Chapter 5 Section 3 Cell Division

“The Cell Cycle” The cell cycle is the regular sequence of growth and division that cells undergo One “parent” cell divides into two identical “daughter” cells The length of the cell cycle depends on the type of cell. Some cells such as the human brain cells, never divide.

The Cell Cycle-Stage 1: Interphase Interphase is the period before cell division occurs The cell grows to its mature size, makes a copy of its DNA, and prepares to divide into two cells. Growth *during the 1st part of interphase, the cell doubles in size and produces all the structures needed to carry out its function

Stage 1: Interphase Growth *the cell enlarges its endoplasmic reticulum, makes new ribosomes, and produces enzymes. Both mitochondria and chloroplasts make copies of themselves. The cell matures and grows to its full size.

Stage 1: Interphase DNA Replication-The next part of interphase *replication-the cell makes a copy of its nucleus in this process * DNA is a nucleic acid found in the chromatin in a cell’s nucleus * DNA is the genetic material that holds all the information that a cell needs to carry out its functions *Each daughter cell must have a complete set of DNA to survive *At the end of DNA replication, the cell contains two identical sets of DNA-one for each daughter cell

Interphase

Stage 2: Mitosis Mitosis-the stage in which the cell’s nucleus divides into two new nuclei. During mitosis, one copy of DNA is distributed into each of the two daughter cells. Mitosis is divided into 4 parts or phases: Prophase, Metaphase, Anaphase, and Telophase

Prophase During this stage the chromatin in the cell’s nucleus begins to condense and coil, like fishing line wrapping around a ball. The condensed chromatin looks like tiny rods which doubled during DNA replication Each double rod of condensed chromatin is called a chromosome Each rod/strand of the chromosome is called a chromatid. The two strands are held together by a structure called a centromere

Prophase

Metaphase Metaphase-the chromosomes line up across the center of the cell. Each chromosome attaches to a spindle fiber at its centromere, which still holds the chromatids together.

Metaphase

Anaphase The centromere split and the two chromatids separate. The chromatids move along the spindle fiber to opposite ends of the cell The cell becomes stretched out as the opposite ends pull apart.

Anaphase

Telophase The chromosomes begin to stretch out and lose their rod-like appearance . Occurs at the two regions at the ends of the cell. A new nuclear membrane forms around each region of chromosomes

Telophase

Stage 3: Cytokinesis During cytokinesis, the cytoplasm divides, distributing the organelles into each of the two new cells. Cytokinesis usually starts about the same time as telophase.

Cytokinesis *Animal Cells -the cell membrane squeezes together around the middle of the cell. The cytoplasm pinches into two cells with about half of the organelles in each daughter cell. *Plant Cells – a plant cell’s rigid cell wall can’t squeeze together, instead a structure called a cell plate forms across the middle of the cell. The cell plate gradually develops into new cell membranes between the 2 daughter cells, then new cell walls then form around the cell membranes.

Cytokinesis

The Entire Cell Cycle

DNA Replication Ensures that each daughter cell will have all of the genetic information it needs to carry out its activities. James Watson and Francis Crick, figured out the structure of DNA.

The Structure of DNA DNA molecules look like a twisted ladder, or spiral staircase. Often called a “double helix” The two sides of the DNA ladder are made up of molecules of a sugar called deoxyribose, alternating with molecules known as phosphates.

The Structure of DNA Each rung of the DNA ladder is made up of a pair of molecules called nitrogen bases. Four kinds of nitrogen bases: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C) Adenine only pairs with Thymine Guanine only pairs with Cytosine

The Structure of DNA

The Replication Process DNA replication begins when the two sides of a DNA molecule unwind and separates or “unzips” between the paired nitrogen bases on each rung. New bases pair with the base on each strand. As a result, two identical DNA molecules form.