Chapter 8 Bio

Cell division is at the heart of the reproduction of

cells and organisms because cells originate only from pre-existing cells

Some organisms reproduce through asexual reproduction, producing

offspring that are all genetic copies of the parent and identical to each other.

Parthenogenesis is a form of reproduction in which an egg

can develop into a embryo without being fertilized by a sperm. Parthenogenesis is derived from the Greek words for “virgin birth,” and several insect species including aphids, bees, and ants are known to reproduce by Parthenogenesis.

Other organisms reproduce through sexual reproduction, creating a variety of

offspring

Prokaryotic cells reproduce asexually by

binary fission, a term that means “dividing in half.”

In typical prokaryotes, most genes are carried on one circular DNA molecule that with associated

proteins, constitutes the organism’s chromosome (bacteria has about 3000 genes)

As the cell replicates its single chromosome,

• the copies move apart

• the plasma membrane pinches inward, and

• more cell wall is made, which eventually divides the parent cell into two daughter cells

A eukaryotic cell has many more genes than a prokaryotic cell, and they are grouped into

multiple chromosomes in the nucleus

Each chromosome contains

one long DNA molecule (human cells have around 21,000 genes)

Individual chromosomes are visible under a light microscope only when

the cell is in the process of “dividing,” otherwise, chromosomes are thin, loosly packed chromatin fibers too small to be seen

Before a cell starts dividing, the chromosomes

duplicate, producing sister chromatids that are joined together along their lengths by proteins, most closely at a region called the centromere

Cell division involves the separation of sister chromatids and results in

two daughter cells, each containing a complete and identical set of chromsomes

The cell cycle is an ordered sequence of events that

extends from the time a cell is first formed from a dividing parent cell until its own division

Mitosis distributes duplicated chromosomes into

2 daughter nuclei

1. After the chromosomes are coiled up, a

2. The sister chromatids then

1. mitotic spindle made of microtubules, move the chromosomes to the middle of the cell

2. separate and move to opposite poles of the cell, at which point two new nuclei form

Cytokinesis, in which the cell divides in two, overlaps

the end of mitosis

In animal cells, cytokinesis occurs when

a cell constricts, forming a cleavage burrow

In plant cells, cytokinesis occurs when a

membranous plate forms and then splits the cell in two

In laboratory cultures, most normal cells divide only when

attached to a surface

The cultured cells continue dividing until

they touch one another

Most animal cells divide only when stimulated by

growth factor, and some do not divide at all

A set of proteins within the cell controls the

cell cycle

Signals affecting critical checkpoints in the cell cycle determine

whether a cell will go through the complete cycle and divide

The binding of a growth factors to specific receptors on the plasma membrane is usually necessary for

cell division

Cancer cells divide excessively to

form masses called tumors

Malignant tumors can

invade other tissues

Radiation and chemotherapy are effective as cancer treatments because

they interfere with cell division

• Taking such data into account may improve outcomes of cancer treatment

The somatic (body) cells of each species contain a

specific number of chromosomes; for example human cells have 46, consisting of 23 pairs of homologous chromosomes

Meiosis, like mitosis, is preceded by chromosome duplication, but in meiosis, the cell divides

twice to form four daughter cells.

• The first division, Meiosis I, starts with the pairing of homologous chromosomes

• In crossing over, homologous chromosomes exchange corresponding segments

Meiosis I separates the members of each homologous pair and produces

two daughter cells, each with one set of chromosomes.

Meiosis II is essentially the same as mitosis:

• In each of the cells, the sister chromatids of each chromosome separate

• The result is a total of four haploid cells.

Both mitosis and meiosis begin with diploid cells that have chromosomes duplicated during the previous interphase

• Mitosis produces two genetically identical diploid somatic cells

• Meiosis produces four genetically unique haploid gametes

Each chromosome of a homologous pair differs at

many points from the other member of the pair

Random arrangements of chromosomes pairs at metaphase I of meiosis lead to

many different combinations of chromosomes in eggs and sperm

• Random fertilization of eggs by sperm greatly increase this variation

The differences between homologous chromosomes comes from the fact that

they can bear different versions of genes at corresponding loci

Crossing over is an

exchange of corresponding segments between nonsister chromatids of homologous chromosomes

Genetic recombination, which results from crossing over prophase I of meiosis, increases

variation still further

An abnormal chromosome count is the result of

nondisjunction (which can happen during either meiosis I or meiosis II)

To prepare a karyotype, white blood cells are:

• isolate,

• stimulated to grow

• arrested at metaphase, and

• photographed under a microscope

The chromosomes are arranged into ordered pairs so that any

chromosomal abnormalities can be detected

Trisomy 21, the most common chromosome abnormality, results in

a condition called Down syndrome

Nondisjunction of the sex chromosomes during meiosis can result in

individuals with a missing or extra X or Y chromosome

• In some cases (such as XXY), this leads to syndromes that can affect the health of the individual

• In other cases (such as XXX), the body is normal

Nondisjunction can produce polypeptide organisms, organisms with

extra set of chromosomes

• Such errors in cell division can be important in the evolution of new species

• Polypeptide is beneficial in crop plants

Chromosome breakage can lead to rearrangements-

• deletions,

• duplications,

• inversions, and

• translocations

  • These conditions can produce genetic disorders or, if the changes occur in somatic cells, cancer.