Cell Division Flashcards
Cancer cells
start out as normal body cells,
undergo genetic mutations,
lose the ability to control the tempo of their own division, and
run amok, causing disease.
In a healthy body, cell division allows for
growth,
the replacement of damaged cells, and
development from an embryo into an adult.
In sexually reproducing organisms,
eggs and sperm result from
mitosis and
meiosis.
Cell division
is reproduction at the cellular level,
requires the duplication of chromosomes, and
sorts new sets of chromosomes into resulting pair of daughter cells.
Cell division is used
for reproduction of single-celled organisms,
growth of multicellular organisms from a fertilized egg into an adult,
repair and replacement of cells, and
sperm and egg production.
Living organisms reproduce by two methods.
Asexual reproduction
Sexual reproduction
Asexual reproduction
produces offspring that are identical to the original cell or organism and
involves inheritance of all genes from one parent.
Sexual reproduction
produces offspring that are similar to the parents, but show variations in traits and
involves inheritance of unique sets of genes from two parents.
Prokaryotes
reproduce by binary fission (“dividing in half”).
The chromosome of a prokaryote is
a singular circular DNA molecule associated with proteins and
much smaller than those of eukaryotes.
Binary fission of a prokaryote occurs in three stages:
duplication of the chromosome and separation of the copies,
continued elongation of the cell and movement of the copies to opposite ends, and
division into two daughter cells.
Eukaryotic cells
are more complex and larger than prokaryotic cells,
have more genes, and
store most of their genes on multiple chromosomes within the nucleus.
Eukaryotic chromosomes are composed of chromatin
onsisting of
one long DNA molecule and
proteins that help maintain the chromosome structure and control the activity of its genes.
To prepare for division, the chromatin becomes highly compact and
visible with a microscope.
Before a eukaryotic cell begins to divide
it duplicates all of its chromosomes, resulting in
two copies called sister chromatids
joined together by a narrowed “waist” called the centromere.
When a cell divides, the sister chromatids
separate from each other, now called chromosomes, and
sort into separate daughter cells.
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.
The cell cycle consists of two stages, characterized as follows:
Interphase: duplication of cell contents G1—growth, (overall cell and organelles) S—duplication of chromosomes (DNA synthesis) G2—growth, (make proteins for division) Mitotic phase: division Mitosis Karyokinesis—division of the nucleus Cytokinesis—division of cytoplasm
Mitosis progresses through a series of stages:
prophase, metaphase, anaphase, and telophase. Cytokinesis begins in anaphase and ends in telophase.
A mitotic spindle is
required to divide the chromosomes, composed of microtubules, and produced by centrosomes, structures in the cytoplasm that organize microtubule arrangement and contain a pair of centrioles.
Interphase
The cytoplasmic contents double,
two centrosomes form,
chromosomes duplicate in the nucleus during the S phase, and
nucleoli, sites of ribosome assembly, are visible.
Prophase
In the cytoplasm microtubules begin to emerge from centrosomes, forming the spindle.
In the nucleus
chromosomes coil and become compact and
nucleoli disappear.
Metaphase
The mitotic spindle is fully formed.
Chromosomes align at the cell equator.
Anaphase
Sister chromatids separate at the centromeres.
Daughter chromosomes are moved to opposite poles of the cell
Telophase
The cell continues to elongate.
The nuclear envelope forms around chromosomes at each pole, establishing daughter nuclei.
Chromatin uncoils and nucleoli reappear.
The spindle disappears.
During cytokinesis
the cytoplasm is divided into separate cells.
In animal cells, cytokinesis occurs as
a cleavage furrow forms from a contracting ring of microfilaments, interacting with myosin, and
the cleavage furrow deepens to separate the contents into two cells.
In plant cells, cytokinesis occurs as
a cell plate forms in the middle, from vesicles containing cell wall material,
the cell plate grows outward to reach the edges, dividing the contents into 2 cells,
each cell now possesses a plasma membrane and cell wall.
Cell division is controlled by
growth factors, proteins necessary for mitosis are made during G2
density-dependent inhibition, in which crowded cells stop dividing, and
anchorage dependence, the need for cells to be in contact with a solid surface to divide.
The cell cycle control system
is a cycling set of molecules in the cell that
triggers and
coordinates key events in the cell cycle.
Checkpoints in the cell cycle can
stop an event or signal an event to proceed.
A tumor
is an abnormally growing mass of body cells.
Benign tumors remain at the original site.
Malignant tumors spread to other locations, called metastasis.
In humans, somatic cells have
23 pairs of homologous chromosomes and
one member of each pair from each parent
The human sex chromosomes
X and Y differ in size and genetic composition.
The other 22 pairs of chromosomes are autosomes with the same size and genetic composition.
Homologous chromosomes
are matched in
length,
centromere position, and
gene locations.
A locus (plural, loci) is the position of a gene.
Different versions of a gene may be found at the same locus on maternal and paternal chromosomes.
An organism’s life cycle is the sequence of stages leading
from the adults of one generation
to the adults of the next.
Humans and many animals and plants are diploid
with body cells that have
two sets of chromosomes,
one from each parent.
Meiosis is a process that converts diploid nuclei to haploid nuclei
Diploid cells have two homologous sets of chromosomes.
Haploid cells have one set of chromosomes.
Meiosis occurs in the sex organs, producing gametes—sperm and eggs.
Fertilization
is the union of sperm and egg.
The zygote
has a diploid chromosome number, one set from each parent.
All sexual life cycles include an alternation between
a diploid stage and
a haploid stage.
Producing haploid gametes prevents the chromosome number from doubling in every generation.
Meiosis
is a type of cell division that produces haploid gametes in diploid organisms.
Two haploid gametes combine in fertilization to restore the diploid state in the zygote.
Meiosis and mitosis are preceded by the duplication of chromosomes.
meiosis is followed by two consecutive cell divisions and
mitosis is followed by only one cell division.
Because in meiosis, one duplication of chromosomes is followed by two divisions, each of the four daughter cells produced has a haploid set of chromosomes.
Meiosis I – Prophase I
events occurring in the nucleus.
Chromosomes coil and become compact.
Homologous chromosomes come together as pairs by synapsis.
Each pair, with four chromatids, is called a tetrad.
Nonsister chromatids exchange genetic material by crossing over.
Meiosis I – Metaphase I
Tetrads align at the cell equator.
Meiosis I – Anaphase I
Homologous pairs separate and move toward opposite poles of the cell.
Meiosis I – Telophase I
Duplicated chromosomes have reached the poles.
A nuclear envelope re-forms around chromosomes in some species.
Each nucleus has the haploid number of chromosomes.
Meiosis II follows meiosis I without chromosome duplication.
Each of the two haploid products enters meiosis II.
Meiosis II – Prophase II
chromosomes coil & become compact (if uncoiled after telophase I).
nuclear envelope, if re-formed, breaks up again.
Meiosis II – Metaphase II
duplicated chromosomes align at equator.
Meiosis II – Anaphase II
Sister chromatids separate and
chromosomes move toward opposite poles.
Meiosis II – Telophase II
Chromosomes have reached the poles of the cell.
A nuclear envelope forms around each set of chromosomes.
With cytokinesis, four haploid cells are produced.
Mitosis and meiosis both
begin with diploid parent cells that
have chromosomes duplicated during the previous interphase.
However the end products differ.
Mitosis produces two genetically identical diploid somatic daughter cells.
Meiosis produces four genetically unique haploid gametes.
Genetic variation in gametes results from
independent orientation (assortment) at metaphase I and random fertilization.
Independent orientation at metaphase I
There is an equal probability of the maternal or paternal chromosome facing a given pole.
The number of combinations for chromosomes packaged into gametes is 2n where n = haploid number of chromosomes.
Random fertilization – The combination of each unique sperm with each unique egg increases genetic variability.
Separation of homologous chromosomes during meiosis can lead to genetic differences between gametes.
Homologous chromosomes may have different versions of a gene at the same locus.
One version was inherited from the maternal parent and the other came from the paternal parent.
As homologues move to opposite poles during anaphase I, gametes will receive either the maternal or paternal version of the gene.
Genetic recombination
is the production of new combinations of genes due to crossing over
Crossing over
is an exchange of corresponding segments between separate (nonsister) chromatids on homologous chromosomes.
Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over.
Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids.
A karyotype
is an ordered display of magnified images of an individual’s chromosomes arranged in pairs.
Karyotypes
are often produced from dividing cells arrested at metaphase of mitosis and
allow for the observation of
homologous chromosome pairs,
chromosome number, and
chromosome structure.
Trisomy 21
involves the inheritance of three copies of chromosome 21 and
is the most common human chromosome abnormality.
Characteristic symptoms include:
mental retardation,
characteristic facial features,
short stature,
heart defects,
susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and
shortened life span.
The incidence increases with the age of the mother.
Nondisjunction
is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during
meiosis I, if both members of a homologous pair go to one pole or
meiosis II if both sister chromatids go to one pole.
Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes.
