Chapter 10: Cell Growth and Division Section 11.4: Meiosis Cell Division

Vocabulary: Asexual Reproduction Sexual Reproduction Mitosis Meiosis Diploid Haploid Chromatin Chromatid Centromere Histone Nucleosomes

Asexual Reproduction: The production of genetically identical offspring from a single parent

Sexual Reproduction: Offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow.

Mitosis: Division of the nucleus of a eukaryotic cell Followed by cytokinesis – division of the cytoplasm The two daughter cells are identical to the original cell

Meiosis: Process that produces gametes from a diploid cell. A reductive division of the nucleus that produces four haploid gametes.

Diploid: Cells having two sets of chromosomes 2N = 46 Examples: All human body cells Except reproductive cells (sperm and egg)

Haploid: Cells containing only one set of chromosomes N = 23

Chromatin: DNA and protein (histones) in the nucleus of a non-dividing cell

Chromatid: One of the two identical parts of a replicated chromosome

Centromere: Point of attachment between sister chromatids

Histone: Protein molecule that DNA wraps around during chromosome formation

Nucleosomes: The beadlike structures formed by DNA and histone molecules

Section 10.1 Cell Growth, Division, and Reproduction

THINK ABOUT IT When a living thing grows, what happens to its cells? What are some of the difficulties a cell faces as it increases in size?

Limits to Cell Size There are 2 main reasons why cells do not grow indefinitely: 1. Larger cells place more demands on their DNA When a cell is small, the information stored in DNA is able to meet all of the cell’s needs It’s like putting a Volvo engine in a Hummer… just doesn’t have the power to make it go.

Limits to Cell Size 2. Larger cells can’t move enough nutrients and waste across the cell membrane Function of the cell membrane: help exchange materials between outside and inside of the cell. A huge cell is going to need lots of food, water, and oxygen and produce lots of wastes that would have to travel through the cell and across the membrane.

Limits to Cell Size The larger a cell becomes, the more demands the cell places on its DNA. In addition, a larger cell is less efficient in moving nutrients and waste materials across its cell membrane.

Information “Overload” Compare a cell to a growing town. The town library has a limited number of books. As the town grows, these limited number of books are in greater demand, which limits access. A growing cell makes greater demands on its genetic “library.” If the cell gets too big, the DNA would not be able to serve the needs of the growing cell.

Exchanging Materials Food, oxygen, and water enter a cell through the cell membrane. Waste products leave in the same way. The rate at which this exchange takes place depends on the surface area of a cell.

Exchanging Materials The rate at which food and oxygen are used up and waste products are produced depends on the cell’s volume. The ratio of surface area to volume is key to understanding why cells must divide as they grow.

Ratio of Surface Area to Volume Imagine a cell shaped like a cube... As the length of the sides of a cube increases, its volume increases faster than its surface area, decreasing the ratio of surface area to volume. If a cell gets too large, the surface area of the cell is not large enough to get enough oxygen and nutrients in and waste out.

Traffic Problems To use the town analogy again, as the town grows, more and more traffic clogs the main street. It becomes difficult to get information across town and goods in and out. Similarly, a cell that continues to grow would experience “traffic” problems. If the cell got too large, it would be more difficult to get oxygen and nutrients in and waste out.

Division of the Cell Before a cell grows too large, it divides into two new “daughter” cells in a process called cell division. Before cell division, the cell copies all of its DNA. It then divides into two “daughter” cells. Each daughter cell receives a complete set of DNA. Cell division reduces cell volume. It also results in an increased ratio of surface area to volume, for each daughter cell.

Cell Division and Reproduction Asexual ReproductionSexual Reproduction A single parent Genetically identical offspring Prokaryotic and eukaryotic single-celled organisms and many multicellular organisms Simple, efficient, & effective way for an organism to produce a large number of offspring. Fusion of two sex cells – one from each of two parents Genetically diverse offspring Most animals and plants, and many single-celled organisms Genetic diversity helps ensure survival of species when environment changes

Section 10.2 The Process of Cell Division

Chromosomes Prokaryotic Eukaryotic

Chromosomes The genetic information that is passed on from one generation of cells to the next is carried by chromosomes. Every cell must copy its genetic information before cell division begins. Each daughter cell gets its own copy of that genetic information. Cells of every organism have a specific number of chromosomes.

Prokaryotic Chromosomes Prokaryotic cells lack nuclei. Instead, their DNA molecules are found in the cytoplasm. Most prokaryotes contain a single, circular DNA molecule, or chromosome, that contains most of the cell’s genetic information.

Eukaryotic Chromosomes In eukaryotic cells, chromosomes are located in the nucleus, and are made up of chromatin. Chromatin is composed of DNA and histone proteins. DNA coils around histone proteins to form nucleosomes. The nucleosomes interact with one another to form coils and supercoils that make up chromosomes.

The Prokaryotic Cell Cycle Binary fission is a form of asexual reproduction during which two genetically identical cells are produced. For example, bacteria reproduce by binary fission. Binary FissionBinary Fission (1:02)

The Eukaryotic Cell Cycle The eukaryotic cell cycle consists of four phases: G1, S, G2, and M. Interphase is the time between cell divisions. It is a period of growth that consists of the G1, S, and G2 phases. The M phase is the period of cell division. Eukaryotic Cell Cycle | Biology | Genetics (4:19)

G 1 Phase: Cell Growth In the G 1 phase, cells increase in size and synthesize new proteins and organelles.

G 0 Phase Cells that leave the cell cycle Do NOT copy their DNA Do NOT prepare for cell division Example: cells of the central nervous system

S Phase: DNA Replication In the S (or synthesis) phase, new DNA is synthesized when the chromosomes are replicated.

G 2 Phase: Preparing for Cell Division In the G 2 phase, many of the organelles and molecules required for cell division are produced. McGraw Hill Control of Cell Cycle McGraw Hill - How the Cell Cycle Works

M Phase: Cell Division In eukaryotes, cell division occurs in two stages: mitosis and cytokinesis. Mitosis is the division of the cell nucleus. Cytokinesis is the division of the cytoplasm. Mitosis (1:29)

Important Cell Structures Involved in Mitosis Chromatid – each strand of a duplicated chromosome Centromere – the area where each pair of chromatids is joined Centrioles – tiny structures located in the cytoplasm of animal cells that help organize the spindle Spindle – a fanlike microtubule structure that helps separate the chromatids

Prophase During prophase, the first phase of mitosis, the duplicated chromosome condenses and becomes visible. The centrioles move to opposite sides of nucleus and help organize the spindle. The spindle forms and DNA strands attach at a point called their centromere. The nucleolus disappears and nuclear envelope breaks down. This is the longest stage of mitosis.

Metaphase During metaphase, the second phase of mitosis, the centromeres of the duplicated chromosomes line up across the center of the cell. The spindle fibers connect the centromere of each chromosome to the two poles of the spindle. This is the shortest phase of mitosis.

Anaphase During anaphase, the third phase of mitosis, the centromeres are pulled apart and the chromatids separate to become individual chromosomes. The chromosomes separate into two groups near the poles of the spindle.

Telophase During telophase, the fourth and final phase of mitosis, the chromosomes spread out into a tangle of chromatin. A nuclear envelope re-forms around each cluster of chromosomes. The spindle breaks apart, and a nucleolus becomes visible in each daughter nucleus. Chromosomes uncoil. Stages of Mitosis (4:30)

Cytokinesis Animal CellsPlant Cells The cell membrane is drawn in (cleavage furrow)until the cytoplasm is pinched into two equal parts. Each part contains its own nucleus and organelles. In plants, the cell membrane is not flexible enough to draw inward because of the rigid cell wall. A cell plate forms between the divided nuclei that develops into cell membranes. A cell wall then forms in between the two new membranes. McGraw Hill - Mitosis and Cytokinesis McGraw Hill - Mitosis and Cytokinesis Animation

The Stages of the Cell Cycle Mitosis (6:10)

Video clips and Animations Mitosis (1:29) Eukaryotic Cell Cycle | Biology | Genetics (4:19) Stages of Mitosis (4:30) Mitosis (6:10) Meiosis (5:27) PBS (Mitosis vs Meiosis) Interactive