MCQs......... Revise and have a fast check on your lessons.
Topic: Reproduction in Flowering Plants
1. Which of the following is not an out breeding device?
     a) Hermaphrodite flowers
     b) self incompatibility
     c) Production of monoecious flowers
     d) Non synchronisation of pollen release and stigma receptivity
2. The type of flower shown in the diagram is-
     
           

    
      a) Hypogynous          
      b) Epigynous        
      c) Perigynous        
      d) None of these
3. The transfer of pollengrains from the anther to the stigma of the different flower of the same plant       is called-
       a) Autogamy       
       b) Geitonogamy     
       c) Xenogamy       
       d) Polygamy
4. Light, non sticky pollengrain and well exposed stamens are the charecteristics of which of the         
    following. 
       a) Anemophilly   
       b) Hydrophilly    
       c) Zoophilly      
       d) Chiropterophilly
5. PEN is formed from the fusion of -
        a) Two polar nuclei
        b) Two antipodal cells
        c) Male gamete and polar nuclei   
        d)female gamete and polar nuclei
6. The residual persistent nucellus present in some seeds  are called-
        a) Perisperm      
        b) Pericarp     
        c) Pericycle       
        d) Perichondrium
7. The function of filiform apparatusin reproduction is to -
         a) provide nutrition to the female gamete
         b) stimulate division of male gamete.
         c) Guide the pollen tube into the synergid
         d) None of these
8. The flowers with exposed anthers and stigma are called-
          a) Unisexual flowers
          b) Bisexual flowers
          c) Cleistogamous flowres
          d) Chasmogamous flowers  
9. Scutellum is -
          a) cotyledon in monocots
          b) Endosperm in monocots
          c) Cotyledon in dicots
          d) Endosperm in dicots
10. Apomixis is the mechanism to produce
          a) Fruits without fertilisation
          b) Organisms without fertilisation
          c) Seeds without fertilisation 
          d) None of these.

Taken the distance between two consecutive base pairs- 0.34nm,

if the length of DNA double helixin a typical mammalian cell,  
         =The total no. of base pairs x the distance between two consecutive base pairs

         = approximately 2.2 metres.

Wonderful and efficient packaging system of such a long polymer  in a cell..............

   

     PACKAGING OF DNA

PACKAGING OF DNA HELIX  IN PROKARYOTES

      In prokaryotes with no well defined nucleus the DNA ( Negatively charged) is organized in large loop held by certain positively charged proteins in a region called nucleoid.      

PACKAGING OF DNA IN EUKARYOTES

In Eukaryotes, the negatively charged DNA is wrapped around positively charged histone octamer into a structure called nucleosome.

1. A typical nucleosome consists of the negatively charged DNA wrapped around the positively charged histone octamer.
2. Histone monomer is a basic protein rich in basic amino acids – Lysines and Arginines. Both these amino acids carry positive charges in their side chains. 8 molecules of  Histone proteins(monomers H-2A,H-2B,H-3 and H-4 )  are organized to form Histone Octamer.
3. A negatively DNA helix consisting of 200 bp is wrapped around the positively charged Histone octamer  to form a structure called Nucleosome.
4. The wrapping of DNA around Histone  is further stabilized by the help of H1 histone.  
5. Nucleosomes constitute the repeating unit of the repeating unit of the structure called chromatin. The nucleosomes in chromatin are seen as ‘beads – on – string’
6. The beads on string structure in chromatin is packaged to form chromatin fibres that are further coiled and condensed at metaphase stage of cell division to form chromosomes.
7. The packaging of chromatin at higher level requires additional set of proteins that collectively referred to as Non- histone Chromosomal (NHC) Proteins.

         DIAGRAMS SHOWING ORGANISATION OF HISTONE MONOMERS AND DNA TO FORM NUCLEOSOME,     CHROMATIN AND CHROMOSOME 

        

Sl. No.	EUCHROMATIN	HETEROCHROMATIN
1	In a typical nucleus , some region of chromatin are loosely packed and stains lightly , are referred to as Euchromatin.	The chromatin that is more densely packed and stains dark are called as Heterochromatin.
2	Euchromatin is said to be transcriptionally active chromatin.	Heterochromatin is transcriptionally inactive chromatin.

   

DNA – The Wonder Molecule

The  DNA :-

• Ø  DNA is a long polymer of Deoxyribonucleotides.
• Ø  The length of DNA is usually defined as number of nucleotides  or base pairs (bp) present in it.
• Ø  The length of DNA (No. of base pairs ) is a unique characteristic of an organism.

Example :-        i) Bacteriophse  - ΦX174 has 5386 nucleotides / base pairs (bp)

                        ii) Bacteriophase – Lambda (λ) has 48502 bp

                        iii) Escherechia coli  has 4.6 X 10⁶ bp

                        iv) Haploid content of human DNA is 3.3 X 10⁹ bp

     STRUCTURE OF A POLYNUCLEOTIDE CHAIN

    Two types of Polynucleotide chains :- DNA &  RNA

   DNA : Deoxyribo Nucleic Acid ( Sugar is – Deoxyribose ; two  – H is present at 2’position of  Carbon)

   RNA : Ribo Nucleic Acid( Sugar is – Ribose ; one –H and one –OH group is present at 2’ position of        

              Carbon

Ø  Polynucleotide = Polymer of Nucleotide

Ø  Nucleotide = Nitrogenous Base +  Pentose Sugar (Deoxyribose / Ribose ) +  Phosphate Group

Ø  Nucleoside = Nitrogenous Base +  Pentose Sugar (Deoxyribose / Ribose )

Ø  Nitrogenous Bases : Two Types  PURINES   &  PYRIMIDINES

Ø  PURINES : Adenine (A ) and  Guanine ( G )

Ø  PYRIMIDINES : Thymine ( T ) , Cytosine ( C )  and  Uracil ( U  - present only in RNA )

Ø  Four Nitrogenous Bases :  A , T , G, & C are present in DNA

Ø  Four Nitrogenous Bases  : A , U , G  & C  are present in RNA

CHEMICAL BONDS IN A NUCLEOTIDE

Ø   A nitrogenous base is linked to a pentose sugar through a N-glycosidic linkage to form a nucleotide

Ø  A phosphate group is linked to 5’-OH of a nucleoside through Phosphoester linkage

Ø   Two nucleotides are linked through 3’- 5’ phosphodiester linkage to form a polynucleotide.Many nucleotides are joined together through phosphodiester linkages to form polynucleotides chain.

NITROGENOUS  BASE	PENTOSE SUGAR	NUCLEOSIDE	PHOSPHATE  GROUP	NUCLEOTIDE
ADENINE	DEOXYRIBOSE	DEOXYADENOSINE	PHOSPHATE	DEOXYADENOSINE MONOPHOSPHATE
GUANINE	DEOXYRIBOSE	DEOXYGUANOSINE	PHOSPHATE	DEOXYGUANOSINE MONOPHOSPHATE
THYMINE	DEOXYRIBOSE	DEOXYTHYMIDINE	PHOSPHATE	DEOXYTHYMIDINE MONOPHOSPHATE
CYTOSINE	DEOXYRIBOSE	DEOXYCYTIDINE	PHOSPHATE	DEOXYCYTIDINE MONOPHOSPHATE

SOME FACTS WHICH HELPED IN DISCOVERING THE DOUBLE HELICAL STRUCTURE OF DNA

1. Friedrich Meischer (1869) first identified DNA as an acidic substance present in the nucleus . He named it as nuclein.
2. Maurice Wilkins and Rosalind Franklin (   )  gave important data by  X-ray diffraction of DNA .
3. Erwin Chargaff proposed that  the ratios between Adenine and Thymine = 1  and

                                                            the ratios between Guanine and Cytosine = 1

Based on these facts James Watson and Francis Crick (1953) proposed famous Doble Helix Model for the structure of DNA.

SALIENT FEATURES OF THE DOUBLE – HELIX STRUCTURE OF DNA

It is made of two polynucleotide chains, where the backbone is constituted by sugar – phosphate and the bases project inside.

1.     
The two chains have antiparallel polarity . It means , if one chain has the polarity 

       5’ - 3’, the other has 3’  -  5’

2.      The bases in two strands are paired through hydrogen bond ( H-bonds) forming base pairs (bp).

Adenine is bonded with Thymine through a double bond; Guanine is bonded with Cytisine through a triple bond. Always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix.

3.      The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm and there are roughly 10 bp in each turn. Consequently the distance between abp in a helix is approximately equal to 0.34 nm.

4.      The plane of one base pair stacks over the other in a  double helix. This in addition to H-bonds , confers stability of the helical structure.

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   Stem cell technology is a rapidly developing field that combines the efforts of cell biologists, geneticists, and clinicians and offers hope of effective treatment for a variety of malignant and non-malignant diseases. So this topic is integrated with Class XII syllabus. Let's have few glimpses of the same......

STEM CELL TECHNOLOGY

Definition of Stem Cells:

•      A cell that has the ability to continuously divide and  differentiate (develop) into various other kind(s) of cells/tissues

•      These are Unspecialized and Undifferentiated cells.

FACTS

•  The body is made up of different kinds of specialised cells such as muscle cells, nerve cells, fat cells and skin cells.

•  All cells in the body come from stem cells.

•  A stem cell  is a cell that is not yet specialised.

•  The process of specialisation is called differentiation.

•  Once the differentiation pathway of a stem cell has been decided, it can no longer become another type of cell on its own.

In mammals, there are two broad types of stem cells: 

1. Embryonic stem cells, which are isolated from the inner cell mass of blastocysts, and 

2.   Adult stem cells, which are found in various tissues.

•      In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing adult tissues. In a developing embryo, stem cells can differentiate into all the specialized cells—ectoderm, endoderm and mesoderm but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.

There are three known accessible sources of autologous adult stem cells in humans:

1. Bone marrow, which requires extraction by harvesting, that    is, drilling into bone (typically the femur or iliac crest).

2. Adipose tissue (fat cells), which requires extraction by liposuction.

3. Blood, which requires extraction through apheresis, wherein blood is drawn from the donor (similar to a blood donation), and passed through a machine that extracts the stem cells and returns other portions of the blood to the donor.

·        Stem cells can also be taken from umbilical cord blood just after birth. Of all stem cell types, autologous harvesting involves the least risk.

·        By definition, autologous cells are obtained from one's own body, just as one may bank his or her own blood for elective surgical procedures.

•      Adult stem cells are frequently used in various medical therapies (e.g., bone marrow transplantation). Stem cells can now be artificially grown and transformed (differentiated) into specialized cell types with characteristics consistent with cells of various tissues such as muscles or nerves.

•      Embryonic cell lines and autologous embryonic stem cells generated  through somatic cell nuclear transfer or dedifferentiation have also been proposed as promising candidates for future therapies.

Embryonic stem cells

•      Totipotent: (toti = total ; potent = capacity)

•      The state of the cells after the first few divisions of the fertilised egg.

•      Have the capacity to become all cell types  plus placenta.

•      Pluripotent:  (pluri = many ; potent = capacity)

•      At blastocyst stage cells become   pluripotent.

•      Have the capacity to become all cell types but not placenta.

•      Cells of most interest to research scientists.

Adult stem cells

•  Multipotent.

•  Can develop into cells that are closely related.

•  Limited number of several cell types.

•  Make all cell types from the tissue they come from.

•  Found in many parts of the body.

•  Can self-renew over a lifetime.

Tissue stem cells

•  Often known as adult stem cells

•  Also includes stem cells isolated from foetal and cord blood

•  Reside in most tissues of the body where they are involved  in repair and replacement

•  Generally very difficult to isolate

•  Already used to treat patients (haematological malignancies,  diseases of the immune system)

Embryonic stem cells

•    Derived from  embryos

•    Can be grown indefinitely in the laboratory in an unspecialized state.

•    Retain ability to specialize into many different tissue type known as  Pluripotent.

•  Can restore function in animal models following transplantation.

Induced pluripotent stem cell

•  Derived from adult cells

•  Can be grown indefinitely in culture in an undifferentiated  state

•  Similar properties to  embryonic stem cells as can  differentiate into many  different tissue types –  pluripotent

Pros and Cons to induced pluripotent stem cell technology

Prons

    Cells would be genetically identical to patient or donor of     skin cells (no immune rejection!)

     Do not need to use an embryo

Cons:

          Cells would still have genetic defects

         One of the pluripotency genes is a cancer gene

        Viruses might insert genes in places we don’t want them causing mutations)

Science is discovering the unknown

•      Stem cell field is still in its infancy

•      Human embryonic stem cell research is a decade  old, adult stem cell research has 30-year head start

•       Holds hope for curing or improving treatments for  70+ diseases

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