Ch. 6 Cell Division and DNA Replication

Focuses on the processes of eukaryotic cell division, including mitosis and cytokinesis, as well as DNA replication.

Early Interphase:
- Chromosomes appear as uncondensed structures; nuclear envelope is intact.
Late Interphase:
- Duplicated chromosomes are present, with sister chromatids attached at the centromere.

Prophase:
- Chromosomes become condensed and visible, nuclear membrane begins to dissolve.
- Mitotic spindle forms at opposite ends (poles) of the cell.
Metaphase:
- Sister chromatids align at the metaphase plate (equatorial plane of the cell).
Anaphase:
- Spindle fibers retract, pulling sister chromatids to opposite ends of the cell.
- Microtubules connected to centromeres facilitate this movement.
Telophase:
- Chromosomes start to de-condense as nuclear membranes re-form around each set of chromosomes.

The process involves the final division of the cytoplasm.
In animal cells:
- Contractile microfilaments form a cleavage furrow that pinches the cell into two.

Mitosis: Deals with the division of the nucleus, results in two genetically identical daughter nuclei.
Cytokinesis: Focuses on cytoplasmic division, resulting in two separate cells.

Necessary for:
1. Growth of an organism.
2. Repair and maintenance of tissues.
3. Asexual reproduction in unicellular organisms.

Requires extraordinary accuracy during the process:
- Replication rates: Can reach approximately 1000 nucleotides per second, taking nearly 8 hours for replication in a human cell.
- Rare copying errors may lead to mutations.
- DNA replication is performed by proteins working together known collectively as the replication machine.
- Every time a cell divides, it must duplicate a substantial amount of DNA.

Antiparallel Strands: The strands run in opposite directions (5'-Prime opposite to 3'-Prime).
Base-pairing: Each strand serves as a template for synthesizing a new strand.
- The newly synthesized strand consists of one ‘parent’ strand and one newly synthesized strand. This process is known as semiconservative replication.

Initiation:
- Begins at the replication origin, where initiator proteins bind to specific sequences in the DNA.
Formation of Replication Forks:
- Two replication forks form at each origin, moving in opposite directions.
Nucleotide Addition:
- DNA is synthesized from the 5’ to 3’ direction, with the enzyme DNA polymerase adding nucleotides to the 3' end of the growing strand.
- The energy for nucleotide polymerization comes from the cleavage of phosphate bonds in the incoming nucleoside triphosphate.
Leading and Lagging Strands:
- DNA polymerase synthesizes continuously on the leading strand but discontinues on the lagging strand, creating small DNA segments known as Okazaki fragments.

Self-correction:
- DNA polymerase contains separate domains for polymerization (P site) and editing (E site), allowing for proofreading.
Primase Role:
- Primase adds RNA primers to provide a free 3' end for DNA polymerase to begin synthesis.
Joining of Okazaki Fragments:
- After removal of RNA primers by nucleases, repair polymerase replaces RNA with DNA, and fragments are joined by DNA ligase.
Torsional Stress:
- The unzipping of DNA at replication forks causes torsional stress, which is relieved by topoisomerase that creates a break in one strand ahead of the replication fork.

Telomeres: These structures prevent the ends of chromosomes from shortening with each DNA replication.
- There is no enzyme that replaces the RNA primer at the end of the lagging strand, leading to gradual shortening.
- Telomerase adds telomere repeats to extend the template strand, enabling completion of chromosome copying.
Variability:
- Telomere length varies by cell type and age. Cells may reduce telomerase activity as they age, leading to telomere shortening, while others maintain telomerase activity.