CHROMOSOME

Chromatid- the strand of a chromosome that becomes visible during meiosis or mitosis.

Chromatin – the less tightly coiled DNA-protein complex

Centromere – the region of the chromosome that holds the two sister chromatids together during mitosis.

SEX 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.

HUMAN KARYOTYPE

HISTONE AND NON-HISTONEThe 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

CHROMOSOME

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).

HUMAN KARYOTYPE

CELL 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.

CELL CYCLE STAGES

Interphase-the cell carries on its regular activities.

The three stages: G1 phaseS phaseG2 phase.

Mitotic StageStages of mitosis:

karyokinesis (division of the nucleus)

cytokinesis (division of the cytoplasm).

Interphase and the Cell Cycle

The 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

CELL CYCLE

PURPOSE 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 Chromosomesinto Dividing Cells

KARYOKINESIS CYTOKINESIS

This 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

MAJOR 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

MAJOR 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

PROMETAPHASE and METAPHASE

Prometaphase refers to the period of chromosome movement and the term metaphase is applied strictly to the chromosome configuration following migration

Metaphase

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.

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Major EventsFor 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

Major 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

Plants Animals

In 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

Telophase

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.

Again. . .

What are the roles of the following during cell division?

a. Cohesin (in cell division)

b. Shugoshinc. Microtubuled. Separase

CONTROL OF CELL DIVISION

IN EUKARYOTES, PROTEINS

REGULATE THE PROGRESS OF CELL

DIVISION AT CERTAIN

CHECKPOINTS.

CELL 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.

DNA 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.

MITOSIS 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.

PROKARYOTIC CELL DIVISION

MOST PROKARYOTES REPRODUCE BY BINARY FISSION, IN WHICH TWO

IDENTICAL CELLS ARE PRODUCED FROM ONE CELL.

Meiosis Reduces the ChromosomeNumber from Diploid to Haploid

in Germ Cells and Spores

OVERVIEW OF MEIOSIS

Three (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.

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Second,unlike mitosis,

members of each homologous pair of chromosomes pair up, undergoing synapsis.

ThirdCrossing over occurs

between chromatids of synapsed homologs

Leptotene 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

Zygotene 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

Pachytene 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

DIPLOTENE 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

The 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

metaphase I Anaphase I

the 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

Telophase 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.

MEIOSIS II

essential if each gamete or spore is to receive only one chromatid from each original tetrad

The Second Meiotic Division

PROPHASE II METAPHASE II

Each 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

Anaphase IITelophase II

the 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

meiosis