Cell Division - Cell Cycle - Mitosis

 Continuity of life depends on cell division. All cells are produced by divisions of pre-existing cell. A cell born after a division, proceeds to grow by macromolecular synthesis, and divides after reaching a species-determined division size. Growth of a cell is an increase in size or mass which is an irreversible process that occurs at all organizational levels.

Cell cycle:

Cell cycle can be defined as the entire sequence of events happening from the end of one nuclear division to the beginning of the next division. Cells have the property of division and multiplication and consist of three major phases namely mitosis (M phase) or the nuclear division, cytokinesis or the division of the cell and interphase where replication of genetic material occurs. The M phase lasts only for an hour in a period of 24 hour required for a eukaryotic cell to divide. The interphase can be further divided into G1 (gap phase 1), S (synthesis) and G2 (gap phase 2) phases.

 This division of interphase into three separate phases based on the timing of DNA synthesis was first proposed in 1953 by Alma Howard and Stephen Pelc of Hammersmith Hospital, London, based on their experiments on plant meristem cells. Cell cycles can range in length from as short as 30 minutes in a cleaving frog embryo, whose cell cycles lack both G1 and G2 phases, to several months in slowly growing tissues, such as the mammalian liver.

 Cells that are no longer capable of division, whether temporarily or permanently, remain in G0 phase. A cell must receive a growth-promoting signal to proceed from the quiescent stage or G0 into G1 phase and thus reenter the cell cycle

Interphase

 During interphase the chromosomes are not visible with a light microscope when the cell is not undergoing mitosis. The genetic material (DNA) in the chromosomes is replicated during the period of interphase to carry out mitosis and is called S phase (S stands for synthesis of DNA). DNA replication is accompanied by chromosome duplication.

 Before and after S, there are two periods, called G1 and G2, respectively, in which DNA replication does not take place. The order of cell cycle events is G1 → S → G2 → M and then followed by cytokinesis. The G1 phase, S phase and G2 phase together form the interphase .

The interphase is characterized by the following features: The nuclear envelope remains intact. The chromosomes occur in the form of diffused, long, coiled and indistinctly visible chromatin fibres. The DNA amount becomes double. Due to accumulation of ribosomal RNA (rRNA) and ribosomal proteins in the nucleolus, the size of the latter is greatly increased. 

G1 Phase: After the M phase of previous cell cycle, the daughter cells begin G1 of interphase of new cell cycle. G1 is a resting phase. It is also called first gap phase , as no DNA synthesis takes place during this stage. It is also known as the first growth phase , since it involves synthesis of RNA, proteins and membranes which leads to the growth of nucleus and cytoplasm of each daughter cell towards their enhancing size.

 During G1 phase, chromatin is fully extended and not distinguishable as discrete chromosomes with the light microscope. Thus, it involves transcription of three types of RNAs, namely rRNA, tRNA and mRNA; rRNA synthesis is indicated by the appearance of nucleolus in the interphase (G1 phase) nucleus. 

Proteins synthesized during G1 phase (a) regulatory proteins which control various events of mitosis (b) enzymes (DNA polymerase) necessary for DNA synthesis of the next stage and (c) tubulin and other mitotic apparatus proteins. 

G1 phase is most variable as to duration it either occupies 30 to 50 per cent of the total time of the cell cycle. Terminally differentiated somatic cells (end cells such as neurons and striated muscle cells) that no longer divide, are arrested usually in the G1 stage, such a type of G1 phase is called G0 phase.

S phase: During the S phase or synthetic phase of interphase, replication of DNA and synthesis of histone proteins occur. New histones are required in massive amounts immediately at the beginning of the S period of DNA synthesis to provide the new DNA with nucleosomes. At the end of S phase, each chromosome has two DNA molecules and a duplicate set of genes. S phase occupies roughly 35 to 45 per cent time of the cell cycle.

G2 phase: This is a second gap or growth phase or resting phase of interphase. During G2 phase, synthesis of RNA and proteins continues which is required for cell growth. It may occupy 10 to 20 per cent time of cell cycle. As the G2 phase draws to a close, the cell enters the M phase.

Dividing phase: There are two types of cell division possible. Mitosis and meosis. The mitosis (Gr., mitos =thread) occurs in the somatic cells and it is meant for the multiplication of cell number during embryogenesis and blastogenesis of plants and animals. Fundamentally, it remains related with the growth of an individual from zygote to adult stage.

 Mitosis starts at the culmination point of interphase (G2 phase). It is a short period of chromosome condensation, segregation and cytoplasmic division. Mitosis is important for growth of organism, replacement of cells lost to natural friction or attrition , wear and tear and for wound healing. Hence, mitosis is remarkably similar in all animals and plants. It is a smooth continuous process and is divided into different stages or phases.

Mitosis

Mitosis is a process of cell division in which each of two identical daughter cells receives a diploid complements of chromosomes same as the diploid complement of the parent cell. It is usually followed by cytokinesis in which the cell itself divides to yield two identical daughter cells.

The basics in mitosis include:

1. Each chromosome is present as a duplicated structure at the beginning of nuclear division (2n).

2. Each chromosome divides longitudinally into identical halves and become separated from each other.

3. The separated chromosome halves move in opposite directions, and each becomes included in one of the two daughter nuclei that are formed.

 Mitosis is divided into four stages: prophase, metaphase, anaphase and telophase

.  Prophase:

The chromosomes are in the form of extended filaments and cannot be seen with a light microscope as discrete bodies except for the presence of one or more dark bodies (i.e. nucleoli) in the interphase stage. 

The beginning of prophase is marked by the condensation of chromosomes to form visibly distinct, thin threads within the nucleus. Each chromosome is already longitudinally double, consisting of two closely associated subunits called chromatids which are held together by centromere. 

Each pair of chromatids is the product of the duplication of one chromosome in the S period of interphase. As prophase progresses, the chromosomes become shorter and thicker as a result of intricate coiling. At the end of prophase, the nucleoli disappear and the nuclear envelope, a membrane surrounding the nucleus, abruptly disintegrates.

2.  Metaphase:

At the beginning of metaphase, the mitotic spindle forms which are a bipolar structure and consist of fiber-like bundles of microtubules that extend through the cell between the poles of the spindle. Each chromosome attached to several spindle fibers in the region of the centromere. The structure associated with the centromere to which the spindle fibers attach is known as the kinetochore.

 After the chromosomes are attached to spindle fibers, they move towards the center of the cell until all the kinetochores lie on an imaginary plane equidistant from the spindle poles. This imaginary plane is called the metaphase plate. Hence the chromosomes reach their maximum contraction and are easiest to count and examine for differences in morphology. 

The signal for chromosome alignment comes from the kinetochore, and the chemical nature of the signal seems to be the dephosphorylation of certain kinetochore-associated proteins. 

3. Anaphase

the centromeres divide longitudinally, and the two sister chromatids of each chromosome move toward opposite poles of the spindle. Once the centromere divide, each sister chromatid is treated as a separate chromosome. Chromosome movement results from progressive shortening of the spindle fibers attached to the centromeres, which pulls the chromosomes in opposite directions toward the poles. At the completion of anaphase, the chromosomes lie in two groups near opposite poles of the spindle. Each group contains the same number of chromosomes that was present in the original interphase nucleus.

4. Telophase:

In telophase, a nuclear envelope forms around each group of chromosomes, nucleoli are formed, and the spindle disappears. The chromosomes undergo a reversal of condensation until and unless they are no longer visible as discrete entities. The two daughter nuclei slowly goes to interphase stage the cytoplasm of the cell divides into two by means of a gradually deepening furrow around the periphery.

 5.  Cytokinesis:

The chromosomes moved close to the spindle pole regions, and the spindle mid-zone begins to clear. In this middle region of the spindle, a thin line of vesicles begins to accumulate. This vesicle aggregation is an indication to the formation of a new cell wall that will be situated midway along the length of the original cell and hence form boundary between the newly separating daughter cells.

Archaebacteria

Archaebacteria 

living organisms into large groups called kingdoms.

Five– Monera, Protista, Fungi, Plantae and Animalia

In 1996 scientists decided to split Monera into two groups –

 Archae and Eubacteria.

2 groups of baceteria were different in many ways -Domain.

There are 3 Domains – Bacteria, Archea and Eukaryota

Greek; Archaio=ancient, bakterion=a small rod

.Arches are the first cells that originated
On the earth.

Prokaryotic unicellular

 

spherical, rod-shaped, spiral or irregular shape.

They range in their size from 0.1 to over 15µ (microns) in length.

Nutritionally they are chemolithotrophs or oganotrophs.

The cell wall do not contain peptide glycan (muramic acid + D – amino acids)

 but a range of other unique polysaccharides are present.

The 16 µ r RNA molecules differ greatly from other bacterial and eukaryotes.

The ribosomes of these organisms are insensitive to chloramphenicol (antibiotic)

The first amino acid to initiate the new polypeptide chain is methionine, instead of N-formyl methionine.

Habitat

  They usually prefer extreme aquatic or terrestrial habitats.

          They are often present in anaerobic, hypersaline , or high-temperature environments.

 

         A few are symbionts in animal digestive systems
Morphology: 
Archea are diverse in morphology. They
may be spherical, spiral, lobed or pleomorphic. Some are single celled, whereas other form filaments or aggregates.
They range in diameter from 0.1 to 15 micrometres.

Cell walls

either Gram positive or Gram negative.

differ from Eubacteria.

They donot have muramic acid and D-amino acids -characteristic of eubacterial  peptidoglycan.

All archaebacteria resist attact by lysozyme and B- lactam antibiotics such as penicillin

Cell walls

Methanobacterium and some other methanogens -pseudomurein, a peptidoglycan like polymer thas has L –amino acids in its cross-like , N-acetyltalosaminuronic acid instead of N-acetylmuramic acid.

Gram negative archaeobacteria have a layer of protein or glycoprotein outside their plasma membrane.

Membranes

   lacks fatty acids and instead have carbon moieties bonded to glycerol by ether (instead of ester) linkages.

They contain polar lipids such as phospholipids, sulfolipids and glycolipids and non-polar lipids  which are derivatives of the isoprenoid compound sqalene.

In spite of their chemical uniqueness, membranes of most archaebacteria are structurally arranged  to form a typical bilayer like eubacterial

The presence of ether linked lipids is such a unique characteristic of archaebacteria -used as a biomarker for detecting archaebacteria in paleontological studies of rocks, sediment cores and other fossil materials.

Genomes

The genome of archaebacteria consists of a single covalently closed circular DNA molecule much smaller than that of eubacteria.

 Plasmids of different sizes have been reported from several bacterial.

 The 16 µ r RNA molecules differ greatly from other bacterial and eukaryotes.

 The first amino acid to initiate the new polypeptide chain is methionine, instead of N-formyl methionine

 The ribosomes of eubacterial are sensitive to  Chloramphenicol while the archaebacteria are not.

 The  diphtheria toxin affects archaebacterial but not eubacteria. Both these inhibitors affect the translational process in the cell.

Classification

Three distinct groups

  

 the methanogens,

extreme halophiles

extreme thermophiles

The Methanogens

They are strictly obligate anaerobes.

 While producing methane they utilize electrons generated by oxidation of hydrogen or simple organic compounds such as acetate and methanol. 

The habitats like marshes, swamps, pond and lake mud,

the intestinal tract of humans and animal,

 the rumen of cattle and

 anaerobic sludge digester in sewage treatment systems are ideal for these archaebacteria.

Ex- Methanobacterium, Methanospirillum, Methanococcus.

Extreme Halophiles (Halobacteria)

They are obligate halophiles found in salt lakes, the Dead sea, foods preserved by salting and salt production industrial plants.

They grow in salt with concentrations above 15 %.

They are aerobic.

They stain Gram negative and range from rod or disc-shaped cells (Halobacterium) to cocci (Halococcus).

Extreme Halophiles (Halobacteria)

The colonies are red to orange colour due to presence of carotenoids which seem to protect the cells against the damaging effect of sunlight.

There mechanism of photo production of energy is unique as they use bacteriorhodopsin, a special pigment, as photoreceptor.

Ex. – Halobacterium, Halococcus.

Extreme Thermophiles (Hyperthermophiles)

They are gram negative and anaerobic, characterized by a remarkable ability to grow under high acidic conditions at high temperature.

They are Gram negative and anaerobic. Some can survive at near boiling point of water.

Ex. Sulfobolus, Pyrodictium, Thermoplasma

Some of the differences between Archea and other organisms:
1. Archea bacterial cell wall is made up of pseudo peptidoglycon instead of peptidoglycon.

2. Ether linked lipids are present in Archeabacterial cell membrane whereas ester linked lipids in eubacteria.

3. Archeabacteria has single DNA polymerase whereas eubacteria has three DNA polymerases

4. Archea tRNA lacks thymine, instead 1-methyl pseudo uridine is present.

Chromosomal Aberrations or Variations in the structure of Chromosomes

 

Chromosomes are the structures which carry genes in a definite linear order. Chromosomes have a definite structure and organization, which is normally constant from one cell division to the next. They, however, sometimes undergo certain structural modifications which are known which are known as chromosomal aberrations. Since, these modification also result in a change in the organism, they are also called chromosomal mutations.

Breakage Fusion-Bridge Cycles:

Breaks in the chromosome are necessary for any structural changes. After break the chromosome may remain un united. When this happens, the part of the chromosome without the centromere is lost, since, spindle fibres attach only to the centromere.

Sometimes, the broken ends may join immediately and the original chromosome is restored (restitution). At other times, the broken ends may join the segments produced by other breaks (non-restitutional union), resulting in structural changes.

McClinctok studied the broken ends of chromosomes in the maize. A chromosome break occurs, and is followed by duplication of the chromosome during prophase. As a result, two chromatids are formed. The broken ends of the chromatids behave as if they are ‘sticky’, and undergo fusion. This results in the formation of a chromatid with two centromere (dicentric chromatid). During anaphase, spindle fibres are attached to the two centromeres. As a result, the chromatid stretches out, forming a bridge from one pole to the other. This bridge may break, but at different point, producing a deficiency and a duplication.

Chromosomal abberations are of four types : 1. Deletions or Deficiencies, 2. Duplications, 3. Inversions, and 4. Translocations.

Deficiencies:

Loss of chromosomal segment resulting in the loss of genes is called as Deficiency or Deletion. The deleted portion will not survive in the higher organisms if it lacks a centromere, because it will have no capacity for movement in Anaphase. The portion of the chromosome carrying the centromere functions as a genetically deficient chromosome.

Deficiencies can be terminal or interstitial.

If the single break occurs near the end of the chromosome, it is called terminal deletion. Generally, if a terminal break occurs the broken piece attaches to the terminal portion of the same chromosome. When restitution takes place no change is observed in the chromosome.

But sometimes, the broken fragment do not join. If the broken fragment do not contain a centromere, then it will not go to the either of the poles as spindle fibres cannot attach to it. Then this fragment ultimately disappear in the cytoplasm.

Sometimes, two breaks occur at any two points, releasing an intercalary segment. The broken ends of the original chromosomes may again fuse, resulting in an intercalary or interstitial deficiency.

Both terminal and intercalary deficiencies can be observed during pachytene stage of meiosis or in the polytene Chromosomes. Pairing of chromosomes takes place chromomere to chromomere. If a segment is missing from the chromosome, then the homologous chromosome containing this segment will form a loop during pairing. This is called deletion loop.

A deletion represent a quantitative change in the genotype in that it involves the loss of of genetic material, so it would be expected that deficiencies would have deleterious effect on an organism, the effect depending upon the amount of genetic material and it's specific function.

Deletions, unlike, other mutations generally donot revert, or mutate back, to the wild type chromosome.

In deletion heterozygotes, recessive alleles on the normal Chromosomes are expressed because the deletion chromosome is missing the homologous regions. Expression of recessive alleles in such cases, is called pseudo dominance.

A well known example in the humans is the deletion of one half of the short arm of chromosome 5 (5p) which, when heterozygous, causes the Cri- du- chat ( cry - of the - cat) syndrome. Infants with this syndrome  generally have characteristic high-pitched, cat like cry as well as microcephally (small head) and several mental retardation.

 Duplications:
when a chromosomal segment is represented twice, it is called as duplications. duplications originates out of unequal crossing over. There are 4 types of duplications

1. Tanden Duplication:

the duplication is present adjacent to the original chromosomal region in the same order as the original sequence, then this type of duplications are called tandem duplications.. for example, if duplication repeat is ABC, a tandem dulication will be ABC ABC, DEF.

2.Reverse Tandem Duplication:

the duplication is present adjacent to the original chromosomal region but in the reverse order of the original sequence, then this type of duplications are called Reverse Tanden Duplications. For example, if the duplication repeat is ABC, a reverse tandem duplication will be ABC CBA DEF.

3. Displaced Duplication:

The duplicated chromosomal segment is located on the same chromosome but away from the original segment i.e., the duplicated region is not present adjacent to the original segment., resulting in a displaced duplication.

4. Transposed Duplication:

The duplicated chromosomal segment is located on a non-homologous chromosome.

when an individual is heterozygous for a duplication and normal chromosome, the duplicated region doesnot have a homologous segment to pair with in Meiosis. As a result a loop of the duplicated region may develop.

one of the classical examples of duplication in Drosopohila is Bar eye character. the wild type eye is large and has an average 779 facets or ocelli. it has two bar region, one on each X chromosome. by the study of gaint salivary gland chromosomes, it could be demonstrated that 'Bar' character was due to a duplication in region 16A of X-chromosome. the normal males have 16A region represented once, Bar-males has two Bar regions with elongated eyes with 358 facets and Bar-Double males has three Bar regions with 45 facets.

in females, the wild B^+/B⁺ has nearly 800 facets in each of its eyes; in the heterozygote B/ B⁺  , the number is reduced to about 360; while the homozygous B/B, the eye is reduced to about 70 facets, arranged in long, narrow eye structure..

As might be expected, duplications are observed more frequently in nature and are less likely to be lethal to the individual than are deficiencies.

Inversions:

Most of the homologous chromosomes in a population have genes in the same sequence. However, sometimes the sequence of genes in a chromosome is altered by the rotation of a chromosome segment through 180⁰. Alterations in the sequence of genes is called inversions.

So, in inversions a chromosome segment contains genes in a sequence which is reverse of normal. For example if a chromosome having gene alignment a b c d e f g h i, breaks at two points b and g and the middle segment c d e f g undergoes inversion, then an inverted chromosome segment if formed with the sequence a b g f e d c h i.

Due to inversions there is no increase or decrease in number of genes. Only re-arrangement of genes occur.  Inversions are of two types:

Paracentric Inversion:

When both the breaks in the chromosome occur on the same side of the centromere, i.e., the inverted segment does not contain centromere, then such type of inversions are called paracentric inversions.

Pericentric Inversion: (peri=around).

The inverted segment contains the centromere i.e., it involves the one break on either side of the centromere.

Inversions arise by the formation of loops on a chromosome. Breaks may occur at the point of intersection of the loops. Reunion of the broken ends takes place in a new combination, and an inversion results. The paracentric inversion results in an acrocentric chromosome just like the non-inverted chromosome, but that the pericentric inversion results in a metacentric chromosome, because the position of the centromere has been changed.

Individual heterozygous for an inversion can be recognized by the presence of inversion loop in meiotic pachytene chromosomes. These structures occur because of the affinity of the two homologous, and the only way two homologous can pair is if one twists on itself and makes a loop while the other makes a loop without twist.

In paracentric inversion, a single cross over or an odd number of cross overs in inverted region results in the formation of a dicentric chromosome (having two chromosomeres) and an acentric fragment (with no centromere). The dicentric chromosome and acentric fragments are observed at anaphase-I, in the form of anaphase bridge and fragment. Crossing over within and outside inversion lead to various kinds of deficiencies and duplications.

Inversions reduce the frequency of recombination between any two genes. Thus, they are called cross over suppressors.

Translocations:

 When a segment of one chromosome is detaches and joins to a non-homologous chromosome, it is called translocation. It is different from crossing over, which involves an interchange between segments of homologous chromosomes.

There is no addition or loss of genes during translocation, only a re-arrangement i.e., change in the sequence and position of a gene, occurs.

Translocations are of the following types:

Simple translocations:

A single break occur in a chromosome. The broken segment gets attached to one end of a non-homologous chromosome. This type of translocations are very rare.

Shift translocation:

A chromosome breaks at two places. A simple break occurs in the other non-homologous chromosome. The broken part gets inserted interstitially in a non homologous chromosome.

Reciprocal translocation:

A segment from one chromosome is exchanged with a segment from another non-homologous chromosome, such type of translocation are called as reciprocal translocations. As a result, two translocation chromosomes are simultaneously achieved.

Reciprocal translocations are of two types:

a.      Homozygotic translocation:

Tranlocation involves both the members of each of the homologous chromosomes. This type of translocation occurs rarely, and exhibit normal meiotic behaviour. Hence, they are difficult to detect cytologically unless morphologically dissimilar chromosomes are involved.

b.     Heterozygotic translocation:

Translocation involves only one member of each of the homologous pair.

Normal pairing into bivalents will not be possible among chromosomes involved in heterozygotic translocation. Due to pairing between homologous segments of chromosomes, a cross shaped (+) figure involving four chromosomes will be observed during pahcytene.