DNA Replication & Cell Cycle

Preparing and Dividing the DNA Each chromosome consists of two identical chromatids, joined by a centromere In human mitosis, 46 chromosomes form 92 sister chromatids Each chromatid splits from sister and enters one of the two daughter cells Now it is considered to be a single chromosome. chromosomes have telomeres (protective end caps) With every DNA replication, the telomeres on its DNA become shorter. No replication if telomeres are too short. DNA replication must occur before a cell can do cell division The chromatin shorten and thicken into chromatids Dividing the DNA

DNA double helix shape two nucleotide strands they run anti-parallel to each other (in opposite directions) held together by 2 types of bonds

DNA Bonding 1. phosphodiester bonds - link the phosphate of one nucleotide to the sugar of the other to create the sides 2. hydrogen bonds – connect the complimentary nitrogen bases to create the rungs

DNA Replication “copying of DNA’ semi-conservative - each new DNA strand formed is half an old strand and half a new strand.

DNA Replication Steps: 1. DNA strand is pulled apart by the enzyme DNA helicase. 2. DNA polmerase (enzyme), adds nucleotides to each strand which produces two new, identical DNA strands. 3. The strands wrap back up and the cell is ready to divide (mitosis!!!)

The Cell Cycle During the cell cycle, the cell lives, grows, duplicates its DNA, prepares for cell division, and then divides into two daughter cells. The cell cycle

The Cell Cycle Eukaryotic cells go through a growing stage and a division stage. 1. Interphase – growing stage Regular cell activities take up 90 percent of the cell cycle increases protein supply creates new organelles grows larger 3 parts: G 1 & G 2 – gap phase, either side of S phase S – synthesis phase, DNA is replicated

2. Mitosis – M phase 2 processes: mitosis and cytokinesis. 1. mitosis - nucleus and genetic material duplicated and distributed equally between two daughter cells (PMAT). a) Prophase: sister chromatids visible centrioles move to either side of cell spindle fibres attach to chromatids nuclear envelope breaks down

b) Metaphase chromatids attached to spindle fibres lined up across centre of cell

c) Anaphase chromatids separate from sisters to become single chromosomes spindle fibres pull chromo to opposite ends of cell

d) Telophase chromosomes begin to uncoil and form chromatin spindle fibers disappear nuclear envelopes reappear around daughter chromosomes

2. Cytokinesis in animals occurs along the cleavage furrow in plants, occurs along a cell plate (more rigid). two genetically identical daughter cells are produced. Each has a single nucleus, correct amount of DNA, cytoplasm and organelles, and a plasma membrane. Daughter cells Cytokinesis