Heredity and Continuity of Life: DNA, Genes, and Protein Synthesis

DNA Distribution and Structural Organization

• Deoxyribonucleic acid ($DNA$) exists in different structural forms and locations depending on the cell type:

  • In eukaryotes, DNA is found in the nucleus as a double-stranded molecule bound to histone proteins, forming chromosomes. It is also present in mitochondria ($mtDNA$) and chloroplasts ($cpDNA$).

  • In prokaryotes, DNA occurs as unbound circular molecules within the cytosol.

• DNA is a nucleic acid macromolecule responsible for carrying the genetic code and directing all cellular activities. It stores hereditary information to be passed across generations.

• Ribonucleic acid ($RNA$) is typically single-stranded and shorter than DNA. Its primary roles involve reading and translating the genetic sequence stored in DNA to carry out protein synthesis.

The Molecular Composition of Nucleotides

• Nucleic acids are composed of monomers known as nucleotides. Each nucleotide consists of three fundamental components:

  • A phosphate group, which forms the sugar-phosphate backbone.

  • A five-carbon ($pentose$) sugar: deoxyribose in DNA and ribose in RNA.

  • A nitrogenous base: Adenine ($A$), Guanine ($G$), Cytosine ($C$), Thymine ($T$) in DNA only, or Uracil ($U$) in RNA only.

• The pentose sugar carbons are numbered $1'$ to $5'$:

  • The phosphate group is attached to the $5'$ carbon.

  • The nitrogenous base is attached to the $1'$ carbon.

• Nucleotides join to form polynucleotide chains. Two chains run in opposite directions, described as being antiparallel ($5'$ to $3'$ and $3'$ to $5'$).

• Nitrogenous bases are classified by structure:

  • Purines: Adenine and Guanine (double-ring structure).

  • Pyrimidines: Cytosine, Thymine, and Uracil (single-ring structure).

• Complementary base pairing occurs via weak hydrogen bonds:

  • $A$ pairs with $T$ (or $U$) using $2$ hydrogen bonds.

  • $G$ pairs with $C$ using $3$ hydrogen bonds.

DNA Replication Mechanics

• Replication is a semi-conservative process, meaning each new DNA molecule consists of one original template strand and one newly synthesized strand.

• The process occurs in the nucleus of eukaryotes and involves three distinct phases:

  • Unwinding and Unzipping: The enzyme helicase breaks the weak hydrogen bonds between nitrogenous bases, exposing the strands at the replication fork.

  • Synthesis: The enzyme DNA polymerase builds the new complementary strand by adding nucleotides in the $5'$ to $3'$ direction. This means it moves along the template strand in the $3'$ to $5'$ direction.

  • Priming: Primase creates a starting point (primer) because DNA polymerase requires a pre-existing chain to begin.

• Synthesis is directional:

  • Leading strand: Synthesized continuously toward the replication fork ($5'$ to $3'$).

  • Lagging strand: Synthesized discontinuously away from the fork in segments known as Okazaki fragments, which are later joined.

• DNA polymerase also performs proofreading to identify and remove incorrect nucleotides.

Genomic Organization and the Genetic Code

• A gene is a specific region of DNA that contains instructions for producing a protein or functional RNA molecule. Its location on a chromosome is called the locus.

• The genome represents the complete set of genetic material in a haploid set of chromosomes for an organism.

• The genetic code is universal and based on three-letter sequences of nucleotides:

  • In DNA, these are called triplets.

  • In mRNA, these are called codons.

• Each codon corresponds to one of $20$ different amino acids. Specific codons initiate (START) or terminate (STOP) the translation process. The start codon is typically $AUG$ (Methionine).

Coding and Non-Coding DNA Components

• Approximately $1\%$ of human DNA consists of protein-coding genes ($exons$). The remaining $99\%$ is non-coding DNA.

• Exons: Portions of a gene expressed and kept in the final RNA sequence.

• Introns: Non-coding sections within a gene removed during RNA processing.

• Functional Elements of Non-coding DNA:

  • Centromeres: Link sister chromatids.

  • Telomeres: Repetitive sequences at chromosome ends that prevent deterioration.

  • Functional RNA genes: Code for molecules like $tRNA$ or $rRNA$.

  • Regulatory sequences: Include promoters, enhancers, and silencers that control transcription.

  • Satellite DNA: Tandemly repeating sequences used in DNA profiling.

Protein Synthesis: Transcription and RNA Processing

• Gene expression is the process of using genetic information to synthesize functional products like proteins. It occurs in three main stages:

• 1. Transcription (Nucleus):

  • Initiation: RNA polymerase binds to the promoter region (often containing a TATA box) and separates the DNA strands.

  • Elongation: RNA polymerase moves along the template strand, adding complementary RNA nucleotides ($A, U, G, C$) to produce primary RNA.

  • Termination: The enzyme reaches a termination sequence and detaches, releasing the primary transcript.

• 2. RNA Processing:

  • A $5'$ cap (modified guanosine) is added to facilitate ribosome attachment.

  • A Poly-A tail (adenine chain) is added to the $3'$ end.

  • Splicing: Introns are removed, and exons are joined together to form mature mRNA, which then exits the nucleus.

Protein Synthesis: Translation and Polypeptide Formation

• 3. Translation (Ribosome):

  • Initiation: A ribosome attaches to the $5'$ end of mRNA. A $tRNA$ molecule with the anticodon $UAC$ carries Methionine to the start codon $AUG$.

  • Elongation: The ribosome moves along the mRNA. Successive $tRNA$ molecules bring specific amino acids based on codon-anticodon matching. Amino acids are linked by peptide bonds to form a polypeptide chain.

  • Termination: The process stops when a stop codon is reached. The polypeptide is released for folding into a functional protein.

Gene Regulation and Expression Control

• Gene regulation involves mechanisms that increase or decrease the production of gene products to ensure they are available only when needed.

• Transcription factors are proteins coded by regulatory genes that control expression by blocking or accelerating the attachment of RNA polymerase.

• Phenotypic expression can be regulated during transcription, translation, through the products of other genes, or by environmental factors (e.g., glucose levels affecting insulin gene expression).

• Tumour-suppressor genes (like the gene for the $p53$ protein) inhibit cell division to prevent cancer. If $p53$ is inactive (as seen in $50\%$ of all cancers), the risk of uncontrolled cell growth increases.

Master Regulatory Genes and Epigenetics

• Master regulatory genes, such as $Hox$ genes, control the development of the entire body plan (e.g., head-to-tail orientation) by turning other regulatory genes on or off during embryonic development. $SRY$ genes specifically influence sex determination.

• Epigenetics refers to mechanisms that alter gene expression without changing the DNA sequence. This includes:

  • Histone modification: Changing how DNA coils around histones.

  • DNA Methylation: The addition of methyl groups to DNA to influence coiling and accessibility.

Categorization of Genetic Mutations

• A mutation is a change in the nucleotide sequence of DNA. They can be spontaneous or induced by mutagens.

• Somatic mutations: Occur in body cells; affect only the individual and are not heritable.

• Germline mutations: Occur in gametes (sperm/ova); can be passed to offspring.

• Point Mutations: Changes to a single base.

  • Synonymous (Silent): No change to the amino acid sequence.

  • Non-synonymous: Results in an amino acid replacement (e.g., sickle cell anaemia).

• Frameshift Mutations: Caused by insertions or deletions ($indels$). These shift the reading frame, altering all subsequent codons and often resulting in non-functional proteins (e.g., cystic fibrosis).

Mutagens and Environmental Influences

• Mutagens increase the rate of mutation above the spontaneous level ($1 \times 10^{-6}$ base pairs). Carcinogens specifically increase the incidence of cancer.

• Physical Mutagens: Ionising radiation (X-rays) and non-ionising radiation ($UV$). UV radiation can cause thymine dimers (bases on the same strand bonding together).

• Chemical Mutagens: Pesticides or base analogs that substitute for nitrogenous bases, leading to misreading or faulty pairing.

• Biological Mutagens: Viruses (Hepatitis B, $HPV$) and bacteria that integrate their DNA into the host genome.

• Heat Mutagens: Can break the bond between the sugar and the base; linked to declines in male fertility.

Aneuploidy and Chromosomal Non-Disjunction

• Non-disjunction occurs when homologous chromosomes (Meiosis I) or sister chromatids (Meiosis II/Mitosis) fail to separate correctly.

• Aneuploidy is the resulting condition of having an abnormal number of chromosomes.

• Specific conditions include:

  • Down Syndrome ($Trisomy 21$): Three copies of chromosome $21$ ($47$ total chromosomes).

  • Klinefelter Syndrome ($XXY$): An extra $X$ chromosome in males ($47$ total).

  • Patau Syndrome ($Trisomy 13$): Three copies of chromosome $13$ ($47$ total).

  • Turner Syndrome ($Monosomy X$): A missing or altered $X$ chromosome in females ($45$ total).

Questions & Discussion

• Recall the building blocks of DNA and distinguish between the functions of DNA and RNA.

• Illustrate nucleotide structure and explain the ladder metaphor for double-helix DNA.

• Determine complementary strands for given sequences (e.g., $5'-ATTCCGTA-3'$ pairs with $3'-TAAGGCAT-5'$).

• Analyze base composition data (e.g., Humans: $A=30.4\%$, $T=30.1\%$, $G=19.6\%$, $C=19.9\%$) to support the theory of complementary base pairing.

• Define allele, gene, and genome.

• Explain why frameshift mutations are typically more damaging than substitutions.

• Describe the life cycle stages of transcription and translation (initiation, elongation, termination).

• Contrast spermatogenesis and oogenesis in terms of haploid and diploid cells.