Introduction to Basic Genetics

Basic Genetics

• Genetics is the study of inherited traits and how they are passed from one generation to the next.
• Genes are the functional units of heredity. They are distinct sections of DNA, providing instructions for specific traits.
• These instructions are encoded within the DNA base sequences.
• Genes encode RNA and proteins; specifically, they contain the instructions for building RNA molecules and protein molecules.
• Alleles are variations of a gene located on a specific section of a chromosome.
• Homozygotes have identical alleles for a particular gene (e.g., AA or aa).
• Heterozygotes have different alleles for a particular gene (e.g., Aa).
• Dominant alleles are expressed even if only one copy is present.
• Recessive alleles are only expressed when two copies are present.
• Genotype is the genetic makeup of an organism, specifying the combination of alleles for a trait.
• Phenotype is the observable physical characteristic or trait. It's the result of the genetic material (genotype).

Useful Terms

• The genetic structure that encodes a trait or the material basis for trait inheritance.
• Visible physical characteristic of an organism.

Where Can Be DNA Found in Plant Cells?

• Every cell in a plant contains the same genetic information.
• Cellular organelles such as the nucleus, chloroplasts, and mitochondria contain hereditary material.
• Gene transfer between different organelles in cells is typical.

Organisation of Cellular DNA

• DNA is a nucleic acid that stores genetic information.
• It was discovered by Friedrich Meischer in 1869.
• DNA has three main components: bases (adenine, cytosine, guanine, thymine), pentose (2-deoxy-β-d-ribose), and phosphorus acid.
• The two strands of DNA are antiparallel.
• Opposite bases are connected by hydrogen bonds.

Organisation of Cellular DNA - Basic Proteins Involved in DNA Unwinding

• Histone proteins are basic proteins involved in DNA unwinding.
• There are five types of histone proteins: H2A, H2B, H3, H4, and H1.
• These histone proteins form a core structure (nucleosome), around which DNA is wrapped.
• A nucleosome is a structural unit consisting of DNA wrapped around histone proteins.
• The length of DNA in a human cell is approximately 2 meters.
• DNA folding is essential in packaging and preventing tangling, breakage, or clumping.

Organisation of Cellular DNA - Chromosome

• Centromere regions play a role in chromosome pairing and displacement in cell division.
• The centromere acts as a binding site for microtubules.
• Cohesin proteins link homologous chromosomes.
• Telomere regions are located at the ends of chromosomes.
• Telomeres assist in stabilizing chromosome ends and prevent the loss of sequence ends.
• Tandem repeats form the telomere region.

Organisation of Cellular DNA - Structure of a Gene

• Promoter region is a transcriptional regulatory stage for RNA polymerase recognition.
• Coding region is the segment of the gene that codes for proteins.
• Exons are gene segments that code for proteins.
• Introns are gene segments that do not code for proteins.
• Termination sequence is a regulatory region that signals the end of transcription.

Organellar DNA Organization - Plastid Genome (ptDNA)

• Plant cells have chloroplasts, whose number ranges from 10-100 per cell, and their size is approximately 5-10 µm.
• Plastid genome, cpDNA, typically contains approximately 120-130 genes, including around 80 protein-coding genes and different rRNA and tRNA genes.
• The genome is comparatively small (0.1-0.001%), but it can sometimes take up 5-10% of the total DNA in a cell.
• Various plastid types have the same genetic composition.

Plastid Genome - cpDNA

• Circular DNA typically contains repetitive inverted regions (IRA and IRB).

Organellar DNA Organization - Plastid Genome (ptDNA)

Organisation of Mitochondrial Genome

• Mitochondria are cellular organelles for energy production and ATP synthesis.
• They have a double membrane system (internal and external membranes).
• Usually they have a bacterial shape (sphere or cylinder).

Mitochondrial Genome (mtDNS)

• Mitochondrial genome is more diverse than the plastid genome.
• Its size varies from 100-3000 kb, which is significantly more diverse than the size range for plastid DNA.
• It is usually circular and double-stranded.
• Coded genes are separated into "housekeeping" genes (for rRNA, tRNA, and proteins) and other genes required for mitochondrial function, including ATP synthase, NADPH dehydrogenase, etc.

Cell Cycle (in Plants ~29 Hours)

• The cell cycle is the series of events that occur in eukaryotic cells between two divisions.
• It comprises interphase, mitosis, and cell division.
• Interphase stages include G1 (first growing stage), G0 (waiting stage), S (DNA duplication), G2 (second growing stage), and the M (mitosis and cytokinesis) stage.
• Cyclin and cyclin-dependent kinase (CDK) proteins are key regulators of the cell cycle.

Cell Cycle – S Phase – DNA Replication

• DNA replication (in the S phase) is a semi-conservative process, using both strands as templates.
• Replication involves three phases: initiation (formation of the replication fork), elongation (DNA synthesis in the 5'-3'), and termination (at telomere sections).
• Essential enzymes involved include DNA polymerase, DNA primase, helicase and topoisomerase.

Cell Cycle – M Phase – Division

• Mitosis is cell division in somatic cells that involves duplication and separation of chromatin to form two daughter cells with an identical number of chromosomes as the parental cell.
• The stages include interphase, prophase, metaphase, anaphase, telophase, and cytokinesis.

Cell Division - Cytokinesis

• Cytokinesis marks the physical division of the cytoplasm, starting during anaphase.
• It involves the formation of a cell wall or other structure to separate the two daughter cells.

Cell Division – Meiosis

• Meiosis is a cell division process for gametes (sex cells) and involves two stages each comprising several phases (e.g., prophase, metaphase, anaphase, telophase).
• The first stage involves pairing homologous chromosomes and crossing over.
• The second stage involves pulling homologous chromosomes to opposite poles of the cell, followed by a mitotic-like division.

Meiosis – Crossing Over

• During meiosis I, homologous chromosomes exchange segments of DNA through crossing over (allele substitution)
• This leads to genetic variation.