Unit 2 “Cells” Part 3

Introduction Organisms grow in size by increasing both the size and number of its cells A single cell grows, divides into two cells, then two cells grow, divide into four cells, etc.

The Cell Cycle The regular sequence of growth and division that cells undergo There are 3 stages: Interphase, Mitosis, and Cytokinesis

Understanding check

Interphase First stage of the cell cycle The period before cell division Cell grows, makes a copy of its DNA, and prepares to divide into two cells (“daughter cells”) Growth Cell grows to its full size and produces structures it needs (new ribosomes, copies of mitochondria and chloroplasts)

Copying DNA Replication: the process in which a cell makes an exact copy of its DNA in the nucleus At the end of replication, the cell contains two identical sets of its DNA Preparing for Division Once DNA is replicated, cell prepares for division Cell produces structures that it will use to divide into two new cells

Mitosis Second stage of the cell cycle Stage during which the cell’s nucleus divides into two new nuclei One copy of the DNA is distributed into each of the two daughter cells Divided into four parts, or phases: prophase, metaphase, anaphase, and telophase

1. Prophase Chromatin in nucleus condenses to form double-rod structures (c hromosomes) each chromosome has two rods b/c the cell’s DNA has replicated, and each rod in a chromosome is an exact copy of the other Each individual rod is called a chromatid and they are held together by a centromere

2. Metaphase Chromosomes line up across the center of the cell Each chromosome attaches to a spindle fiber at its centromere

3. Anaphase Centromeres split; the two chromatids separate One chromatid is pulled by its spindle fiber to one end of the cell; other chromatid moves to the opposite end of the cell Cell stretches out as the opposite ends are pushed apart

4. Telophase Chromosomes begin to stretch out and lose their rod-like appearance A new nuclear membrane forms around each area of chromosomes

Understanding Check

Cytokinesis Third stage and final stage of the cell cycle Division of the cytoplasm occurs; organelles are distributed into each of the two new cells Usually starts about the same time as telophase Two new daughter cells are formed; each has the same number of chromosomes as the original parent cell In plant cells, a cell plate forms and this turns into new cell membranes Each cell now enters interphase, and the cycle begins again