CHROMOSOME Chromatid- the strand of a chromosome that becomes visible during meiosis or - PowerPoint Presentation

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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.

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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.

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HUMAN KARYOTYPE

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HISTONE 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

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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).

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HUMAN KARYOTYPE

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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.

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CELL 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).

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Interphase

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

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CELL CYCLE

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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 Chromosomes

into Dividing Cells

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KARYOKINESIS

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

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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

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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

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PROMETAPHASE and METAPHASE

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

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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 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

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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

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Plants

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

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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

.

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Again. . .

What are the roles of the following during cell division?Cohesin (in cell division)Shugoshin

Microtubule

Separase

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CONTROL OF CELL DIVISION

In eukaryotes, proteins regulate the progress of cell division at certain checkpoints.

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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.

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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.

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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.

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PROKARYOTIC CELL DIVISION

Most prokaryotes reproduce by binary fission, in which two identical cells are produced from one cell.

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Meiosis Reduces the Chromosome

Number from Diploid to Haploidin Germ Cells and Spores

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OVERVIEW OF MEIOSIS

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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

.

.

Second,

unlike mitosis, members of each homologous pair of chromosomes pair up, undergoing

synapsis

.

Third

Crossing over occurs between

chromatids

of

synapsed

homologs

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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

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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

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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

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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

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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

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metaphase 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

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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.

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MEIOSIS II

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

The Second Meiotic Division

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PROPHASE 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

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Anaphase 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

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meiosis