental conditions are optimal, allowing the organism to colonise an area rapidly.
• Disadvantages:
  • No genetic variation: Because all offspring are identical clones, any change in environmental conditions (such as a new pathogen or climate fluctuation) will affect every individual in the exact same way. If one is vulnerable, the entire population could be wiped out.

Sexual Reproduction:

• Advantages:
  • Genetic variation: Offspring inherit a unique combination of alleles. If environmental conditions change, some individuals within the population are likely to possess characteristics that help them survive. Over time, this drives natural selection and evolution.
• Disadvantages:
  • Requirement to find a mate: Finding a suitable mate requires a significant expenditure of time and energy, and can also expose the organism to predators or sexually transmitted infections.

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2. Meiotic Cell Division

To facilitate sexual reproduction, organisms must produce specialized sex cells called gametes.

Before looking at how gametes are made, we must define two key genetic states:

• Diploid (2n): Cells containing two complete sets of chromosomes—one set inherited from each parent. Human body cells are diploid and contain 46 chromosomes (23 pairs).
• Haploid (n): Cells containing only one single set of chromosomes. Gametes are haploid and contain 23 chromosomes in humans.

The Role of Meiosis

Meiosis is a specialized form of cell division that occurs exclusively in the reproductive organs (testes and ovaries in animals). Its role is to produce four genetically different haploid daughter cells from a single diploid parent cell.

When two haploid gametes fuse during fertilisation, they form a diploid cell (a zygote) with the correct double set of chromosomes ($23+23=46$ chromosomes in humans). Without meiosis, the chromosome number would double with every generation!

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3. The Structure of DNA

To understand how genetics works, we must study the physical molecule that holds all the instructions: DNA (Deoxyribonucleic Acid).

The Nucleotide: The Building Block of DNA

DNA is a polymer, meaning it is a large macromolecule made of many repeating units called monomers. In DNA, these monomers are called nucleotides.

Each nucleotide consists of three distinct parts joined together:

1. A phosphate group
2. A sugar group (specifically deoxyribose)
3. One of four different nitrogenous bases attached to the sugar

There are four bases in DNA: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

The Double Helix and Complementary Base Pairing

As illustrated in the diagram above, DNA is composed of two individual strands coiled around one another to form a double helix.

• The structural "sides" of the ladder are made of alternating sugar and phosphate groups, known as the sugar-phosphate backbone.
• The "rungs" connecting the two strands are formed by the nitrogenous bases.
• The two strands are held together by weak hydrogen bonds between complementary bases.

These bases pair up in a highly specific way called complementary base pairing:

• Adenine (A) always pairs with Thymine (T)
• Cytosine (C) always pairs with Guanine (G)

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4. Core Practical: Extracting DNA from Fruit

You must be able to describe how to extract DNA from plant tissue (such as strawberries or kiwi fruit) and explain the scientific purpose behind each step.

Step-by-Step Method

1. Mash the fruit: Peel and mash the fruit in a plastic bag. This mechanically breaks open the tough plant cell walls.
2. Add extraction buffer: Mix in a solution of liquid detergent (washing-up liquid) and salt.
   • Detergent: Dissolves the lipids in the cell membranes and nuclear membranes, releasing the DNA into the solution.
   • Salt: Encourages the DNA molecules to clump together so they are easier to extract.
3. Heat and filter: Incubate the mixture in a water bath at 60 °C for 15 minutes, then filter it using filter paper and a funnel. The filtration removes large insoluble cell debris, leaving a clear liquid filtrate containing the dissolved DNA.
4. Precipitate the DNA: Slowly pour ice-cold ethanol down the side of the test tube containing the filtrate.
   • DNA is insoluble in cold ethanol.
   • A white, cloudy, stringy precipitate of DNA will form at the boundary layer between the filtrate and the ethanol. This can be spooled out using a glass rod.

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5. Protein Synthesis (Higher Tier Only)

How does the order of bases in a gene actually translate into a physical characteristic? The order of bases in DNA determines the order of amino acids in a protein, and these fold to produce specifically shaped proteins, such as enzymes.

How the DNA Code Works

1. Triplets (Codons): Every three bases in a gene (a triplet) codes for one specific amino acid.
2. Polypeptides: The cell reads these triplets in order to link amino acids together into a long chain called a polypeptide.
3. Protein Folding: Once the polypeptide chain is built, it folds into a precise three-dimensional shape. The shape of the protein is determined by the precise order of its amino acids.
   • For example, an enzyme must fold to form a specifically shaped active site that matches its substrate perfectly.

The Two Stages of Protein Synthesis

Because DNA is a massive molecule, it cannot leave the protection of the nucleus. However, proteins are synthesised by ribosomes in the cytoplasm. To solve this, the cell creates a temporary, portable copy of the gene called mRNA (messenger RNA). This process occurs in two stages:

Stage 1: Transcription (In the Nucleus)

1. RNA polymerase (an enzyme) binds to a region of non-coding DNA located directly in front of the gene.
2. The DNA double helix unzips, and RNA polymerase moves along the coding DNA template strand of the gene.
3. As it moves, RNA polymerase links free RNA nucleotides together to build a complementary, single-stranded molecule called mRNA.
   • Note: RNA does not contain Thymine (T). Instead, it uses Uracil (U). Therefore, Adenine (A) on the DNA template pairs with Uracil (U) on the mRNA.
4. The completed mRNA molecule detaches, exits the nucleus through a nuclear pore, and enters the cytoplasm.

Stage 2: Translation (At the Ribosome)

1. The mRNA strand binds to a ribosome in the cytoplasm.
2. The ribosome reads the mRNA bases in groups of three. A triplet of bases on mRNA is called a codon.
3. tRNA (transfer RNA) molecules bring the correct amino acids to the ribosome.
   • Each tRNA molecule has an anticodon (a complementary triplet of bases) on one end and a specific amino acid attached to the other.
   • The tRNA anticodon binds to its complementary mRNA codon.
4. The ribosome joins the adjacent amino acids together with peptide bonds, building a growing polypeptide chain.
5. This continues until the ribosome reaches a 'stop' codon. The chain is released and folds into its final active protein.

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6. Genetic Variants and Phenotype (Higher Tier Only)

A mutation is a change in the sequence of DNA bases, resulting in a genetic variant. These mutations can occur in both coding and non-coding sections of DNA.

1. Variants in Coding DNA

If a mutation occurs in the coding DNA of a gene:

• The base sequence of the mRNA transcript will be altered.
• This can change the codon, causing a different amino acid to be delivered by tRNA during translation.
• A different amino acid sequence changes how the polypeptide folds, altering the three-dimensional shape of the protein.
• If the protein is an enzyme, the shape of its active site might change. If the substrate can no longer bind, the enzyme loses its activity.

2. Variants in Non-Coding DNA

Not all DNA codes for proteins. Some regions of DNA are non-coding and control how genes are expressed.

• If a mutation occurs in the non-coding DNA located in front of a gene where RNA polymerase normally binds:
• It can alter the structure of the binding site, making it easier or harder for RNA polymerase to attach.
• If it makes it harder to bind, less mRNA will be transcribed, resulting in less of the protein being produced.
• If it makes it easier to bind, more mRNA is transcribed, leading to an overproduction of the protein.

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