Cell Division Mitosis and Meiosis

In Prokaryotes The DNA is duplicated prior to cell division

In Prokaryotes In most prokaryotes, the rest of the process of cell division is a simple matter of separating the contents of the cell into two parts.

In Eukaryotes In eukaryotes, cell division is more complex and occurs in two main stages. The first stage, division of the cell nucleus, is called mitosis (my-TOH-sis). The second stage, division of the cytoplasm, is called cytokinesis (sy-toh-kih-NEE-sis).

Asexual Reproduction Reproduction by mitosis is classified as asexual since the cells produced by mitosis are genetically identical to the parent cell. Mitosis is also the source of new cells when a multicellular organism grows and develops.

Chromosomes In eukaryotic cells, the genetic information that is passed on from one generation of cells to the next is carried by chromosomes. Chromosomes are made up of DNA—which carries the cell's coded genetic information—and proteins.

Chromosomes The cells of every organism have a specific number of chromosomes. Fruit flies have 8 chromosomes Human cells have 46 chromosomes Carrot cells have 18 chromosomes

Chromosomes Chromosomes are not visible in most cells except during cell division At the beginning of cell division the chromosomes condense into compact, visible structures that can be seen through a light microscope.

Chromosomes Well before cell division, each chromosome is replicated, or copied. Because of this, each chromosome consists of two identical “sister” chromatids (KROH-muh-tidz) When the cell divides, the “sister” chromatids separate from each other. One chromatid goes to each of the two new cells.

Cell Cycle Eukaryotes have separate periods of cell division with intervals of growth and protein synthesis. This is represented as the Cell Cycle. Interphase is the longest phase of the cycle, followed by cell division and cytokinesis.

Interphase = Time between cells divisions. Normal Cell Functioning

Early Prophase

Prophase – DNA condenses into chromosomes Nuclear envenlope & nucleolus disintegrate, spindles appear Spindles attach to chromosome at centromere

Metaphase- Spindles move chromosomes to cell equator

Anaphase Spindles contract pulling chromosomes to opposite ends. Centromeres split to separate sister chromatids apart.

Telophase Single stranded chromosomes stop moving & unwind. Nuclear envelope and nucleolus reappear. Spindles disintegrate.

Chromosome Number Most eukaryotes have two sets of chromosomes: one set from each parent. Cells that have two sets of chromosomes are diploid (2n). Cells with one set are haploid (n). Thus diploid organisms have two copies of each chromosome. These chromosome pairs are known as homologues or Homologous Chromosomes

Karyotype = A photomicrograph of an individuals chromosomes Chromosomes are pasted with their homologous partner

Sex Determination Females XX Males XY

Sexual Reproduction = Meiosis Gamete = sexual cells. EX: egg and sperm. Haploid in chromosome number Somatic Cell = non sex cells always diploid. EX: skin, muscle, nerve, bone

How do You get haploid gametes? Meiosis = the process of nuclear division that reduces the number of Chromosomes in the original cell to the haploid number Requires 2 cell divisions

Meiosis 1 Prophase 1 = Nuclear envelope and nucleolus disappear. Chromatin condenses into chromosomes. Chromosomes partner up into Homologous pairs. Crossing over of non sister chromosomes may occur to exchange genetic information

Meiosis 1 continued Metaphase 1 = Homologous chromosome pairs line up along the cell equator Anaphase 1 = Homologous chromosome pairs split up and move as spindle fibers contract, toward opposite cell poles

Meiosis 1 continued Telophase 1 = chromosomes stop moving begin to unwind back into chromatin and nuclear envelope and nucleolus reform. Cell splits into 2 daughter cells that must have further reduction of the chromosomes number.

Meiosis 2 – Each daughter cell follows the path. Prophase 2 = Nuclear envelope and nucleolus disappear. Chromosomes reform. Metaphase 2 = CHromsomes line up along cell equator. Anaphase 2 = Sister chromatids separate from each other.

Meiosis 2 continued Telophase 2= sister chromatids reach opposite poles of cell, unwind back into chromatin. Nuclear envelope and nucleolus reform. Cytokinesis = Result in 4 non identicle , HAPLOID gametes.

End Results of Mitosis versus Meiosis Mitosis = 2 identical, DIPLOID, daughter cells Meiosis = 4 non – identical , HAPLOID gametes

Males vs Females Males = spermatogenesis, 4 haploid Females = oogenesis, results in 1 large haploid egg and 3 small polar bodies.