The Cell Cycle Topic 2.5

Cell Cycle The cell cycle consists of Interphase and Cell division

Cell Division There are two forms of cell division: – Mitosis Produces two genetically identical nuclei – Meiosis Is a reduction division. Produces nuclei that has half the chromosome number (haploid) than the original chromosome number (diploid) nucleus. We will discuss meiosis later

Interphase Interphase consists of three distinct phases: – G1 Phase : Growth and copies of chloroplasts and mitochondria – S Phase : Replication of DNA (synthesis phase) – G2 Phase : Preparation for cell division (DNA condenses from chromatin to chromosomes, microtubules form, organelles increase in size)

Terminology Chromatid – A single molecule of DNA. Sister Chromatids – Identical molecules of DNA joined by a centromere. The Centromere is the point at which two sister chromatids join. During mitosis sister chromatids are referred to as one chromosome.

Prophase

Mitosis Mitosis consists of four phases: – Prophase – Metaphase – Anaphase – Telophase After telophase the last phase of the cell cycle (cytokinesis) is completed. Mitosis is responsible for growth of organisms, development of embryos, tissue repair, and asexual reproduction in unicellular organisms..

Prophase Chromatin fibers become tightly coiled to form chromosomes (supercoiling) Nuclear envelope degenerates Mitotic spindles begin to form Centromeres begin to attach to spindles Centrosomes move towards opposite poles of the cell

Prophase

Metaphase Chromosomes move towards the equator of the cell. (metaphase plate) Chromosomes centromeres will align on the metaphase plate Centrosomes are at opposite poles

Metaphase

Anaphase Sister chromatids split apart Chromosomes move toward the opposite poles of the cell. Centromeres move first since they are attached to the spindle fibers. By the end of this phase the poles of the cell should be identical.

Anaphase

Telophase Chromosomes are at each pole Nuclear membrane begins to reform Chromosome starts to elongate Nucleoli reappear Spindle fibers disappear

Telophase

Cytokinesis Once nuclear division is complete the cell membrane will pinch off in the center to form two new cells. This occurs in animal cells Plant cells must form a cell plate at the equator of the cell. (deposits of cellulose and pectin)

Cytokinesis in animals

Cytokinesis in Plants

Plant cell Mitosis

Control of the Cell Cycle A group of proteins called cyclins are responsible for controlling the cell cycle Bind to enzymes (cyclin- dependent kinases) Enzymes then add a phosphate onto other proteins which then become activated and do their job

Tumor formation Abnormal groups of cells Can develop at any stage of life and in any part of the body If the cells adhere to each other and do not invade nearby tissue, they are considered benign If the cells become dethatched and move elsewhere (metastasis), they are considered malignant

Tumor formation

Cancer Due to malignant tumors (uncontrolled mitosis) Caused by mutagens (capable of producing a mutation in the DNA) Also known as carcinogens (x-rays, u-v rays, viruses) Genes that can cause cancer are called oncogenes Genes that are involved in the cell cycle

Smoking and Cancer Correlation vs. Causation Correlation is a relationship between 2 variables Smoking and cancer have a positive correlation; the more cigarettes smoked during a day, the higher the death rate from cancer Correlation does not necessarily mean causation