Hereditary material, also called genetic material, is the chemical substance that stores and transmits genetic information from one generation to the next. DNA (deoxyribonucleic acid) is the primary genetic material in most organisms, while RNA (ribonucleic acid) serves as genetic material in some viruses.

Location of Genetic Material

In eukaryotic cells (plants, animals, fungi, protists), genetic material is located inside a membrane-bound nucleus. DNA is organized into chromatin threads—long DNA molecules wrapped around histone proteins. During cell division, chromatin condenses into visible chromosomes. Human cells have 46 chromosomes (23 pairs).

In prokaryotic cells (bacteria), there is no nucleus. The genetic material is a single circular DNA molecule located in the cytoplasm within a region called the nucleoid. Many bacteria also contain small circular DNA molecules called plasmids.

Chemical Composition of Genetic Material

Both DNA and RNA are polymers of nucleotides. Each nucleotide has three components:

• Pentose sugar: Deoxyribose in DNA; ribose in RNA. Deoxyribose lacks an oxygen atom on carbon 2.
• Phosphate group: Attached to the 5-carbon of the sugar, giving nucleic acids their acidic nature.
• Nitrogenous bases: Two types—purines (double-ringed: Adenine A, Guanine G) and pyrimidines (single-ringed: Cytosine C, Thymine T in DNA, Uracil U in RNA).

Types of RNA

Structure of DNA

DNA is a double-stranded helix with two anti-parallel strands running in opposite directions (5'→3' and 3'→5'). The key features are:

• Complementary base pairing: Adenine (A) pairs with Thymine (T) via two hydrogen bonds; Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.
• Sugar-phosphate backbone: Each strand's backbone consists of alternating deoxyribose sugars and phosphate groups.
• Base pairs are 0.34 nm apart; one complete turn contains 10 base pairs.

This structure is stable and allows one strand to serve as a template for replication or transcription.

DNA Replication

DNA replication produces two identical copies of DNA from one original molecule. The process is semi-conservative—each new DNA molecule contains one old strand and one newly synthesized strand.

Steps of DNA replication:

1. Unwinding (helicase): Helicase enzyme unwinds the double helix at the replication fork, breaking hydrogen bonds between base pairs.
2. Stabilization (SSB proteins): Single-stranded binding proteins stabilize the unwound strands.
3. Leading strand synthesis: DNA polymerase III adds nucleotides continuously in the 5'→3' direction toward the replication fork.
4. Lagging strand synthesis: DNA synthesis is discontinuous, producing Okazaki fragments (100-200 nucleotides each). Primase adds RNA primers; DNA polymerase III extends each fragment.
5. Primer removal and gap filling: DNA polymerase I removes RNA primers and replaces them with DNA nucleotides.
6. Ligation: DNA ligase joins Okazaki fragments into a continuous strand.

Key enzymes and their roles:

Significance of DNA Replication

DNA replication ensures genetic continuity during cell division. It is the basis of inheritance, allowing parent cells to pass complete genetic information to daughter cells. Errors in replication (mutations) introduce genetic variation essential for evolution.

Protein synthesis converts the genetic information in DNA into functional proteins through two main processes: transcription and translation.

Transcription

Transcription is the synthesis of mRNA from a DNA template in the nucleus.

1. Initiation: RNA polymerase binds to the promoter region of the gene.
2. Elongation: RNA polymerase moves along the template strand (3'→5'), adding complementary RNA nucleotides (A pairs with U in RNA).
3. Termination: RNA polymerase stops at termination signals, releasing the mRNA transcript.

In eukaryotes, the pre-mRNA undergoes processing:

• 5' capping: Addition of 7-methylguanosine cap
• 3' polyadenylation: Addition of poly-A tail (~200 adenine nucleotides)
• Splicing: Removal of introns (non-coding regions) and joining of exons (coding regions)

Translation

Translation occurs on ribosomes in the cytoplasm. The mRNA sequence is read in triplets called codons, each specifying an amino acid.

Steps of translation:

1. Initiation: mRNA binds to the small ribosomal subunit. The initiator tRNA (carrying methionine, codon AUG) binds to the start codon.

2. Elongation:

   • The ribosome moves along mRNA, reading each codon
   • Corresponding tRNA molecules bring amino acids
   • Peptide bonds form between adjacent amino acids
   • Empty tRNAs are released

3. Termination: When a stop codon (UAA, UAG, or UGA) is reached, release factors terminate translation. The polypeptide chain is released.

The Genetic Code

The genetic code has these key features:

• Triplet: Each amino acid is specified by three bases (codon). With 4 bases, 64 codons are possible (4³ = 64).
• Degenerate: Most amino acids are coded by more than one codon. For example, threonine is coded by ACU, ACC, ACA, ACG.
• Universal: The same code applies to nearly all organisms (exceptions in mitochondria).
• Non-overlapping: Codons are read sequentially without overlap.
• Punctuated: AUG is the start codon; UAA, UAG, UGA are stop codons.

Gregor Mendel established the fundamental laws of inheritance through experiments with pea plants (Pisum sativum).

Mendel's First Law: Law of Segregation

Each characteristic is controlled by a pair of alleles. During gamete formation, the pair separates so each gamete receives only one allele.

Key terms:

• Dominant allele: The allele that expresses its phenotype in a heterozygote (represented by capital letter, e.g., R)
• Recessive allele: The allele whose effect is masked in a heterozygote (represented by lowercase letter, e.g., r)
• Genotype: The genetic makeup (e.g., RR, Rr, rr)
• Phenotype: The physical appearance (e.g., red or white flowers)

Monohybrid Cross

A cross involving one characteristic (e.g., flower color).

Worked Example: Cross a pure-breeding red-flowered plant (RR) with a pure-breeding white-flowered plant (rr).

Self-cross the F₁ plants:

This 3:1 ratio demonstrates that the red allele is dominant over white.

Test Cross

To determine whether a dominant phenotype is homozygous (RR) or heterozygous (Rr), cross it with a homozygous recessive (rr).

• If all offspring show dominant phenotype → parent is homozygous (RR)
• If offspring show 1:1 ratio of dominant:recessive → parent is heterozygous (Rr)

Dihybrid inheritance involves two characteristics simultaneously.

Mendel's Second Law: Law of Independent Assortment

Alleles of different genes assort independently during gamete formation. This explains why new combinations appear in the F₂ generation.

Worked Example: Cross pure-breeding round yellow seeds (RRY) with pure-breeding wrinkled green seeds (rry).

Self-cross F₁ (RrYy × RrYy):

Using a Punnett square with gametes: RY, Ry, rY, ry

F₂ Phenotypic ratio:

• 9 round yellow (R-Y-)
• 3 round green (R-yy)
• 3 wrinkled yellow (rrY-)
• 1 wrinkled green (rryy)

The 9:3:3:1 ratio is the classic dihybrid ratio. Each character independently shows a 3:1 ratio, and the combination produces the observed ratio.

Non-Mendelian Inheritance

Some traits deviate from Mendelian ratios:

Incomplete dominance: Neither allele is dominant; heterozygote shows intermediate phenotype (e.g., red × white → pink in snapdragons). F₂ ratio = 1:2:1.

Codominance: Both alleles are expressed equally (e.g., red and white spotted flowers; human ABO blood groups).

Epistasis: One gene masks the effect of another. Examples include:

• Recessive epistasis (9:3:4): In Labrador dogs, ee genotype produces yellow coat regardless of B/b alleles.
• Dominant epistasis (12:3:1): In squash, white color (W-) masks yellow/green.

Understanding inheritance principles has practical applications in Tanzania. For example, cattle keepers in regions like Mwanza and Shinyanga use knowledge of Mendelian genetics to improve herd quality—breeding selectively for desired traits like disease resistance or high milk yield. Similarly, healthcare workers apply understanding of sickle-cell anemia inheritance (a recessive condition more common in malaria-prone areas) to provide genetic counseling to families, helping them understand the 25% risk when both parents carry the trait.