Introduction to Genetics

Introduction to Genetics

• Genetics studies inheritance, including how traits are passed from one generation to the next, the movement of genes within and between populations, and the molecular basis of inheritance.
• Traditionally, genetics is divided into three disciplines: transmission genetics, molecular genetics, and population genetics.

The Importance of Genetics

• Has applications in agriculture, pharmaceuticals, biotechnology, and medicine.

The Cell Cycle and Cell Division

• The cell cycle represents the life stages a cell goes through from one division to the next.
• This process is essential to genetics as it ensures the transfer of genetic information from parent to daughter cells.
• When cells divide, DNA replicates and grows, both of which need to occur in a coordinated manner.

Phases of the Cell Cycle

• The cell cycle is divided into interphase and M Phase (Mitosis phase).
• Interphase is the period between two successive M phases; it is divided into G1, S, and G2 phases.
• M phase is the actual cell division phase, encompassing karyokinesis (separation of daughter chromosomes) and cytokinesis (division of cytoplasm).

Interphase

• The interphase is the longest phase of the cell cycle, taking up over 95% of the cycle's duration.
• While considered a resting phase, it marks the point where the cell prepares for division through cell growth and orderly DNA replication.

G1 Phase

• The G1 phase is the longest phase in the cell cycle, lasting around 9 hours.
• During G1, the cell grows, synthesizes proteins needed for cell division, and performs its essential functions.
• The cell also duplicates its centrosomes.
• Some cells may enter a quiescent stage (G0) before reaching the G1/S checkpoint, where they are arrested and maintain a constant size.

S Phase

• The S phase is the period during which DNA replication occurs.
• The amount of DNA doubles in the cell, increasing from 2C to 4C.
• However, the chromosome number remains the same.
• In animal cells, DNA replication also takes place in the nucleus, and the centrioles replicate within the cytoplasm.

G2 Phase

• The G2 phase involves the synthesis of proteins required for mitosis, with cell growth continuing.
• The duplication of the centrosome is completed during this stage.

Regulation of the Cell Cycle

• The cell cycle is controlled by a series of checkpoints and growth factors that regulate cell proliferation.
• Checkpoints act as biochemical circuits that monitor the cell's physiological state and signals from the environment.

Checkpoints of the Cell Cycle

• The Restriction checkpoint (G1/S checkpoint): This checkpoint occurs in the G1 phase and assesses cell size and its interactions with the environment. Cells that do not receive sufficient growth signals will not pass this point and may undergo programmed cell death.
• DNA damage checkpoints: During the G1, S, and G2 phases, DNA damage checkpoints ensure the repair of damaged DNA or trigger cell apoptosis.
• The unreplicated DNA checkpoint(G2/M checkpoint): This checkpoint, located in the G2 phase, prevents entry into mitosis before complete DNA synthesis occurs.
• The spindle-assembly checkpoint (the metaphase checkpoint): This checkpoint during mitosis ensures all chromosomes attach correctly to the mitotic spindle before entering anaphase.
• The chromosome-segregation checkpoint: This checkpoint occurs during telophase and prevents cytokinesis until all chromosomes are properly separated.

M Phase

• This phase lasts for approximately 1–2 hours in most mammalian cells.
• It encompasses the division (mitosis) of the cell, resulting in two genetically identical daughter cells from the original cell.
• Mitosis is further divided into six stages: prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis.

Mitosis

• Prophase: chromosomes condense.
• Prometaphase: nuclear envelope breaks down, microtubules attach to chromosomes.
• Metaphase: chromosomes align at the metaphase plate.
• Anaphase: sister chromatids separate and move to opposite poles.
• Telophase: the nuclear envelope reforms around each set of chromosomes.
• Cytokinesis: cytoplasm divides.

Genetic Importance of the Cell Cycle

• The cell cycle ensures the proper replication and segregation of genetic material.
• The result is two identical daughter cells, each carrying the complete set of genetic information.