12th Class Biology Chapter 7 Cell Cycle Short Questions Answer

The cell under goes a sequence of changes which involve period of growth replication of DNA followed by cell division.This sequence of changes is called cell cycle

In the case of human cell  average cell cycle is about 24 hours.

Chromatin is a network of very fine threads which can be visualized but using histologic stains for DNA.

Higher plants lack visible centrioles instead they have its analogous region from which the spindle microtubules radiate.

The cells composing a malignant tumor or cancer divide more rapidly mostly invade surrounding tissues get into body circulatory system and set up areas of proliferation away from their site of original appearance.This spread of tumor cells and establishment of secondary areas of growth is called as metastasis.

Meiosis takes place in diploid cells only in animals at the time of gamete formation while in plants when spores are produced.

It is the exchange of segments between non-sister chromatids of homologous chromosomes due to chiasmata formation.

Crossing over and random assortment of chromosomes are two significant happening of meiosis.

Males with 48 chromosomes with 49 chromosomes and male with 47 chromosomes are also observed.

The affected individuals have one missing X chromosomes.Individuals have female appearance with short stature webbed neck without ovaries and complete absence of germ cells.

Internal programmer of events and sequence of morphological changes by which cell commits suicide is collectively called as apoptosis.

Interphase can further be divided into G₁-phase S-phase and G₂-phase.

G₁ is the period of extensive metabolic activity in which cell normally grows in size specific enzymes are synthesized and DNA base units  are accumulated for the DNA synthesis.

Post-mitotic cell can exit the cell cycle during G₁ entering a phase called G_o and remain fore days weeks or in some cases even the life time of the organism without proliferating further.

Following the G₁  is the S-phase  during which the DNA is synthesized and chromosomes number is doubled.

In G₂ phase the cell prepares  for division i.e.energy storage for chromosome movements mitosis specific proteins RNA and microtubule subunits synthesis are accomplished.

It is the type of cell division which ensures the same number of chromosomes in the daughter cells as that in the parent cells.

i)Karyokinesis which involves the division of nucleus. ii) Cytokinesis that refers to the division of the whole cell.

The specialized microtubule structure including aster and spindle is called mitotic apparatus.

Three sets of microtubules originate from each pair of centrioles i.e.astral microtubules kinetochore kinetochore microtubules and polar microtubules.

Each chromosomes is visible having two sister chromatids attached at centromere.Towards the end of prophase nuclear envelope disappears and nuclear material is relesed is relesed in the cytoplasm neucleoli disappear.Mitotic apparatus is orgenized.Cytoplasm becomes more viscous.

The centromere has special area the kinetochore with specitic base arrangement and special proteins where kinetochore fibres of mitotic apparatus attach.

The centromere has special area the kinetochore with specific base arrangement and special proteins where kinetochore fibres of mitotic apparatus attach.

At cytokinesis in plants a membrane structure called phragmoplast is formed from vesicles of golgi comples.These vesicles line up in the centre of the dividing cell where they fuse to form phragmoplast at the and of telophase.

Any malignant growth r tumour from an abnormal and uncontrolled division of body cells is known as cancer.The cells involved in formation of tumour are called cancer cells.

When cancer cells produce new cells which continue to proliferate in uncontrolled fashion an unwanted clone of cells called tumour is formed which can expand un-definitely.

Benign tumours ii) Malignant tumours

Benign tumours are of small size and localized called benign.The cells in this type usually behave like the normal cells and have little deleterious effects.

The cells composing a malignant tumour or cancer divide more repidly mostly invade surrounding tissues get into the body’s circulatory system and set up areas of proliferation away from their site of original appearance.

Cancer cells can be distinguished from normal cells because they are less differentiated than normal cells exhibit the characteristics of rapidly growing cells that is high nucleus to cytoplasm ratio prominenet nucleoli and many mitosis.

Cancer is caused mainly by mutations in somatic cells.The cancer results from the accumulation of as few as three to as many as twenty mutations in genes that regulate cell division.

Meiosis is the special type of cell division in which the number if chromosomes in daughter cells reduces to half as compared to the parent cell.

This is very prolonged phase and differs from the prophase of mitosis,because in this chromosomes behave as homologous pairs.Each diploid cell has two chromosomes of each type one member from each parent.

The chromosomes which are similar but not necessarily identical are called as homologous chromosomes.

Leptotene zygotene pachtene diplotene and diakinesis.

The pairing of homologous chromosomes during zygotene stage is called synapsis.

Each paired but not fused complex structure of homologus chromosomes is called as bivalent or tetrad.

Pachytene may lasts for days weeks for days weeks or even years whereas leptotene and zygotene can last only for few hours.

The paired homologous chromosomes repel each other and begin to separatebut still remain united by their point interchange which is called chiasmata.

The condensation of chromosomes reaches to its maximum.At the same time separation of the homologous chromosomes is completed but still they are united at one point more oftenly at ends.Nucleoli disappear.

Nuclear membrane disorganize at the beginning of this phase.Spindle fibres orignate and the kinetochore fibres attach to the kinetochore of homologous chromosome from each pole and arrange bivalents at the equator.

In anaphase I of meiosis the sister chromatids are not separated only homologous chromosomes get separated.While in anaphase of mitosis the sister chromatids are separated.

Meiosis usually takes place at the time of sexual cells formation thus reducing the number of chromosomes to half in each which is restored after fertilization and maintains chromosome number constant generation after genaration.

In non-disjunction chromosomes fail to segregate during Anaphase and Telophase and do not finish with equal distribution of chromosome among all the daughter nuclei.This results either increase in the number of chromosomes causing serious phusical social and mental disorders.

The non-disjunction in which autosomal chromosomes fail to segregate is called autosomal non-disjunction.

It occurs in man during which 21st chromosome fails to segregate resulting gemete with 24 chromosomes.This gamete fertilizes normal gemete the new individual will have 47 chromosomes.

Yes the chances of teenage mother having downs syndrome child s one in many thousands forty years old mother one in hundred chance and by forty-five the risk is tree times greater.

The affected individuals have flat broad face squint eyes with the skin fold in the inner corner and protruding tongue mental retardation and defective development of central nervous system.

The non-disjunction in which sex chromosomes fail to segregate is called sex chromosomal non-disjunction.

The cell death due to tissue damage is called necrosis,during which the typical cell swells and bursts releasing the intracellular contents which can damage neighbouring cells and cause inflammation.

The sequence of changes in a cell involving period of growth, replication of DNA followed by cell division is called cell cycle.

The two basic phases of cell cycle are the interphase and the mitotic phase.

These are the G₁-phase during which cell grows in size and synthesizes specific enzymes; S-phase during which DNA is synthesized and the chromosome number is doubled; G₂-phase during which the cell is prepared for division.

These are mitosis, 30 minutes G₁, 9 hours; S-phase, 10 hours; and G₂, 4.5 hours.

The duration of cell cycle in yeast cells is only 90 minutes.

In an animal cell, the centrosome is the region of the cytoplasm surrounding the centrioles.

During mitotic cell division the number of chromosomes in the daughter cells remains the same as in the parent cell where as in meiotic cell division it is reduced to one half of the parent cell.

Karyokinesis is the division of the nucleus which is followed by the division of the cytoplasm known as cytokinesis.

Karyokinesis can be distinguished for convenience into four phases viz, prophase, metaphase, anaphase and telophase.

Due to the replication of DNA during interphase, each prophase chromosome consists of two coiled filaments which are known as the chromatids.

The mitotic apparatus in animal cells is formed of spindle, asters and centrioles where as in plant tells it consists of only the spindle.

The spindle fibers are chemically made of mainly a protein, the tubulin along with traces of RNA.

At the metaphase stage the chromosomes attach themselves in the middle (equator) of the spindle in a circle looking like a plate, known as the equatorial plate.

The chromosomal set of an individual (or a species) with all its characteristics, such as number of chromosomes, their shapes, size and position of centromeres is called karyotype.

A chromosome having the centromere located at its one end is called telocentric chromosomes.

The centromere of the acrocentric chromosome lies very near to one end with the result that one arm of the chromosome is very small or even imperceptible. In sub metacentric chromosome the centromere lies at some distance from one end so that the two arms of the chromosome are unequal.

In metacentric chromosome the centromere lies at the center and both arms of the chromosome are equal.

The chromatids separate as independent chromosomes at the anaphase stage of mitosis.

At late telophase the actin and myosin at equatorial region of the cell get activated, form a contractile ring followed by a cleavage furrow which deepens dividing the parent cell into two daughter ones.

In the plant cell, a membranous partition called phragmoplast is formed at the equator of the spindle from vesicles which originate from the golgi complex during metaphase and fuse together at the end of the telophase. Later cellulose and pectin are deposited to form new cell wall.

It is the mitosis which is involved in asexual reproduction in plants and animals.

The tumour is a swelling a mass of abnormal tissue which resembles the normal tissue in structure but performs no useful function and grows at the expense of the body. The tumour may be benign/innocent or malignant causing metastasis. The malignant tumour is also called cancer.

‘X’ and ‘Y’ chromosomes are known as the sex chromosomes because they help in the identification of the male and the female organisms.

They are called autosomes.

Non-disjunction is mistake which sometimes occurs during meiosis in which the homologous chromosomes of a pair (or more) fail to separate from each other.

Klinefelter’s syndrome is a hereditary defect in which a human male possesses an extra X-chromosome.

The cause of Down’s syndrome is the presence of an extra autosome in the cells of the affected person.

The cause of Turner’s syndrome is the absence of one X-chromosome in the afflicted human female whose sex chromosomes are XO.

The affected male humans become physically mature but remain sexually immature.

Internal programme of events and sequence of morphological changes by which cells commit suicide are collectively called as apoptosis.