Bio exam 2 pt. two

Cells reproduce because

• Make more organisms

• Damage repair

• Growth

• Tissue Renewal

Before Division…

all organisms must replicate DNA

Prokaryote Cell Division

Binary Fission

Eukaryote Cell Division

Mitosis

Steps of Binary Fission

• Circular DNA replicated

• Replicated strands attach to plasma membrane

• Plasma membrane elongates, separating DNA

• Plasma membrane grows inward at center of parent cell separating into 2 daughter cells

• Cytokinesis – separation of cytoplasm

Daughter cells are identical

Eukaryotic Cell Cycle Phases

• Interphase

• Mitotic phase

Interphase

Time of normal growth and cell division

Mitotic Phase

Replicated DNA & cytoplasm are split and the cell divides

Mitosis

division of the nucleus

Cytokinesis

division of the cytoplasm

G1 Phase

• Most of the cell’s cycle is spent here

• Primary growth period

  • Cell doubles in size

  • Organelles duplicate

  • Ribosomes, RNA, enzymes, etc. synthesized

G0 Phase (Resting phase)

• G₀ phase is occupied by non-dividing cells (cells not actively preparing to divide)

• Some cells can stay in this ‘dormant’ state permanently (some cardiac, muscle, nerve cells)

• Other cells may just be temporary and can leave and re-enter the active cell cycle if a signal is received

S Phase

• S phase: DNA replication occurs

  • Identical copies of the DNA molecules (sister chromatids) are joined at the centromere

  • Centrosomes produce the mitotic spindle that moves chromosomes

DNA replication begins when

DNA unwinds at “special sites”

These are called origins of replication  (location of DNA separation)

• Prokaryotes (one origin)

• Eukaryotes (Several)

The Replication Fork at the origins of replication

• DNA elongation occurs at the replication fork

  • Catalyzed by enzymes called DNA polymerases (DNA Pol)

DNA Polymerase

They add nucleotides to the 3’ end of the new growing strand

Torsion

• The state of being twisted

• serves as a challenge for DNA replication

DNA replication is initiated by…

Priming DNA synthesis

Replication Protein: Topoisomerase

Relieves torsional/topological stress caused by unwinding; prevents excessive supercoiling.

Replication Protein: Helicase

Unwinds/separates the DNA double helix at the replication fork.

Replication Protein: Single-strand binding proteins (SSB)

Bind separated DNA strands and keep them from re-pairing

Replication Protein: Primase

makes RNA primers that provide a starting point for DNA synthesis.

Replication Protein: DNA polymerase I

Removes/replaces RNA primers with DNA in the replication process described in lecture.

Replication Protein: DNA ligase

Seals breaks between DNA fragments, forming a continuous strand.

Leading Strand

Synthesized continuously.

Needs one RNA primer.

Continuous addition to 3′ end.

Lagging Strand

Synthesized discontinuously.

Each Okazaki fragment needs its own primer.

Short sections = Okazaki fragments that are later joined.