Ch 3d part 4 Lecture Video

Cells spend most of their time in interphase before division.
Interphase is where normal growth and activities occur before cell division begins.
S Phase: Key phase where DNA replication occurs to prepare for mitosis.

G1 Phase (Gap 1)
- Cell growth and metabolic activities take place.
- If a cell does not divide, it enters G0 Phase, especially in post-mitotic cells like neurons.
S Phase (Synthesis)
- DNA replication occurs.
- DNA strands are copied to ensure each new cell has the same genetic information.
G2 Phase (Gap 2)
- Organelles double in number and prepare for cell division.
- Ensures the cell is ready for mitosis.

Process of cell division that produces identical daughter cells.
Starts with 46 chromosomes (in humans) and ends with two cells, each with 46 chromosomes.
Important for: skin cells, muscle cells, bone cells, etc.

Prophase
- Chromosomes condense and become visible.
- The mitotic spindle begins to form and the nuclear envelope starts breaking down.
Metaphase
- Chromosomes align at the cell's equator.
- Spindle fibers attach to chromosomes.
Anaphase
- Sister chromatids are pulled apart to opposite poles of the cell.
- The acronym "A for Anaphase, A for Apart" illustrates this process.
Telophase
- Nuclear envelopes reform around separated chromatids.
- Chromosomes de-condense back into chromatin.
Cytokinesis
- Cytoplasm divides, leading to two separate cells (cleavage furrow formation).
- Ends mitosis and completes cell division.

Occurs before mitosis to ensure both daughter cells receive identical genetic material.
Semiconservative model: each new double helix consists of one old and one new strand.

DNA Helicase unwinds and unzips the DNA double helix forming replication forks.
DNA Polymerase: Enzyme that synthesizes new DNA strands by pairing nucleotides according to base-pairing rules (A with T; C with G).
Leading Strand: Continuously synthesized in the 5' to 3' direction.
Lagging Strand: Synthesized discontinuously in short segments (Okazaki fragments) due to the directionality of replication.
DNA Ligase: Enzyme that joins Okazaki fragments on the lagging strand.

Adenine (A) pairs with Thymine (T) in DNA, while in RNA, Thymine is replaced by Uracil (U).
Guanine (G) pairs with Cytosine (C).

Occurs in the nucleus, converting DNA to RNA.
mRNA synthesized based on DNA templates, following base pairing rules (A with U, C with G).
Introns removed, and exons expressed in the final mRNA product.

Takes place in the cytoplasm at ribosomes.
tRNA brings amino acids to the ribosome, corresponding to codons on the mRNA.
Polypeptide chain is assembled based on the sequence of codons until a stop codon is reached.
Three stop codons signal the termination of protein synthesis (UAA, UAG, UGA).

Checkpoints in the cell cycle ensure healthy DNA and proper division.
G1 Checkpoint: Evaluates DNA integrity before replication.
G2 Checkpoint: Verifies the correctness of DNA replication.
M Checkpoint: Confirms that all chromosomes are correctly aligned and separated before division.

Cells have a finite life span due to telomere shortening and cumulative damage from free radicals and chemical insults.
Telomeres: Protective caps on chromosome ends that shorten with each cell division.
Telomerase: Enzyme that can extend telomeres, mostly active in germ cells and cancer cells.

All cells carry the same DNA, but gene expression differentiates their functions (e.g., skin cell vs. nerve cell).
Cells exhibit hyperplasia (increased cell numbers) and atrophy (decreased cell size).
Apoptosis: Programmed cell death to remove damaged or unnecessary cells.

Key biological processes of DNA replication and protein synthesis drive cellular function and life.
Healthy cell division and replication mechanisms are critical in biology, impacting growth, repair, and development.