Chromatin the less tightly coiled DNAprotein complex Centromere the region of the chromosome that holds the two sister chromatids together during mitosis SEX CHROMOSOMES amp AUTOSOMES Sex chromosomes are chromosomes that determine the sex of an organism and they may also carry genes ID: 910393
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Slide1
CHROMOSOME
Chromatid- the strand of a chromosome that becomes visible during meiosis or mitosis. Chromatin – the less tightly coiled DNA-protein complexCentromere – the region of the chromosome that holds the two sister chromatids together during mitosis.
Slide2SEX CHROMOSOMES & AUTOSOMES
Sex chromosomes are chromosomes that determine the sex of an organism, and they may also carry genes for other characteristics. In humans, sex chromosomes are either X or Y.
XX for female
XY for male
The autosomes are all the other chromosomes in an organism except for sex chromosomes.
In humans two of the 46 are sex chromosomes, and the remaining 44 chromosomes are autosomes.
Slide3HUMAN KARYOTYPE
Slide4Slide5Slide6HISTONE AND NON-HISTONE
The DNA in eukaryotic cells wraps tightly around proteins called histones
.
Histones
help maintain the shape of the
chromosomes
and aid in the
tight packaging of DNA.
Nonhistone
proteins are generally involved in
controlling the activity
of specific regions of the DNA
Slide7CHROMOSOME
Each chromosome contains a constricted region called the
centromere
,
whose location establishes the general appearance of each chromosome
chromosomes are classified as
metacentric
,
submetacentric
,
acrocentric
, or
telocentric
on the basis of the centromere location
The shorter arm, by convention, is shown above the
centromere
and is called the
p arm (
p, for “petite”
).
The longer arm is shown below the
centromere
and is called the
q arm (
q because it is the next letter in the alphabet).
Slide8Slide9HUMAN KARYOTYPE
Slide10CELL CYCLE
The cell cycle is an orderly set of stages that take place between the time a cell divides and the time the resulting daughter cells also divide.
Slide11CELL CYCLE STAGES
Interphase -the cell carries on its regular activities.The three stages:
G1 phase
S phase
G2 phase.
Mitotic Stage
Stages of mitosis:
karyokinesis (division of the nucleus)
cytokinesis (division of the cytoplasm).
Slide12Slide13Interphase
and the Cell CycleThe events that occur from the completion of one division until the completion of the next division constitute the cell cycle.Interphase : G1 phase, S-phase & G2 phase
Slide14CELL CYCLE
Slide15PURPOSE OF MITOSIS
In some single-celled organisms, such as protozoans and some fungi and algae, mitosis provides the basis for asexual reproduction.
foundation for the development and growth of the organism (zygote).
In adult
multicellular
organism, mitotic activity is the basis for wound healing and other forms of cell replacement in certain tissues.
Mitosis Partitions Chromosomes
into Dividing Cells
Slide16KARYOKINESIS
CYTOKINESISThis process is quite complex and requires great precision.
The chromosomes must first be
exactly replicated
and then
accurately partitioned
.
The end result is the production of two daughter nuclei, each with a chromosome composition identical to that of the parent cell.
This less complex process requires a mechanism that partitions the volume into two parts, then encloses each new cell in a distinct plasma membrane.
As the cytoplasm is reconstituted, organelles either replicate themselves, arise from existing membrane structures, or are synthesized
de novo (anew) in each cell.
MITOSIS
Slide17Slide18MAJOR EVENTS
In animal cells, migration of two pairs of centrioles to opposite ends of the cell (plant cells do not have
cemtrioles
).
the nuclear envelope begins to break down and gradually disappears.
the nucleolus disintegrates within the nucleus
the diffuse chromatin fibers have begun to condense, until distinct threadlike structures, the chromosomes, become visible.
PROPHASE
Slide19MAJOR EVENTS (cont)
Near the end of prophase the chromosomes become apparent and is actually a double structure split longitudinally except at a single point constriction, the centromere
.
The two parts of each chromosome are called
sister
chromatids
PROPHASE
Slide20Slide21Slide22PROMETAPHASE and METAPHASE
Prometaphase refers to the period of chromosome movement and the term metaphase is applied strictly to the chromosome configuration following migration
Slide23Metaphase
During metaphase, the kinetochore fibers move the chromosmes to the center of the dividing cell.
Once the chromosomes are in the center of the cell, each of them is held in place by the
kinetochore
fibers.
.
Slide24Major Events
For complete disjunction:During this phase, sister
chromatids
of each chromosome, held together only at their
centromere
regions,
disjoin (separate)
from one another—an event described as
disjunction—
and
are pulled to opposite ends of the cell.
(1),
shugoshin
must be degraded, reversing its protective role; (2), the cohesin complex holding the centromere region of each sister chromosome is then cleaved by
separase
; and
(3) sister
chromatids
of each chromosome are pulled toward the opposite poles of the cell
Anaphase
Slide25Slide26Major Events
At its beginning, two complete sets of chromosomes are present, one set at each pole. The most significant event of this stage is
cytokinesis
, the division or partitioning of the cytoplasm
Animal cells and plant cells differ in
cytokinesis
Telophase
Slide27Plants
AnimalsIn plant cells, a cell plate is synthesized and laid down across the region of the metaphase plate.
Animal cells, however undergo a constriction of the cytoplasm, much as a loop of string might be tightened around the middle of a balloon.
Plant and Animal Cells’
Cytokinesis
Slide28Slide29Telophase
Major events (cont)In each new cell, the chromosomes begin to uncoil and become diffuse chromatin once again, while the nuclear envelope reforms around them, the spindle fibers disappear, and the nucleolus gradually reforms and becomes visible in the nucleus during early
interphase
.
Slide30Again. . .
What are the roles of the following during cell division?Cohesin (in cell division)Shugoshin
Microtubule
Separase
CONTROL OF CELL DIVISION
In eukaryotes, proteins regulate the progress of cell division at certain checkpoints.
Slide32Slide33CELL GROWTH CHECKPOINT
1. Cell Growth Checkpoint. Proteins at this checkpoint control whether the cell will divide.
Hint: if the cell is healthy and has grown to a suitable size during G1 phase, protein will initiate DNA synthesis (S phase). If conditions are not favorable for DNA synthesis, the cell cycle will stop at this point.
Slide34DNA SYNTHESIS CHECKPOINT
2. DNA Synthesis (G2) checkpoint.DNA repair enzymes check the results of DNA replication. If this checkpoint is passed, proteins will signal the cell to begin the molecular processes that will allow the cell to divide mitotically.
Slide35MITOSIS CHECKPOINT
3. Mitosis checkpoint. If a cell passes this checkpoint, proteins signal the cell to exit mitosis. The cell enters into the G1 phase, the major growth phase of the cell cycle, once again.
Slide36Slide37PROKARYOTIC CELL DIVISION
Most prokaryotes reproduce by binary fission, in which two identical cells are produced from one cell.
Slide38Meiosis Reduces the Chromosome
Number from Diploid to Haploidin Germ Cells and Spores
Slide39OVERVIEW OF MEIOSIS
Slide40Three (3) Characterized Events in Prophase I
FirstFirst, as in mitosis, chromatin present in interphase thickens and coils into visible chromosomes.And, as in mitosis, each chromosome is a double structure, held together by the molecular complex called
cohesin
.
.
Second,
unlike mitosis, members of each homologous pair of chromosomes pair up, undergoing
synapsis
.
Third
Crossing over occurs between
chromatids
of
synapsed
homologs
Slide41Leptotene Stage
the interphase
chromatin material begins to condense
, and the chromosomes, though still extended, become
visible
.
Along each chromosome are
chromomeres
,
localized condensations that resemble beads on a string.
homology search,
which precedes and is essential to the initial pairing of
homologs
.
PROPHASE 1 - LEPTONEMA
Slide42Zygotene Stage
The chromosomes continue to shorten and thicken. During the process of homology search, homologous chromosomes undergo initial alignment with one another -
rough pairing
is complete by the end of
zygonema
.
structures called
lateral elements
are visible between
paired
homologs
.
synaptonemal
complex
begins to form between the homologs (for pairing of homologs)It is upon completion of zygonema
that the paired
homologs
are referred to as bivalents.
PROPHASE I - ZYGONEMA
Slide43Pachytene Stage
the chromosomes continue to coil and shorten, further development of the synaptonemal complex between bivalent for more intimate pairing.
each homolog is now evident as a double structure, providing visual evidence of the earlier replication of the DNA of each chromosome (bivalent contains 4-member
chromatids
)
PROPHASE I - PACHYNEMA
Slide44DIPLOTENE STAGE
more apparent that each tetrad consists of two pairs of sister chromatids
. Within each tetrad, each pair of sister
chromatids
begins to separate.
However, one or more areas remain in contact where
chromatids
are intertwined. – (
chiasma
is an area for crossing-over)
PROPHASE I - DIPLONEMA
Slide45The chromosomes pull farther apart, but
nonsister chromatids remain loosely associated at the
chiasmata
.
As separation proceeds, the
chiasmata
move toward the ends of the tetrad.
during this final
substage
, the nucleolus and nuclear envelope break down
the two
centromeres
of each tetrad attach to the recently formed spindle fibers
PROPHASE I - DIAKINESIS
Slide46metaphase I
Anaphase Ithe chromosomes have maximally shortened and thickened.
The terminal
chiasmata
of each tetrad are visible and appear to be the major factor holding the
nonsister
chromatids
together.
Each tetrad interacts with spindle fibers, facilitating its movement to the metaphase plate.
cohesin
is degraded between sister
chromatids
, except at the
centromere region, which, as in mitosis, is protected by a shugoshin complex. Then, one-half of each tetrad (a dyad) is pulled toward each pole of the dividing cell.
Metaphase, Anaphase, and
Telophase
I
Slide47Telophase I
reveals a nuclear membrane forming around the dyads and the cells go directly to meiosis II without chromosome replication.
In general, meiotic
telophase
is much shorter than the corresponding stage in mitosis.
Slide48MEIOSIS II
essential if each gamete or spore is to receive only one chromatid from each original tetrad
The Second Meiotic Division
Slide49PROPHASE II
METAPHASE IIEach dyad is composed of one pair of sister chromatids attached by the common
centromeric
region.
the
centromeres
are positioned on the equatorial plate.
the
shugoshin
complex is degraded and separates the
centromeres
MEIOSIS II
Slide50Anaphase II
Telophase IIthe sister chromatids
of each
dyad are pulled to opposite poles.
one
member of each pair of homologous chromosomes present at each pole.
each chromosome is now a monad
four haploid gametes may result from a single meiotic event
Slide51meiosis