Chromosomes

 




Chromosomes:

Chromosomes are typically  present in the nucleus and which become visible as thick thread like structures during cell division. These are not visible in active nucleus due to their high water content. Most of the chromosomes in a cell are called autosomes. In addition there are one or two of sex chromosomes or heterosomes which carry genes for determining  of sex.Chromosomes are thread like structure found in the nucleus as chromatin material. It becomes visible during cell division. Each chromosome  is made up of two chromatids joined together at a point centromere. Bead like structure found on chromosome is called gene. Genes are made up of DNA ( deoxyribo nucleic acid) which are the carrier of genetic information  from generation to generation. In some viruses  RNA  is the genetic material called retrovirus. In prokaryotes there is only one chromosome, like bacteria and virus.

☆ Chromosomes are made up of DNA  and protein.

The term chromosome was suggested by Waldeyer ( 1888). W. Fleming ( 1879) described the splitting chromosomes and coined the term chromatin for stainable material of the nucleus. Chromosomes are capable of self_ reproduction and play a role in heredity, mutation, variation and evolutionary  development  of the species.

The chromosome number is constant for each species. It is known to be lowest in Ascaris megalocephala ( only two) and highest in the Eupagurus schotensis ( 254). In case of plant  the  Ophioglossum reticulatum ( 1260) have the highest number of chromosomes. 

Eukaryotic cell possess many chromosome. A particular kind of species have definite number of chromosome in their cell, which are in pair known as diploid. The set of unpaired chromosome is called haploid.  Gametes have haploid set of chromosome.

The size also varies from species  to species and remains constant for each species. The length  varies from 0.2 to 50 microns. The diameter varies  from 0.2_ 20 microns. The glant chromosomes  ( Lampbrush chromosome) is the largest chromosome.

What are the 4 types of chromosomes?

Types of chromosomes :



Each chromosome has a clear no _ stainable zone known as centromere  or kinetochore. The centromere divide the chromosomes into arms. The position of the centromere give different shapes to the chromosomes. 

(1) Telocentric:

These are rod shaped chromosomes with centromere at one end or terminal. Eg. Some species  of protozoa and mammals. 

 

2): Acrocentric:

These chromosomes are also rod_ shaped, but the centromere  is towards one end subterminal  and  so the chromosome has a short and a long arm. Eg. Grasshopper,  man ect.

 (3) Sub_ metacentric: 

These chromosomes are 'J' or 'L' shaped .The centromere is near the centre ( submedian) and so the two arms are unequal in length.


(4) Metacentric: 


These are ' V'shaped chromosomes .The centromere  is at the centre ( median) and so the two arms are equal in length.  Eg. Trillium, Tradescantia  and man.



Structure of chromosome: 

The chromosomes  are composed of chromonema and centromere. The chromonema is composed of two, four or more  strands according to the species.The strands of chromonema are coiled in two different ways: 

(a) Paranemic coiling: The strands can be separated easily. 

(b) Plectonemic coiling: The strands of chromonema are inter _ twined and they cannot be separated easily.  Besides these, chromosomes show major and minor coils during mitosis and meiosis. 


Structure of a typical chromosome 

Chromomeres:

The chromonema show alternate thick and regions. The thick_ bead like structures are called the chromomeres. They are formed due to super_ imposed coiling of the strands.

Primary constriction ( Centromere) and secondary  constriction ( Nucleolar organ_
niser):

The chromosome have a clear region containing a small granule. This is centromere or kinetochore. Its position is fixed for particular chromosome. The place at which the centromere is located is known as primary constriction. During nuclear division,  the microtubules of the spindle apparatus are attached only to the centromeres.

The chromosomes also possess  secondary  constriction too. This is referred  as Nucleolar organiser, because  they are concerned with the formation  of nucleolus. 

Satellite Chromosomes or Sat Chromosomes:

The part beyond nucleolar organiser is very short and appears like spheres or knob_ like ( satellite). Chromosomes bearing  satellites are called sat_ chromosomes. These are relatively DNA deficient regions. There are atleast two sat _ chromosomes in each diploid nucleus. 


Telomere

The extreme ends of the chromosomes are known as telomeres. It prevents the ends of the chromosomes from sticking together. These are specially  modified regions of chromosomes for attachment to nuclear envelope. 

Heterochromatin and Euchromatin:

During interphase and prophase certain chromosomal  segments appear denser and darker than the rest. The chromosomal  substance of the former is called Heterochromatin. While that of the rest is called euchromatin.  These two may stain differently but both consist of identical chromatin. In heterochromatin region of the  chromosome, chromatin is tightly coiled and condensed  whereas the euchromatin  chromatin is comparatively uncoiled  and swollen. Heterochromatin is inert in genetic and metabolic activities while euchromatin is active in these functions:


Model of Constitutive Heterochromatin in the meta phase chromosome of a mammal 

Karyotype and Idiogram:

A set of chromosomes of an individual or species is called karyotype . The karyotype of a species represented by a diagram called idiogram.


Functions

1): Chromosomes contain Genes which are vehicle of hereditary. 

2): Chromosomes control synthesis of structural proteins and thus help in cell division and cell growth.

3): They control cellular differentiation. 

4): Chromosomes control cell metabolism.

5): Chromosomes can replicate themselves or produce their carbon copies for passage to daughter cell and next generation. 

6): SAT chromosomes produce nucleoli for the synthesis  of ribosomes. 

7): Sporophytic and gametophytic characteristics are maintained by diploid and haploid set of chromosomes. 

8): chromosomes form a link between  the offspring and the parents. 

9): Sex is determined by sex chromosomes ( e.g X and Y and X or O).

10): Chromosomes introduce variations bu forming recombinats during crossing over.

11): Any change in the structure of genes  causes mutation. 

Functions of Nucleus:

i) The nucleus controls all the activities of the cell. 

ii): The chromosomes bear the genes which contain DNA and protein. The DNA of the nucleus controls the synthesis of all proteins. The genes control heredity and transmission of characters from one generation to another.

iii) Synthesis  of ribosomes occurs in the nucleus.

iv) Plasma and membranes of other cell organelles arise from nuclear membrane. 

v) Nucleus plays an important role in cell division. 


GIANT CHROMOSOMES:

(a) Polytene chromosomes ( salivary gland chromosomes)

(b)  Lampbrush chromosomes 

Polytene Chromosomes:

Balbiani (1881) first reported,  these chromosomes in the salivary glands of the Chironomous, hence called the salivary  gland chromosomes, the name Polytene was suggested by Kollar due to the occurrence of many chromonemata in them. These also occurs in trachea, fat cells and malpighian tubules of many insects. The enormous  size of the Polytene chromosomes is achieved by the duplication  of the chromonemata for 9 to 10 times and duplicated chromonemata are also separated.  The polytene chromosomes consists of closely coiled or associated  homologous pairs of the chromosomes,  this sort of associated is known as somatic pairing.

The polytene  chromosomes are the permanent  prophase chromosomes which contain 1000 times more DNA than the somatic chromosomes. The chromosomes bear alternating dark and light transverse bands along their length.  The polytene chromosomes develop swelling at particular points called Chromosomal puffs or Balbiani rings. The puffs represent regions where the tightly coiled chromosomal in a band. Puffs are active genes and represent sites of mRNA synthesis (  transcription).







Lampbrush Chromosomes:

These chromosomes were discovered by Ruckert in 1892. However , first description of Lampbrush chromosomes was given by Fleming ( 1882)on amphibian oocytes.  These present in oocytes of many vertebrates such as fishes, amphibians, reptiles and birds can be seen best during  diplotene stage of meiosis. These are larger than polytene chromosomes and are composed of a main axis and lateral loops. The axis consists  of two chromatids. The lateral loops are the lateral projections from the chromonema of the chromatids. There loops give the appearance like the brushes used to clean glass chimneys of kerosene  lamps, hence called lambrush chromosomes. 

The loops are rich in RNA and protein and are concerned with the synthesis of proteins and yolk.



Vacuoles  and Non_ living  Inclusions of the cell:

In the cytoplasm  of the cell there are spaces filled with a fluid ( cell sap) known as vacuoles.  These are lined by a membrane called the tonoplast.  The cell sap contains minerals , sugars,  amino acids, proteins,  water soluble pigments ( Anthocyanin), waste products etc. in solution or ad crystalline deposits.  The tonoplast is semipermeable membrane,  which enables the vacuole to segregate and store nutrients and waste products. It also helps in rapid exchange of solutes and gases between the cytoplasm and adjoining fluids.

Non_ living or Metabolically Inactive Materials:

There are collectively known as paraplasm, deutoplasm or ergiastic  substances. They are formed during the metabolism of the cell. The three types of non_ living  inclusions are: food substances, secretory and excretory substances. 

Food Substances:

These are present in the form of starch grains, fat droplets and aleurone grains containing  stored protein. In animal cells glycogen  is stored in the liver and muscle cells.


Secretory Substances:

These include colouring matters like anthocyanin, nectar secreted by nectaries, essential oils etc.


Excretory Substances: These are alkaloids, tannin, resins, gums, latex, minerals salts formed as byproduct of  metabolism . Mineral crystals are usually in the form of calcium salts.

(a) Raphides In the petiles  of Colocasia, calcium crystals occur in the form of fine needle _ like  structures. 

(b) Cytoliths: In the leaves of Ficus elastica calcium crystals are formed in the form of a bunch of grapes.


Number of chromosomes in Different Organisms:


Pigeon     80              Dog   78  Horse    64

Chimpanzee  48    potato   48  Human    46 

Rabbit          44      Wheat    42   Cat       38 

Frog            26       Tomato  24     Pea  14 

House fl    12         Mosquito 6    Ascaris   2

HOW many chromosomes do human have?


Humans typically have 46 chromosomes in their cells. These chromosomes are organized into 23 pairs, with one member of each pair coming from each parent. This includes 22 pairs of autosomes, which determine the majority of an individual's traits, and one pair of sex chromosomes. Males have one X and one Y sex chromosome, while females have two X chromosomes.



What are chromosomes made of?

Chromosomes are made of a combination of DNA (deoxyribonucleic acid) and proteins. DNA is a long, double-stranded molecule that carries the genetic information necessary for the development and functioning of all living organisms. It consists of four nucleotide bases: adenine (A), thymine (T), guanine (G), and cytosine (C), which are arranged in specific sequences.

In the case of chromosomes, DNA is tightly coiled and condensed, allowing it to be packaged into a compact structure. Proteins called histones help organize and compact the DNA by forming a complex called chromatin. The chromatin further condenses and forms visible chromosomes during cell division.

Additionally, chromosomes also contain other proteins and molecules that play roles in DNA replication, gene expression, and the regulation of various cellular processes. These components work together to ensure the proper functioning of the genetic material and the transmission of genetic information from one generation to the next.

How many chromosomes does a female  have?

A typical human female has 46 chromosomes in her cells. These chromosomes exist in 23 pairs, with one member of each pair coming from each parent. Of the 23 pairs, 22 are autosomes and one pair is the sex chromosomes. In females, both sex chromosomes are X chromosomes, denoted as XX.



Apologies for the confusion. A typical human female has 46 chromosomes in her cells. These chromosomes exist in 23 pairs, with one member of each pair coming from each parent. Of the 23 pairs, 22 are autosomes and one pair is the sex chromosomes. In females, the sex chromosomes consist of two X chromosomes, denoted as XX.


 





















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