Cell Growth and Division Mitosis and Meiosis

Cell Growth When an organism grows, the number of cells increase but the size of each cell remains small.

Limits to cell growth DNA “overload”: The larger a cell becomes, the more demands the cell places on its DNA. a.     DNA stores the information that controls how a cell functions b.     When a cell is small, DNA can meet the cells needs c. When a cell is large, it still has only one copy of DNA so it is more difficult for the cell to perform its function

Limits to cell growth 2. Exchanging materials: additionally, large cells have more trouble movingsubstances across the cell membrane. a.  If a cell is too large, it is difficult to get enough oxygen and nutrients in and waste products out b. This is why cells do NOT grow much larger even if the organism does grow large

Division of the Cell Before a cell gets too large, it will divide to form two “daughter” cells Before a cell divides, it makes a copy of its DNA for each daughter cell

Cell Division Cell division in eukaryotes is more complex than in prokaryotes. There are two stages of eukaryotic cell division Mitosis: Division of the cell nucleus Cytokinesis: Division of the cell cytoplasm Unicellular organisms reproduce asexually by mitosis a. The daughter cells are identical to the parents cells 4. Mitosis is how a multicellular organism grows and develops

Chromosomes Chromosomes are made of condensed chromatin. Chromatin consists of DNA and the proteins it is wrapped around. The cells of every organism have a specific number of chromosomes (humans have 46 chromosomes).

3. Chromosomes are only visible during cell division, when they are condensed. The rest of the time the chromatin is spread throughout the nucleus. Before cell division, each chromosome is replicated (meaning copied). a. When a chromosome is replicated, it consists of two identical “sister” chromatids. b. When a cell divides the chromatids separate, and one goes to each of the two new cells. c. Sister chromatids are attached to each other at the spot called the centromere.

The Cell Cycle When a cell is NOT dividing, it is said to be in interphase. The series of events that a cell goes through as it grows and divides is called the cell cycle.

Events of the cell cycle Interphase, when the cell is NOT dividing, has three phases: G1, S, and G2. 1.   G1 phase: period of activity in which cells do most of their growing. a.  Cells increase in size b.  Cells synthesize (make) new proteins and organelles 2.   S phase: DNA (chromosomes) is replicated 3. G2: organelles and molecules required for cell division are produced

2.Cytokinesis, or the division of cytoplasm. M phase is the phase of cell division. This includes: 1.Mitosis, the division of the cell nucleus, which is made up of four segments including prophase, metaphase, anaphase, and telophase. 2.Cytokinesis, or the division of cytoplasm. Stop at 1 minute

M phase G2 phase S phase G1 phase

Mitosis There are four phases in mitosis: 1.     Prophase a. Longest phase in mitosis (take 50- 60% of total time mitosis requires) b.     Chromosomes become visible because they are condensed

e. Nuclear envelope breaks down c.    Centrioles become visible on opposite sides of the nucleus i. The centrioles help organize the spindle, a structure made of microtubules that helps separate the chromosomes ii. Chromosomes attach to the spindle fibers near the centromere iii.      Plant cells do not have centrioles but do have mitotic spindles d.     Nucleolus disappears e.    Nuclear envelope breaks down

2. Metaphase. a. Chromosomes line up in the center of. the cell. b 2. Metaphase a.     Chromosomes line up in the center of the cell b. Microtubules connect to the centromeres

3. Anaphase a.     Centomeres split and the sister chromatids separate b.     The sister chromatids become individual chromosomes c.      Chromosomes move and separate into two groups near the spindle d. Anaphase ends when the chromosomes stop moving

4. Telophase a. Chromosomes change from being condensed to dispersed b. A nuclear envelope forms around each cluster of chromosomes c.   Spindle breaks apart d. Nucleolus is visible in each daughter nucleus Telophase in the midbodies of two daughter cells Start at 1 minute

Cytokinesis Cytokinesis occurs within the cytoplasm of one cell. Cell division is complete when the cytoplasm divides (pinches together to create new cells). In plants, a structure called the cell plate forms between the two daughter nuclei. The cell plate develops into a cell membrane and cell wall.

Cytokinesis In animal cells, the cell membrane is drawn inward until the cytoplasm is pinched into two equal parts. Each part has a nucleus and cytoplasmic organelles. The cleavage of daughter cells is almost complete; this is visualized by microtubule staining

Interphase Cytokinesis Telophase Centrioles Chromatin Interphase Nuclear envelope Cytokinesis Nuclear envelope reforming Telophase Anaphase Individual chromosomes Metaphase Centriole Spindle Chromosomes (paired chromatids) Prophase Centromere Spindle forming

Meiosis

Chromosomes You have 23 different pairs of chromosomes, for a total of 46 chromosomes. One chromosome in each pair came from your mother and one from your father.

Each chromosome in a pair is said to be homologous, meaning that the chromosome from the father has a corresponding chromosome from the mother.

Cells that contain both sets of homologous chromosomes are called diploid. All of your cells except the sex cells (sperm and eggs; also called gametes) are diploid. Gametes are haploid, meaning they contain only one copy of each chromosome. When one sperm and one egg combine their DNA, there are two versions of each chromosome.

haploid haploid diploid

We use “N” to represent the haploid number of chromosomes and “2N” to represent the diploid number of chromosomes. 1. For humans, the haploid number is 23. We write this as N = 23. The diploid number is 46, which we write as 2N = 46. 2. In fruit flies, N = 4 and 2N = 8.

Meiosis Meiosis is the process that divides one diploid (2N) cell to form four haploid (N) cells. This process is a reductional division because the number of chromosomes per cell are cut in half. Meiosis is how gametes are formed.

There are two divisions that occur in meiosis: Meiosis I and Meiosis II. 1. Before meiosis I begins, cells go through Interphase I. 2. This involves DNA replication, forming a duplicate copy of each chromosome. 3. Each chromosome is made of two sister chromatids. centrioles Nucleus with duplicated DNA that is not condensed. cell

Meiosis I is similar to mitosis. a. Prophase I: i.      Centrioles are visible on opposite sides of the nucleus ii.      Nucleolus disappears iii.      Nuclear envelope breaks down spindle centrioles DNA condensed as chromosomes

iv. Homologous chromosomes pair 1. When a pair of chromosomes aligns a tetrad is formed. 2. When chromosomes form a tetrad, they exchange portions of their chromatids in a process called crossing-over. 3. Crossing-over produces new combinations of DNA. tetrad

b.     Metaphase I: i.      Spindle fibers attach to the chromosomes at the centromere spindle centromere centrioles chromosomes (DNA)

c.     Anaphase I: i. Spindle fibers pull the homologous chromosomes toward opposite ends of the cell. ii. Chromosomes move and separate into two groups near the spindle iii. Anaphase ends when the chromosomes stop moving spindle chromosomes (DNA) centrioles

d. Telophase I e. Cytokinesis i. Cytoplasm divides i.      Nuclear membranes form around chromosomes ii.      Daughter nuclei form e. Cytokinesis i. Cytoplasm divides Nucleus centrioles chromosomes (DNA) Nucleus

Meiosis I produces two haploid (N) daughter cells that have only one copy of each chromosome. Each chromosome is made of two sister chromatids.

Meiosis II After Meiosis I, cells enter Meiosis II. No DNA replication occurs between Meiosis I and Meiosis II. Meiosis II separates the sister chromatids.

i. Centrioles are visible on opposite sides of the nucleus a.     Prophase II: i. Centrioles are visible on opposite sides of the nucleus ii. Nucleolus disappears iii. Nuclear envelope breaks down chromosome

i. Chromosomes align like they do in mitosis b.     Metaphase II: i. Chromosomes align like they do in mitosis ii. Chromosomes are attached to the spindle at the centromere. centrioles spindle chromosome

Sister chromatids separate and move towards opposite ends of the cell. Anaphase II: Sister chromatids separate and move towards opposite ends of the cell. centrioles spindle chromosome

f. Meiosis II produces four haploid (N) daughter cells d.     Telophase II: i. Nuclear membranes form around chromosomes ii. Daughter nuclei form e.     Cytokinesis: i. Division of the cytoplasm f.     Meiosis II produces four haploid (N) daughter cells centrioles nucleus chromosome

Gamete formation In male animals, the haploid gametes are called sperm. In female animals, the haploid gametes are called eggs.

Comparing mitosis and meiosis Mitosis produces two genetically identical diploid (2N) cells. Mitosis allows an organism’s body to grow and to replace cells. In organisms that reproduce asexually, new organisms are produced by mitosis. Meiosis produces four genetically different haploid (N) cells. Meiosis produces gametes for use in sexual reproduction.