Cell NCERT XI Biology

Cell-The Unit of Life

(1) Cytology: (G.k. kyios = cell ; logas = study) is the branch of biology which comprises the study of cell structure and function.

(2) Cell is the structural and functional unit of all living beings.

(3) There are two types of cells: plant cell and animal cell.



                                  Plant cell

                                  Animal cell


Cell wall present.

Cell wall absent.


Nucleus usually lies near periphery due to vacuole.

Nucleus present near the centre.


Centrosome is usually absent from higher plant cells, except lower motile cells.

Usually centrosome is present that helps in formation of spindle fibres.


Plastids are present, except fungi.

Plastids are absent.


Mitochondria is generally spherical or oval in shape.

Generally tubular in shape.


Single large central vacuole is present.

Many vacuoles occur, which are smaller in size.


Number of mitochondria from 200 – 2000.

Number of mitochondria is approximately 1600 – 16000 in liver cells.


Cytoplasm during cell division usually divides by cell plate method.

Cytoplasm divides by furrowing or cleavage method.


Plant cells are capable of forming all the amino acids coenzymes and vitamins.

Animal cells cannot form all the amino acids, coenzymes and vitamins.


There is no contractile vacuole.

Contractile vacuole may occur to pump excess water.


Sodium chloride is toxic to plant cells.

Tissue fluid containing sodium chloride bathes the animal cells.


Plant cells are generally well over 100 micrometer long.

Generally much smaller than 100 micrometer


Spindle formed during cell division is anastral.

Spindle formed during cell division are amphiastral.


Lysosomes present in less number.

Lysosomes present in more number.


Chromosomes are larger in size.

Chromosomes are smaller in size.




Important Points:-

(a) Jan Swammerdam: First to see red blood cells of frog.

(b) Marcello Malpighi: Observed small utricles in slice of plant and animal tissue.

(c) N. Grew: Initiated cell concept

(d) Lamarck: All living beings are formed of cells.

(e) Corti: First to point out living substance filled inside the cell. It was called “Sarcode” by Dujardin.

(f) In vivo (in life) study: Study of cells in their natural environment within the intact organism.

(g) In vitro (cultural condition) study: Study of isolated life system in laboratory and cultural condition.

(h) Max Shultze proposed protoplasm theory.

(i) Sachs proposed organismic theory.

(j) Crystallo: colloidal theory (Fischer), substances dispersed and dissolved in water forming both true solution as well as colloidal solution

(4) Difference between Prokaryotic and eukaryotic cells

                                     



                                Prokaryotic cell

                                 Eukaryotic cell


It is a single membrane system.

It is a double membrane system.


Cell wall surrounds the plasma membrane.

Cell wall surrounds the plasma membrane in some protists, most fungi and all plant cell. Animal cell lack it.


Cell wall composed of peptidoglycans. Strengthening material is mureir.

It is composed of polysaccharide. Strengthening material is chitin in fungi & cellulose in others plants.


Cell membrane bears respiratory enzymes.

It lacks respiratory enzymes.


Cytoplasm lacks cell organelles e.g., Mitochondria, ER, Golgi body etc.

Cytoplasm contains various cell organelles.


Ribosomes are 70 S type.

Ribosomes are 80 S type.


There are no streaming movements of cytoplasm.

Cytoplasm show streaming movements.


Endocytosis and exocytosis do not occur.

Endocytosis and exocytosis occur in animal cells.


Mitotic spindle is not formed in cell division.

Mitotic spindle is formed in cell division.


The mRNA does not need processing.

The mRNA needs processing.


Nuclear material is not enclosed by nuclear envelope and lies directly in cytoplasm. It is called nucleoid.

It is enveloped by nuclear envelope. Nucleus is distinct from cytoplasm.


DNA is circular and not associated with histone proteins.

Nuclear DNA is linear and associated with histone proteins extranuclear DNA is circular and protein free.


Replication of DNA occurs continuously throughout cell cycle.

Replication of DNA occurs during S- Phase of cell cycle only.


These have small size (0.5 to 10 micrometer) and have much less DNA.

These are relatively large (10 – 15 micrometer) and have much more DNA.


Sexual reproduction absent but parasexuality present. 

Sexual reproduction is present.


Plasmids and pili occur in many prokaryotes

Example – E. coli

There are no plasmids and pili in eukaryotic cells

Example – Spirogyra, Chlorella


Cell division mostly amitotic.

Cell division is typically mitotic.


Plasma invaginates and from finger like process. Mesosome which take part in respiration

Absent




(5) Difference between primary cell wall and secondary cell wall



                              Primary cell wall

                           Secondary cell wall


Primary wall is laid inner to middle lamella

Secondary wall is laid inner to primary wall.


It is formed in a growing cell.

It is formed when the cells have stopped growing.


It is capable of extension.

Extensibility is absent except in collenchyma cells.


It is single layered.

It is three or more layered.


Cellulose content is comparatively low (5 – 20%).

Cellulose content is comparatively high (20 – 90%).


Cellulose microfibrils are shorter, wavy and loosely arranged.

They are longer, closely arranged straight and parallel.


Protein content up to 5%.

Protein content up to 1%.


Hemicellulose content is high up to 50%.

It is 25% of the total.


Lipid content up to 5 – 10%.

Lipid is absent.


Primary wall is comparatively thin 1 – 5 micrometer

It is comparatively thick 5 – 10 micrometer




(6) Difference between extrinsic protein and intrinsic protein



                            Extrinsic Protein

                                  Intrinsic Protein


These are associated with surface only.

These lie throughout phospholipid matrix and project on both surfaces, also called transmembrane or tunnel protein.


They form about 30% of the total membrane protein.

They form about 70% of total membrane proteins.


Example – Spectrin in red blood cells & ATPase in mitochondria.

Example – Rhodopsin in retinal rod cells.




Cell Wall

(1) Discovery: It was first discovered by Robert Hooke in 1665.

(2) Cell wall is the outer most, rigid, protective, non living and supportive layer found in all the plant cells, bacteria, cyanobacteria and some protists.

                                        

(3) It is not found in animal cells.

(4) Difference between primary cell wall and secondary cell wall



                          Primary cell wall

                        Secondary cell wall


Primary wall is laid inner to middle lamella

Secondary wall is laid inner to primary wall.


It is formed in a growing cell.

It is formed when the cells have stopped growing.


It is capable of extension.

Extensibility is absent except in collenchyma cells.


It is single layered.

It is three or more layered.


Cellulose content is comparatively low (5 – 20%).

Cellulose content is comparatively high (20 – 90%).


Cellulose microfibrils are shorter, wavy and loosely arranged.

They are longer, closely arranged straight and parallel.


Protein content up to 5%.

Protein content up to 1%.


Hemicellulose content is high up to 50%.

It is 25% of the total.


Lipid content up to 5 – 10%.

Lipid is absent.


Primary wall is comparatively thin 1 – 5 micrometer

It is comparatively thick 5 – 10 micrometer




Plasma Memberane

(1) Definition: Every living cell is externally covered by a thin transparent electron microscopic, elastic regenerative and selective permeable membrane called plasma membrane.

                                

(2) It is quasi fluid in nature.

(3) According to Singer and Nicolson it is “protein iceberg in a sea of lipid”.

(4) A cell wall lies external to plasmalemma in plant cells, many monerans, some protists and fungal cells.

(5) Membranes also occur inside the cells.

(6) They are collectively called biomembranes.

(7) The term cell membrane was given by C. Nageli and C. Cramer (1855) for outer membrane covering of the portoplast.

(8) It was replaced by the term plasmalemma or plasma membrane by Plowe (1931).

Protoplasm

(1) Definition: Protoplasm is a complex, granular, elastic, viscous and colourless substance. It is selectively or differentially permeable.

(2) It is considered as “Polyphasic colloidal system”.

(3) Discoveries

(i) J. Huxley defined it as “physical basis of life”.

(ii) Dujardin (1835) discovered it and called them “sarcode”.

(iii) Purkinje (1837) renamed it as “Protoplasm”.

(iv) Hugo Von Mohl (1844) gave the significance of it.

(v) Max Schultz (1861) gave the protoplasmic theory for plants.

Cytoplasm

The substance occurs around the nucleus and inside the plasma membrane containing various organelles and inclusions is called cytoplasm.

                                                     

Mitochondria

(1) Definition: (Gk – mito = thread ; chondrion = granule) Mitochondria are semi autonomous having hollow sac like structures present in all eukaryotes except mature RBCs of mammals and sieve tubes of phloem.

(2) These are absent in all prokaryotes like bacteria and cyanobacteria.

(3) Mitochondria are also called chondriosome, chondrioplast, plasmosomes, plastosomes and plastochondriane.

Plastids

(1) Definition: Plastids are semiautonomous organelles having DNA, RNA, Ribosomes and double membrane envelope which store or synthesize various types of organic compounds as ATP and NADPH + H+ etc.

(2) These are largest cell organelles in plant cell.

(3) History: Haeckel (1865) discovered plastid, but the term was first time used by Schimper (1883).

(4) Difference between chlorophyll a and b



                                         Chl. a

                                      Chl. b


Absorption peak at 430, 662.

It is 453, 642.


Bluish green in colour.

Yellowish green.


Soluble in petroleum, ether.

Soluble in methyl alcohol.


Functional group at C3 position is CH3

Functional group attached to pyrrol ring is CHO.


Present in all green plants excepts autotrophic bacteria.

Present in all green plants except blue green, brown and red algae.


In chloroplast it is 75%.

It is 25%


In reflected light Chl. a shows blood red colour while in transmitted light, it shows blue green colour.

In reflected light it show dull brown colour while in transmitted light, it shows yellowish green colour.




Endoplasmic Reticulum

(1) Definition: It is well developed electron microscopic network of interconnected cisternae, tubules and vesicles present throughout the cytoplasm, especially in the endoplasm.

(2) Discovery: Garnier (1897) was first to observe the ergastoplasm in a cell. The ER was first noted by Porter, Claude, and Fullman in 1945 as a network. It was named by Porter in 1953.

Golgi Complex

(1) Definition: Golgi complex is made up of various membranous system e.g. cisternae, vesicles and vacuoles.

(2) These are also called golgi bodies, golgisomes, lipochondrion, dictyosomes, Dalton complex, idiosomes or Baker’s body.

(3) These are also called “traffic police” of the cell.

(4) Discovery: First observed by George (1867) but it’s morphological details were given by Camillo Golgi (1898), in nerve cells of barn fowl and cat .
                                                   

Molecular Basis of Inheritance C6 XII Biology

 Molecular Basis of Inheritance

DNA

(1) DNA is a long polymer of deoxyribonucleotides.

(2) The length of the DNA depends on the number of nucleotide pairs present in it.

(3) Bacteriophage lambda has 48,502 base pairs.

Central dogma of molecular biology
(1) Crick proposed the Central dogma in molecular biology

(2) It states that the genetic information flows from DNA à RNA à Protein. 

(3) In some viruses like retroviruses, the flow of information is in reverse direction, which is from RNA à DNA à mRNA à Protein.

Structure of polynucleotide chain:

(1) A nucleotide has three components-

(a) A nitrogen base

(b) A pentose sugar (ribose in RNA and deoxyribose in DNA)

(c) A phosphoric acid.

(2) There are two types of nitrogen bases:

(a) Purines (Adenine and Guanine)

(b) Pyrimidines (Cytosine, Uracil and Thymine)

(3) Adenine, Guanine and Cytosine are common in RNA and DNA.

(4) Uracil is present in RNA and in DNA in place of Uracil, Thymine is present.

(5) In RNA, Pentose sugar is ribose and in DNA, it is Deoxyribose.

(6) Based on the nature of pentose sugar, two types of nucleosides are formed - ribonucleoside and deoxyribonucleotides.

(7) Two nucleotides are joined by 3’-5’ Phosphodiester linkage to form dinucleotide.

(8) More than two nucleotides join to form polynucleotide chain.

(9) The two strands of DNA  (called DNA duplex) are antiparallel and complementary, i.i., one in 5’->3’ direction and the other in 3”->5” direction.

History of DNA

(1) DNA is an acidic substance in the nucleus.

(2) It was first identified by Friedrich Meischer in 1869. He named it as ‘Nuclein”

(3) In 1953 double helix structure of DNA was given by James Watson and Francis Crick, based on X-ray diffraction data produced Maurice Wilkins and Rosalind Franklin.

Packaging of DNA Helix

(1) The basic unit into which DNA is packed in the chromatin of eukaryotes.

(2) Nucleosome is the basic repeating structural (and functional) unit of chromatin, which contains nine histone proteins.

(3) Distance between two conjugative base pairs is 0.34nm

(4) The length of the DNA in a typical mammalian cell will be 6.6 X109 bp X 0.34 X10-9 /bp, it comes about 2.2 meters.

(5) The length of DNA is more than the dimension of a typical nucleus (10-6m)

DNA Replication

(1) DNA is the only molecule capable of self duplication so it is termed as a living molecule.

(2) All living beings have the capacity to reproduce because of DNA.

(3) DNA replication takes place in S-phase of the cell cycle. At the time of cell division, it divides in equal parts in the daughter cells.

(4) Delbruck suggested three methods of DNA replication i.e.

(i) Dispersive

(ii) Conservative

(iii) Semi-conservative

(5) The process of DNA replication takes a few minutes in prokaryotes and a few hours in eukaryotes.

RNA

(1) RNA is the first genetic material.

(2) RNA is a non hereditary nucleic acid except in some viruses (retroviruses).

(3) RNA used to act as a genetic material as well as catalyst.

(4) It is a polymer of ribonucleotide and is made up of pentose ribose sugar, phosphoric acid and nitrogenous base (A,U,G,C).

(5) RNA may be of two types – genetic and non-genetic.

Genetic Code

(1) Term genetic code was given by George Gamow (1954). He was the first to propose the triplet code (one codon consists of three nitrogen bases).

(2) The relationship between the sequence of amino acids in a polypeptide chain and nucleotide sequence of DNA or mRNA is called genetic code.

(3) There occur 20 types of amino acids which participate in protein synthesis. DNA contains information for the synthesis of any types of polypeptide chain. In the process of transcription, information transfers from DNA to m-RNA in the form of complementary N2-base sequence.

(4) A codon is the nucleotide sequence in m-RNA which codes for particular amino acid; whereas the genetic code is the sequence of nucleotides in m-RNA molecule, which contains information for the synthesis of polypeptide chain.

(5) 61 out of 64 codons code for only 20 amino acids.

(6) The main problem of genetic code was to determine the exact number of nucleotide in a codon which codes for one amino acid.

Characteristics of genetic code

(1) Triplet in nature

(a) A codon is composed of three adjacent nitrogen bases which specify one amino acid in polypeptide chain.

(b) For example- In m-RNA if there are total 90 N2 – bases. Then this m-RNA determines 30 amino acids in polypeptide chain.

(2) Univerality

(a) The genetic code is applicable universally.

(b) The same genetic code is present in all kinds of living organism including viruses, bacteria, unicellular and multicellular organisms. In all these organisms, triplet code for specific amino acid.

(3) Non-ambiguous

(a) Genetic code is non ambiguous i.e. one codon specifies only one amino acid and not any other.

(b) In this case one codon never code two different amino acids. Exception GUG codon which code both valine and methionine amino acid.

(4) Non-overlapping

(a) A nitrogen base is a constituent of only one codon.

(5) Comma less

(a) There is no punctuation (comma) between the adjacent codon i.e. each codon is immediately followed by the next codon.

(b) If a nucleotide is deleted or added, the whole genetic code read differently.

(c) A polynucleotide chain having 50 amino acids shall be specialized by a linear sequence of 150 nucleotides. If a nucleotide is added in the middle of this sequence, the first 25 amino acids of polypeptide will be same but next 25 amino acids will be different.

(6)  Degeneracy of genetic code

(a) Only two amino acids – tryptophan and methionine are specified by single codon.

UGG for tryptophan

AUG for methionine

(b) All the other amino acids are specified or coded by 2 to 6 codons.

(c) Leucine, serine and arginine are coded or specified by 6-codons.

(d) Degeneracy of genetic code is related to third position (3’-end of triplet codon) of codon. The third base is described as ‘Wobble base’.

Genomics and Human Genome project:

(1) The term genome has been introduced by Winkler in 1920 and the genomics is relatively new, coined by Thomas Rodericks in 1986.

(2) Genomics is the subdiscipline of genetics devoted to the mapping, sequencing and functional analysis of genomes. Genomics is subdivided into following types:

(a) Structural genomics: It is the study of genome structure deals with the complete nucleotide sequences of the organisms.

(b) Functional genomics: It is the study of genome function which includes transcriptome and proteome. Transcriptome is a complete set of RNAs transcribed from a genome while proteome is a complete set of proteins encoded by a genome and aims the determination of the structure and function of all the proteins in living organisms.

(3) The human genome project, sometimes called “biology’s moon shot”, was launched on october 1, 1990 for sequencing the entire human genome of 2.75 billion (2.75 ´ 109 or 2750000 bp or 2750000 kilobase pairs or 2750 megabase pairs) nucleotide pairs.

(4) Two important scientist associated with human genome are Francis Collins, director of the Human Genome Project and J. Craig Venter, founding president of Celera genomics.

(5) The complete sequencing of the first human chromosome, small chromosome 22, was published in December 1999.

Genome of Model organisms



S. No.

Organism

No. of base pair

No. of genes


(1)

Bacteriophage

10 thousand

–


(2)

E. coli

4.7 million

4000


(3)

Saccharomyces cerevisiae

12 million

6000


(4)

Caenorhabditis elegans

97 million

18,000


(5)

Drosophila melanogaster

180 million

13,000


(6)

Human

3 billion

30,000


(7)

Lily

106 billion

–




Prospects and implications of human genome:

(1) The genome project is being compared to the discovery of antibiotics.

(2) Efforts are in progress to determine genes that will revert cancerous cells to normal.

(3) The human genome sequencing not only holds promise for a healthier living. It also holds the prospects of vast database of knowledge about designer drugs, genetically modified diets and finally our genetic identity.

DNA finger printing

(1) Alec Jeffreys et al (1985) developed the procedure of genetic analysis and forensic medicine, called DNA finger printing.

(2) It is individual specific DNA identification which is made possible by the finding that no two people are likely to have the same number of copies of repetitive DNA sequences of the regions.

(3) It is also known as DNA profiling.

(4) The chromosomes of every human cell contain scattered through their DNA short, highly repeated 15 nucleotide segments called “mini-satellites” or variable-number Tandem Repeat (VNTR).

Technique for DNA fingerprinting

(1) Only a small amount of tissues like blood or semen or skin cells or the hair root follicle is needed for DNA fingerprinting.

(2) Typically DNA content of about 100,000 cells or about 1 microgram is sufficient.

(3) The procedure of DNA fingerprinting involves the following major steps:

(i) DNA is isolated from the cells in a high-speed refrigerated centrifuge.

(ii) If the sample of DNA is very small, DNA can be amplified by Polymerase Chain Reaction (PCR).

(iii) DNA is then cut up into fragments of different length using restriction enzymes.

(iv) The fragments are separated according to size using gel electrophoresis through an agarose gel. The smaller fragments move faster down the gel than the larger ones.

(v) Double stranded DNA is then split into single stranded DNA using alkaline chemicals.

(vi) These separated DNA sequences are transferred to a nylon or nitrocellulose sheet placed over the gel. This is called ‘Southern Blotting’ (after Edward Southern, who first developed this method in 1975).

(vii) The nylon sheet is then immersed in a bath and probes or makers that are radioactive synthetic DNA segments of known sequences are added. The probes target a specific nucleotide sequence which is complementary to VNTR sequences and hybridizes them.

(viii) Finally, X-ray film is exposed to the nylon sheet containing radioactive probes. Dark bands develop at the probe sites which resemble the bar codes used by grocery store scanners to identify items.

Applications of DNA fingerprinting

This technique is now used to:

(i) Identify criminals in forensic laboratories.

(ii) Settle paternity disputes.

(iii) Verify whether a hopeful immigrant is, as he or she claims, really a close relative of already an established resident.

(iv) Identify racial groups to rewrite biological evolution