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Eukaryotes (cells with nuclei such as those found in plants, yeast, and animals) possess multiple large linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere, although, under most circumstances, these arms are not visible as such. In addition, most eukaryotes have a small circular mitochondrial genome, and some eukaryotes may have additional small circular or linear cytoplasmic chromosomes.


Normal members of a particular eukaryotic species all have the same number of nuclear chromosomes. Other eukaryotic chromosomes, i.e., mitochondrial and plasmid-like small chromosomes, are much more variable in number, and there may be thousands of copies per cell.

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Chromosomes are of two types on basis of their role :

1 : Autosomes :

An autosome is a chromosome that is not a sex chromosome - that is to say there are an equal number of copies of the chromosome in males and females.
For example, in humans, there are 22 pairs of autosomes.

2 : Sex Chromosomes :

They determine the sex of an organism .
They are different in males and female . In females, the sex chromosomes are the 2 X chromosomes. Males have one X chromosome and one Y chromosome.

Homologous and Non Homologous Chromosomes

Homologous Chromosomes:

The chromosomes that can pair during meiosis forming a sypapse and have the genes for the same traits at the same loci ( alleles may be homo or hetero) are called homologous chromosomes.These are similar in length (other than the sex chromosomes in certain taxa.The holomologous chromosomes have a pair of sister chromatids that synapse and crossing over occurs between interchangeable segments.

Non Homologous chromosomes:

In non homologous chromosomes ,each chromosome of the pair is obtainred from the each parent in diploids and contains all the genepool of that organism .In polyploids, there may be more than two pairs in a set forming polyploidy from a set .

There are four types of chromosomes based upon the position of the centromere.

1) Metacentric : In this type of chromosome the centromere occurs in the centre and all the four chromatids are of equal length.

2) Submetacentric : In this type of chromosome the centromere is a little away from the centre and therefore chromatids of one side are slightly longer than the other side.

3) Acrocentric : In this type of chromosome the centromere is located closer to one end of chromatid therefore the chromatids on opposite side are very long. A small round structure, attached by a very thin thread is observed on the side of shorter chromatid. The small round structure that is a part of the chromatid is termed as satellite. The thin strands at the satellite region are termed as Nucleolar Organiser Region.

4) Telocentric : In this type of chromosome the centromere is placed at one end of the chromatid and hence only one arm. Such telocentric chromosomes are not seen in human cells.

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Chromosomes and DNA
Chromosomes:
Definition
The thread like structure present inside the nucleus bearer of hereditary character in the form of genes is called Chromosomes.

Discovery
Chromosomes were discovered by a German Biologist Walther Flaming in 1882. He placed the larva of salamander in a chemical Perkins Aniline, then he observed that the chromosomes become darker than other organelles of a cell.

Number of Chromosomes
The number of chromosomes varies from species to species.
For Example
 Penicillin => 2 Chromosomes
 Mosquito => 6 Chromosomes
 Drosophila => 8 Chromosomes
 Pea => 14 Chromosomes
 Frog => 26 Chromosomes
 Human => 46 Chromosomes
 Sugar Cane => 80 Chromosomes
 Fern => 100 Chromosomes

Structure of Chromosome
Chromosomes can only be seen when the cells are divide. A chromosome is made up of
1. Chromatid
2. Centiomere

1. Chromatids
Each chromosome consists of two very fine thread like structure called Chromatids.

Chromonema
Each chromatids of a chromosomes consists of one or more fine thread called Chromonema.
Chromomere
Chromonema contain deeper staining regions along their length, giving the threads like appearance of strings of beads. These regions are called Chromomeres.

Types of Chromotids
There are two types of chromatids.
i. Sister Chromatids
2. Non-Sister Chromatids

i. Sister Chromatids
The two chromatids of same chromosome are called Sister Chromatids.
ii. Non-Sister Chromatids
The chromatids of different chromosomes are called Non-Sister Chromatids.

2. Centromere
The region or point in which the chromatids are linker together is called Centromere.

Kinetochore
Centeomere is small spherical zone on the chromosome, within centromere a disc shape protein structure called Kinetochore is present to which the spindle fibres attached during cell division.

Composition of Chromosome
Chromosome are complex structure made up of Deoxyribonuleo protein. This protein composed of
1. DNA
2. Protein
3. RNA

1. DNA
Chromosome have 30-40% DNA. DNA is made up of billion of units called Nucleotide and the nucleotide made up of three components.
 Phosphate group
 Deoxyribsoe sugar
 Nitrogenous base
There are two groups of nitrogenous bases.
i. Purines
ii. Pyrimidines

i. Purines
Nitrogenous base that have double ringed structure called Purines.
Example
 Adenine (A)
 Guanine (G)

ii. Pyrimidines
Nitrogenous structure that have single ringed structure called Pyrimidines.
Example
 Cytosine (C)
 Thymine (T)

2. Protein
Chromosome have 50-65% protein. The most abundant chromosomal protein is called Histones. It is a basic protein.

3. RNA
Chromosome have very less amount of RNA about 1-10%

Ultra Structure of Chromosome
Eukaryotic chromosomes are composed of chromatin, a complex of DNA and protein and less amount of RNA.

Deoxyribonucleic Acid (DNA)
The DNA of a chromosome is one very long double stranded fiber, duplex which extend unbroken through the entire length of chromosome.
A typical human chromosome contain about 140 million nucleotide in its DNA.

Size
If a strand of DNA from a single chromosome were laid out in a straight line, it would more than 7 feet long (2 meter). It is too much long to fit into a cell. In the cell, however, the DNA is coiled, thus fitting into a much smaller space.

Nucleosome
DNA resemble a string of beads. Even 200 nucleotides, the DNA duplex is coiled around a core eight histone proteins forming a complex known as Nucleosome.

Protein
Histones are positively charged proteins due to an abundance of the basic aminoacids, arginine and lysine. They are thus strongly attached to the negatively charged phosphate groups of the DNA.
Further coiling occurs when the string of nucleosomes wraps up into higher order coils called Super Coils.

Heterochromatin
Highly condensed portions of the chromatin are called Heterochromatin.
Euchromatin
The reminder of the chromosome called Euchromatin is condensed only during the cell division.

Karyotype
The particular array of chromosome which an individual possess is called the Karyotype in which chromosomes are paired by matching bonding and arranged by size and shape.

Applications
 Karyotypes show marked differences among species and sometime even among individuals of the some species.
 It help in determining many issue as the culprits of difference offences and settle the disputed paternity.
 It help in detecting genetic abnormalities, arising from extra or lost chromosome.

Types of Chromosome on the Bases of Centromere
There are four types of chromosome according to the position of centromere are as follows.
i. Telocentric Chromosome
ii. Acrocentric Chromosome
iii. Sub-Metacentric Chromosome
iv. Metacentric Chromosome

i. Telocentric Chromosome
When the centromere is placed at the extreme end of the chromsome, the chromosome is called Telocentric Chromosome.

ii. Acrocentric Chromosome
When the centromere is placed near the end of the chromosome, the chromosome is called Acrocentric Chromosome.

iii. Sub-Metacentric Chromosome
Chromosome with unequal arms (chromatids) that resembling the "j" shape are called Sub-meta Centric chromosome.

iv. Meta Centric Chromosome
Chromosome with equal arms and centromere in center is called a Metacentric Chromosome.
There are three more types of chromosomes
i. Homologous Chromosome
ii. Autosomes
iii. Sex Chromosomes

i. Homologous Chromosomes
Those chromosomes which are morphologically similar with same set of genes in which one chromosome comes from male and another from female is called as Homologous Chromosome.

ii. Autosomes
The somatic paired chromosome which contain the genes of various characters except reproductive organs are called Autosomes.
These are same in male and female both.

iii. Sex Chromosome
Those chromosomes in a cell by which the sex of organism can be determined are called Sex-Chromosome.
There are different types of distribution of Sex-Chromosome.
i. XY type distribution
ii. ZW type distribution

i. XY Type Distribution
Females have a pair of similar sex chromosomes called X but males have a mismatch set and X and another chromosome called a Y, so the male XY and the female is XX.

ii. ZW Type Distribution
In some fishes, moths and birds, the male have similar chromosome called Z and the female has different so the male is ZZ and the female is ZW.

Chromosomes as Carrier of Genes
Genes are small bodies found in the chromosomes.
Chromosomes are considered as the carrier of genes.
 The chromosomes can be separately identified visually but the genes are very small units. And so far have not been seen even with the best microscope.
 The chromosomes and gene behave as hereditary units but the genes can not be considered outside the chromosome.
 At the time meiosis, the separation of homologous chromosomes takes place which result in the segregation of gene pairs.
 In the genotype of every individual one member of each pair of genes is contributed by one parent and the other by the other parent.

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i need proper notes for the comparative study of zoology paper 3