LIFESCI 7A Lecture Notes - Lecture 10: Spindle Apparatus, Sister Chromatids, Microtubule Organizing Center
WEEK 10
11.2: Mitotic Cell Divsion
● Mitotic cell division=mitosis+cytokinesis
○ Asexual reproduction in eukaryotes; a parent cell’s DNA is divided and passed to two daugther
cells
● The DNA of eukaryotic cells is organized as chromosomes.
○ One fo the earliest phases of mitosis is the condensing of DNA to short, dense forms identifiable
under a microscope during M phase
○ Karyotype portrait formed by the number and shapes of chromosomes representative of a
species
■ Most of the cells in human body (except gametes) contain 46 chromosomes
● 23 pairs; 22 pairs of homologous chromosomes and one pair of sex
chromosomes
○ homologous chromosomes Pairs of chromosomes, matching in size
and appearance, that carry the same set of genes; one of each pair was
received from the mother, the other from the father.
○ sex chromosome Any of the chromosomes associated with sex.;
XX=female, XY=male
○ ploidy=number of complete sets of chromosomes in a cell
■ haploid Describes a cell with one complete set of chromosomes.
■ diploid Describes a cell with two complete sets of chromosomes.
■ sister chromatids The two copies of a chromosome produced by DNA replication.
● Duplicated chromosomes in S phase=sister chromatids; bound by centromere
○ centromere A constriction that physically holds sister chromatids
together; the site of the attachment of the spindle fibers that move the
chromosome in cell division.
● At the beginning of mitosis, the nucleus of a
human cell contains 46 chromosomes, each
of which is a pair of identical sister
chromatids linked together at the centromere
○ Count chromosomes by counting
centromeres
■ Homologous chromosomes are similar; sister
chromatids are identical
● Prophase: Chromosomes condense and become visible.
○ Chromosomes condense
○ Outside nucleus (in cytosol) cell begins to assemble mitotic spindle
■ mitotic spindle A structure in the cytosol made up predominantly of microtubules that
pull the chromosomes into separate daughter cells.
● Spindles radiate from centrosome on either side of cell
○ centrosome A compact structure that is the microtubule organizing
center for animal cells.
○ Plants have mitotic spindles, but not centrosomes
■ Failure of mitotic spindles to connect to kinecticore
● One of the daughter cells would have an extra copy of that chromosome.
● The sister chromatids would not separate.
● The chromosome would not line up properly at metaphase.
■ Centrosome duplicates and move to opposite sides of cell at the start of prophase
● Define opposite ends of cell that will be split into daughter cells
find more resources at oneclass.com
find more resources at oneclass.com
○ tubulin dimers assemble around them, forming microtubules
○ form the mitotic spindle and later serve as the guide wires for
chromosome movement
● The mitotic spindle is composed of dynamic protein polymers called microtubules.
○ Microtubules: large, tube-shaped structures built out of thousands of individual protein subunits
called tubulin dimers
■ each dimer is made up of two individual polypeptide chains called alpha and beta
■ Can grow/shrink dramatically/rapidly; “dynamic instability”
● Prometaphase: Chromosomes attach to the mitotic spindle.
○ prometaphase The stage of mitosis in which the nuclear envelope breaks down and the
microtubules of the mitotic spindle attach to chromosomes.
○ As the ends of the microtubules encounter chromosomes, they attach to the chromosomes at
their centromeres
■ kinetochore The protein complexes on a chromatid where spindle fibers attach.
● Two located on each side of the constriction; associated with one of two sister
chromatids
● ensures that each sister chromatid is attached to a spindle microtubule radiating
from one of the poles of the cell
○ symmetrical tethering of each chromosome to the two poles of the cell is essential for proper
chromosome segregation
● Metaphase: Chromosomes align as a result of dynamic changes in the mitotic spindle.
○ metaphase The stage of mitosis in which the chromosomes are aligned in the middle of the
dividing cell.
○ Once chromosome is attached to mitotic spindles, microtubules of the mitotic spindle lengthen
or shorten to move the chromosomes into position in the middle of the cell
■ chromosomes are lined up in a single plane that is roughly equidistant from both poles of
the cell
● Anaphase: Sister chromatids fully separate.
○ anaphase The stage of mitosis in which the sister chromatids separate.
○ Centromere splits, each sister chromatid becomes its full chromosome
■ Spindles shorten, pulling chromosomes to opposite poles
■ During anaphase, identical set of 46 chromosomes arrives at each spindle pole
● Telophase: Nuclear envelopes re-form around newly segregated chromosomes.
○ telophase the nuclei of the daughter cells are formed and the chromosomes uncoil to their
original state.
○ Cell prepares for division into two daughter cells
○ microtubules of the mitotic spindle break down and disappear, nuclear envelopes form around
each set of chromosomes
■ Once nucleus forms, chromosomes decondense, less visible under microscope
○ End of mitosis
● The parent cell divides into two daughter cells by cytokinesis.
○ contractile ring In animal cells, a ring of actin filaments that forms at the equator of the cell
perpendicular to the axis of what was the spindle at the beginning of cytokinesis.
■ Ring contracts, pinching the cell into two (similar to binary fission)
● In BF, driven by FtsZ protein, a homolog of tubulin, not by actin
■ Driven by motor proteins
■ Two daughter cells w/ own nuclei and set of 46 chromosomes; enter G1 phase
○ In plants, cytokinesis happens by forming new cell wall
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Asexual reproduction in eukaryotes; a parent cell"s dna is divided and passed to two daugther cells. The dna of eukaryotic cells is organized as chromosomes. One fo the earliest phases of mitosis is the condensing of dna to short, dense forms identifiable under a microscope during m phase. Karyotype portrait formed by the number and shapes of chromosomes representative of a species. Most of the cells in human body (except gametes) contain 46 chromosomes. 23 pairs; 22 pairs of homologous chromosomes and one pair of sex chromosomes. Homologous chromosomes pairs of chromosomes, matching in size and appearance, that carry the same set of genes; one of each pair was received from the mother, the other from the father. Sex chromosome any of the chromosomes associated with sex. Ploidy=number of complete sets of chromosomes in a cell. Haploid describes a cell with one complete set of chromosomes. Diploid describes a cell with two complete sets of chromosomes.
