Chapter 8

The asexual reproduction of a cell by division into two equal, or nearly equal parts. Or division into two daughter cells.

True

Eukaryotes have about 1,000 times more DNA then an average prokaryotic cell.

An integral part of the cell cycle. Results in genetically identical daughter cells. Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA

Most eukaryotes have between 10 and 50 chromosomes in their body cells. Human cells have 47 chromosomes. 23 nearly-identical pairs.

10 and 50

• Are distinct from each other in their characteristics.
• Are represented as x and Y
• Determine the sex of the individual, xx being female, xy being male

• The chromosome occur in pairs. The 2 members of each pair are called homologous chromosomes or homologues.
• A cell possessing two copies of each chromosome (Human body cells)
• Homologous chromosomes are made up of sister chromatids joined at the centromere.

• Look the same
• Control the same traits
• May code for different forms of each trait
• Independent Origin: Each one was inherited from a different parent.

• The centromer is a constricted region of the chromosome containing a specific DNA sequence, to which is bound 2 discs of proteins called kinetochores. 
• Kinetochores serve as points of attachment for microtubules that move that chromosomes during cell division.

A cell possissing a single copy of each chromosome (Human sex cells.)

• In preparation for cell division, Dna is replicated and the chromosomes condense
• Each duplicated chromosome has two sister chromatids, which separate during cell division.
• Because of duplication, each condensed chromosome consists of 2 identical chromaties joined by a centremere. 
• Each duplicated chromosome contains 2 identical DNA molecules (Unless mutation occurred), one in each chromatid

• An ordered, visual representation of the chromosomes in a cell.
• Chromosomes are photographed when they are highly condensed, then photos of the individual chromosomes are arranged in order of decreasing size
• In humans each somatic cell has 46 chromosomes, made up of two sets, one set of chromosomes come from each parent.

• Dna helix wraps around proteins with positive charges called histones.
• The positive charges of the histones counteract the negative charges of the DNA, so that the complex has no charge.

Every 200 nucleotide pairs, the DNA wraps twice around a group of 8 histone proteins to form

• The Nucleosomes are further coiled into a Solenoid. The Solenoid is then arranged into looped domains, which appears as rossettes of looped chromatin around a scaffolding protein.
• Higher order coiling and supercoiling also help condense and package the chromatin into a compact chromosome inside the nucleus.

• G1- Primary growth
• S- Genome Replicated
• G2- Secondary Growth

Mitosis

Cytokinesis

Cells undergo majority of growth. For most organisms, this phase occupies much of the cell's life span

• Each chromosome replicates (Synthesizes) to produce sister chromatids
• -Attached at centromere
• -Contains attachement site (kinetochore)

• Chromosomes begins to condense- Assembles machinery for division such as centrioles. Replication of mitochondria and also synthesis of microtubules are significant.
• •  A nuclear envelope bounds
•    the nucleus.
• • The nucleus contains one or more nucleoli (singular, nucleolus).
• • Two centrosomes have formed by replication of a single centrosome.
• •  In animal cells, each centrosome features two centrioles.
• •  Chromosomes, duplicated during S phase, cannot be  seen individually because they have not yet condensed.

• The light micrographs show dividing lung cells from a newt, which has 22 chromosomes in its somatic cells (chromosomes appear blue, microtubules green, intermediate filaments red). For simplicity, the drawings show only four
• chromosomes.

ØSome haploid & diploid cells divide by mitosis.

• ØEach new cell receives one copy of every chromosome that was present in
• the original cell.

• ØProduces 2 new cells that are both genetically identical to the
• original cell.

• •   The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope.
• •   The nucleoli disappear.
• •   Each duplicated chromosome appears as two identical sister chromatids joined together.
• •  The mitotic spindle begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”).
• • The centrosomes move away from each other, apparently propelled by the lengthening microtubules between them.

• • Metaphase is the longest stage of mitosis, lasting about 20 minutes.
• • The centrosomes are now at opposite ends of the cell.
• •The chromosomes convene on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles. The chromosomes’ centromeres lie on the metaphase plate.
• • For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.
• • The entire apparatus of microtubules is
• called the spindle because of its shape.

• The spindle includes the centrosomes, the spindle microtubules, and the asters. 
• The apparatus of microtubules controls chromosome movement during mitosis. 
• The centrosome replicates, forming two centrosome that migrate to opposite ends of the cell.
• Assembly of spindle microtubles begins in the centrosome, the microtubule organizing center.
• An aster (a radial array of short microtubules) extends from each centrosome.
• Some spindle microtubules attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate. 
• In anaphase, sister chromatids separate and move along the kinetochore microtubles towards opposit ends of the cell

• Anaphase is the shortest stage of mitosis, lasting only a few minutes.
• Anaphase begins when the two sister chromatids of each pair suddenly part. Each chromatid thus becomes a fullfledged chromosome. 
• The two liberated chromosomes begin moving toward opposite ends of the cell, as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first 
• The cell elongates as the nonkinetochore microtubles lengthen
• By the end of anaphase, the two ends of the cell have equivalent- and complete- collections of chromosomes

• • Two daughter nuclei begin to form in the cell.
• • Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the  endomembrane system.
• • The chromosomes become less condensed.
• • Mitosis, the division of one nucleus into two genetically dentical nuclei, is now complete.

•Cleavage of cell into two halves

• –Animal
• cells

• §Constriction belt of actin
• filaments

• –Plant
• cells 

§Cell plate

• –Fungi
• and protists

§Mitosis occurs within the nucleus

• •No cell is able to live forever. Damaged cells can be replaced to some
• degree. Irreversible damage eventually leads to cell death.

• •Human cells are programmed to undergo only so many cell divisions and
• then die. In tissue culture, cell lines divide about 50 times, and then the
• entire population of cells die off. 

• •During embryological life, some cells are programmed to die, a process
• called apoptosis. For example, the human embryo’s hands & feet appear first
• like paddles, but the skin cells between bones die as programmed to form the
• separated toes and fingers.

• •In addition, cells that are frozen for years, when thawed would resume
• where they left off and die on schedule.

•Only cancer cells violate this principle, dividing endlessly.

•Uncontrolled invasive cell growth.  A tumor or cell mass resulting from uncontrollable cell division. Cancer is a gene disorder of somatic tissue, in which damaged genes fail to properly control cell growth and division.

•Metastases are tumor cells that break off, enter the bloodstream and spread to distant areas of the body, forming new tumors at the distant sites.

•Mutation is a damage to DNA or genes.

•There are two classes of growth factor genes that are usually involved in cancer: Proto-oncogens & tumor suppressor genes.

• •Proto-oncogens are genes that encode proteins that stimulate cell division. Mutated
• proto-oncogen become cancer causing genes called Oncogens

•Tumor Suppressor genes encodes proteins that turn off cell division in healthy cells. Mutations to these genes essentially allows uncontrolled cell division.