Cell Growth & Division

Ch. 10

• Cells divide to maintain a workable ratio of volume to surface area. The original cell is called the parent cell and the offspring are called daughter cells. The offspring receive a portion of the cytoplasm, organelles, and hereditary information from the parent.

• Cell size

Limits to Cell Growth: • 1. DNA overload - more demands are placed

on the cell’s DNA2. Material Exchange - the rate at which food & oxygen are used up & waste products are produced become too much for the cell3. Ratio of Surface Area to Volume - volume increases more rapidly that surface area4. Cell Division - 2 new daughter cells form

Diploid Cells

• Diploid cells are also known as 2n because they have the full set of chromosomes. Somatic cells are diploid. The diploid number for humans is 46.

Haploid Cells

• Haploid cells are also known as n because they have half of the cells found in a typical somatic cell. The haploid number for humans is 23. An example of a haploid cell in humans would be sperm or eggs.

Diploid vs. Haploid Division

• Diploid cells undergo cell division known as mitosis while haploid cells undergo cell division known as meiosis. All cells that undergo mitosis have daughter cells with the same number of chromosomes as the parent cell, while cells that undergo meiosis have half the number of chromosomes as the parent cell.

Homologous Chromosomes

• Every body cell contains 2 chromosomes bearing genes for the same set of characteristics, & the 2 members usually have the same size & shape. The members of a pair are called homologous chromosomes – one from the mother & one from the father.

• A locus is a particular site on a chromosome.

Chromosome drawing:

Human Karyotype

Autosomes – 1st 22 pairs of chromosomesSex chromosomes – last pair – determines sex

samples

Karyotype

• Nondisjunction - failure of chromosomes to separate properly in cell division – responsible for Klinefelter Syndrome (XXY) , Turner Syndrome (XO), & Trisomy-21 (Down Syndrome)

Cell Division

• Some cells divide once a day, others are not as frequent. Brain cells and other highly specialized cells (mature muscle cells) don’t divide at all.

THE CELL CYCLE

• 1. Interphase - time in between divisions – cell carries on its usual life activities- metabolism is very high in this stage

• 3 Stages of Interphase: 1) G1 - growth phase – the cell decides if it will divide again – cellular differentiation occurs2) S - DNA replication occurs – chromosomes go from being single to being double (2 sister chromatids)3) G2 - another growth phase – organelles reproduce – prepares for division.

The Cell Cycle

Cell Cycle

• 1) Interphase – 90% of cell’s life• 2) Mitotic Stage – broken up into: • a) Mitosis – splits nucleus & contents

– consists of prophase, metaphase, anaphase, telophase

• b) Cytokinesis – splits cytoplasm

Prophase• Prophase - EARLY – centrioles begin to

migrate to opposite sides of the nucleus, chromatin may become denser, MIDDLE – nuclear membrane starts to break down, nucleolus begins to disasppear, chromosomes appear, centrioles migrate to poles & produce the aster complex that makes the spindle arising from 2 centrosomes, LATE – no nucleolus or nuclear membrane, chromosomes are floating within the cell, spindle apparatus has formed

Metaphase

• Metaphase - chromosomes align themselves along the equator – they are attached to the spindle fibers at the kinetochore.

Anaphase

• Anaphase - splitting of the centromeres , spindle fibers contract & chromatids are divided, these become chromosomes & are pulled to the poles, the chromosomes are V-shaped in middle anaphase & J-shaped in late anaphase, this stage is over when chromosomes reach the poles.

Telophase

• Telophase - this is the reverse of prophase, MIDDLE – begins the formation of the nuclear membrane, the nucleolus reappears, granular chromatin reappears, LATE – there is a complete nuclear membrane, the aster complex disappears, centrioles disappear

CYTOKINESIS• The result of cell division is

the production of 2 identical daughter cells. In animals a cleavage furrow occurs that pinches in the cell. This begins during anaphase. In plants a cell plate forms in the middle of the dividing cell – this eventually forms the cell wall after migrating to the edges of the cell.

Examples

MEIOSIS

• Meiosis occurs in sex cells. It is composed of 2 stages: Meiosis I & Meiosis II. Meiosis consists of 2 nuclear divisions in which the chromosomes divide only once. The result is that mature gametes have only 1 member of each homologous chromosome pair (haploid). When the gametes unite, a zygote is formed.

MEIOSIS I• 1. Prophase I -the 2 members of

each pair of homologous chromosomes come into side-by-side contact . Each chromosome has already replicated to form 2 chromatids, each of which will become a new chromosome. The 2 chromatids are joined at 1 point, the synapsis, so that each will become 4 future chromosomes, & is thus called a tetrad. In this stage, crossing over occurs in which chromatids exchange segments (increases genetic variation).

Metaphase I

• 2. Metaphase I - the chromosomes align on the equator of the cell

Anaphase I

• 3. Anaphase I - unlike mitosis, the centromeres holding the chromatids together do NOT divide – each chromatid is pulled toward each pole (1st Reductive Division occurs)

Telophase I

• 4. Telophase I - the daughter cells contain 1 of each of the homologous chromosomes - total chromosome number is now 23 in humans.

Meiosis I

Meiosis II

MEIOSIS II

• The second meiotic division more closely resembles the events in mitosis. The chromosomes are split at the beginning of meiosis II by division of the centromeres, & single-stranded chromosomes move toward each pole. Each chromatid now exists in a separate cell. The essential difference is that you start with haploid cells. 4 cells are now formed. In the male, all 4 cells are functional sperm, but in the female, only 1 is functional & the others become polar bodies.

Oogenesis

Spermatogenesis

Comparing Mitosis & Meiosis

• Similarities: Both are types of cell division; Chromosomes duplicate only once

• Differences: Mitosis used for somatic cells, meiosis used for sex cells; Mitosis yields 2 diploid genetically identical cells, meiosis yields 4 haploid genetically different cells; Mitosis has 1 division of the nucleus, meiosis has 2

Cell Differentiation

• All cells contain the same DNA. In cell differentiation, some of the DNA is repressed and some is expressed. This allows cells to have different functions.

Cell Differentiation & Regulation

• Stem cells – Unspecialized cells that can differentiate into any type of body cell – 2 types are embryonic & adult

• Apoptosis – programmed cellular death (all cells have a pre-determined life span)

Cell cycle regulators

• 1. Internal regulators - proteins respond to events inside the cell to determine when mitosis occurs – cyclins bind to cyclin-dependent kinases that start stages in the cell cycle2. External regulators - proteins respond to events outside the cell – direct cell to speed up or slow down the cell cycle – cells grow until they touch each other

Types of Cell Regulation

• Density-dependent inhibition – cells stop dividing when they touch each other – based on cell surface proteins (will close wounds)

• Anchorage dependence – cells must be in contact with a solid surface (matrix of tissue) to divide

Cancerous Division

• Tumor – abnormally growing mass of cells• Benign – stays enclosed in a “wall” & remains

at the original site• Malignant – spreads into neighboring tissues

& causes angiogenesis (lures new blood vessel formation)

• Metastasis – spread of cancer through the body via the circulatory/lymphatic system

Types of cancers

• Carcinomas – in coverings of skin/lining of intestines

• Sarcomas – in bone, muscle – in areas that support the body

• Leukemias /lymphomas – in blood-forming tissues (marrow, spleen, lymph nodes)

• Most chemotherapy agents work to freeze or prevent spindle formation.

Cancer Development

• Hypertrophy - cells increase in sizeHyperplasia - cells increase in numberDysplasia - cells change their shape (atypical cells) Neoplasia - producing new cellsApoptosis - is programmed cellular death

Asexual vs. sexual reproduction

• Asexual reproduction only takes one parent. Asexual reproduction is the primary form of reproduction for unicellular organisms. Asexual reproduction is relatively rare among multicellular organisms.

• While asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, sexual reproduction offers a net advantage by allowing more diversity, allowing adaptation to changing environments.

Types of asexual reproduction:

• 1. Budding - the formation of a new organism by the protrusion of part of another organism. This is very common in plants, bacteria, and fungi, but may be found in animal organisms, such as the hydra, as well.

• 2. Binary fission - in single-celled organisms by which one cell divides into two cells of the same size, used by most bacteria. This process results in the reproduction of a living cell by division into two equal or near-equal parts.

• 3. Vegetative propagation - Plants are produced using material from a single parent and as such there is no exchange of genetic material, therefore vegetative propagation methods almost always produce plants that are identical to the parent. Vegetative reproduction uses vegetative plants parts or roots, stems and leaves.

• 4. Spore formation – alternation of generations allows spore formation with meiosis (not true asexual method), some fungi & algae, however, can form spores without meiosis

• 5. Fragmentation – new organism grows from a fragment of the parent (regeneration)

• 6. Parthenogenesis - form of reproduction found in females where growth and development of an embryo or seed occurs without fertilization by males (plants, bees, aphids, wasps, some reptiles, some fish, some amphibians).

Sexual reproduction

• 1. Conjugation - transmission of DNA without fusion of gametes (some bacteria & protists)

• 2. Fertilization - sperm & egg (gametes) join to form embryo