lecture6

Chromosome structure, chemical composition, nucleosome, centromere, telomere, euchromatin, heterochromatin, NOR, satellite chromosome, karyotype, ideogram

    Chromosomes are the darkly stained bodies seen during the metaphase stage of mitosis. Strasburger discovered chromosomes in 1875 and the team chromosome was coined by Waldeyer in 1888. Chromosomes are composed of thin chromatin threads called chromonemata. These chromonemata undergo coiling & super coiling during prophase and it become readily observable by the light microscope. The main features of eukaryotic chromosomes are given below:

• Chromosomes are clearly visible during mitotic metaphase. Hence, they are studies during metaphase.

• Chromosomes bear genes in a linear fashion and thus are concerned with transmission of characters, from generation to generation.

• Chromosomes of eukaryotes are enclosed by a nuclear membrane.

• Chromosomes vary in shape, size and number in different species of plants and animals.

• Chromosomes have property of self-duplication, segregation and mutation.

• Chromosomes are composed of DNA, RNA and histones. DNA is the major genetic constituent of chromosome.

Chromosome shape:

     Chromosome shape is usually observed during anaphase. The shape of chromosomes is determined by the position of centromere, a part of chromosome on which spindle fibres are attached during metaphase. Chromosomes have generally three different shapes, viz, rod shape, J shape and V shape. These shapes are observed when the centromere occupies terminal, sub-terminal and median(middle) position on the chromosomes, respectively.

Chromosome size:

     Chromosome size is measured at mitotic metaphase. It is measure in two ways viz., in length and diameter. Plants usually have longer chromosomes than animals, the maximum length of chromosome is observed during interphase and minimum during anaphase. Chromosome size varies from species to species. Maize chromosomes have the length of 8-12µ. Giant chromosomes have length up to 300µ.

Chromosome number:

• Each species has definite and constant somatic and gametic chromosome number. Somatic chromosome is the number of chromosomes found in somatic cells and it is represented as diploid number (2n). The somatic cells contain two copies of each chromosome (expect sex chromosome) one of which is identical in morphology, gene content and gene order and they are known as homologous chromosomes, Gametic cells or gametes contain one half of the somatic chromosome number which is represented by haploid number (n).

• The genetic chromosome number of a true diploid species is called basic number. It is the minimum haploid chromosome number of any species, which is denoted by x. For example, in wheat, the basic number is 7, whereas the haploid number is 7,14 and 21 for diploid, tetraploids and hexaploidy species, respectively. Thus, haploid chromosome number differs from basic number. Both are same in case of true diploid species but differ in case of poly ploidy species.

• Number of chromosomes varies from 2n=4 (n=2) in Haplopappus gracilis (Compositae) to 2n=> 12000 in some Pteridophytes. In Plant kingdom, chromosome number is higher in dicots than in monocots.

• Chromosomes divided into two transverse parts by its centromere called ‘Arms’. One arm of a chromosome is longer than other (long arm and short arm). Short arm is usually represented by letter “p” – comes form French “petit” meaning small and long arm denoted by “q” (follows p in the alphabet). All human chromosomes have 2 arms – p (short) arm and the q (long) arm.

Chemical composition of Chromosome:

• Major chemical components of the chromosome are DNA, RNA (nucleic acid) and proteins (histones and non-histones).

• DNA being about 35% along with histone protein (These are basic proteins having amino acids such as arginine and lysine) which is 55% thus forming deoxy-ribonucleoprotein comprising 90% of chromosome.

• The remaining 10% part is called residual chromosome and it contains RNA.

• The presence arginine and lysine amino acids make the histone protein, a positively charged particle. The DNA show negative charge due to phosphate.

Nucleosome:

• The basic structural unit of chromatin in eukaryotes, composed of eight histone molecules wrapped by a segment of DNA.

• Nucleosomes occur at intervals along a continuous strand of DNA.

• A polynucleotide part consisting of 146 nucleotide base pairs of the DNA takes two turns on the core of eight histone proteins forming a complex known as nucleosome.

• The negatively charged coil of polynucleotide is strongly attracted by the positively charged histone protein. The nucleosome appears as a beaded structure on the string under electron microscope.

• Two nucleosomes are joined by linker DNA consisting of up to 200 nucleotides. A typical human chromosome consists of about 140 million nucleotides in its DNA which are repeatedly arranged as beaded structures called nucleosomes on a string.

• Nucleosomes pack together and produce a thick chromatin Fiber called nucleosome Fiber. This nucleosome Fiber may supercoil to give rise to 300 Å chromatin Fiber. The supercoiled Fiber is called solenoid.

Chromosome structure:

      The structure of chromosome becomes easily visible during metaphase due to coiling of interphase chromosomes. Each chromosome consists of seven parts, viz.,

• Centromere

• Chromatids

• Secondary constriction & satellite

• Telomere

• Chromomere

• Chromonema

• Matrix

Centromere (Primary constriction):

   It is a localized region of the chromosome with which spindle fibres are attached during metaphase is known as centromeres or primary constriction or kinetochore. Centromere has four important functions, viz.,

• Orientation of chromosomes at metaphase

• Movement of chromosome during anaphase

• Formation of chromatids

• Chromosomes shape.

   Centromere may occupy various positions on the chromosome, viz., terminal, sub-terminal, median etc. Generally, each chromosome has one centromere, but in some cases, the number of centromeres may vary from nil to many. The centromere divides the chromosome in to two arms of varying length.

Chromatid:

   One of the two distinct longitudinal subunits of a chromosome is called chromatid. These subunits of a chromosome get separated during anaphase. Chromatids are of two types viz., sister chromatids and non-sister chromatids. Sister chromatids originate from homologous chromosomes. Chromatids are formed due to chromosome and DNA replication during interphase. Two chromatids of a chromosome are held together by centromere. After separation at anaphase each chromatid becomes a chromosome.

Secondary constriction:

   Some chromosome exhibits secondary constriction in addition to primary constriction. It may be present either in short or long arm of the chromosome.

Satellite chromosome:

    The chromosomal region between secondary constriction and nearest telomere (end of the chromosome) is called as satellite or trapant. The chromosome having satellite is known as satellite chromosomes. The position of secondary constriction in the chromosome is constant. The number of satellite chromosomes in a genome various from species to species.

Telomere:

   The two ends of the chromosomes are known as telomeres. Telomeres are highly stable and they do not fuse or unite with telomeres of other chromosomes. The structural integrity and individuality of the chromosome is maintained by telomeres.

Nuclear Organizer Region (NOR):

   During interphase, nucleolus of the cell is always associated with secondary construction of satellite chromosome. So, the secondary constriction is also called as NOR. The NOR contain several copies of gene coding for ribosomal RNA.

Chromomeres:

   The chromosome of some of the species show small bead like structures called as chromomeres. The distribution of chromomeres in the chromosome is constant. Available evidence indicated that chromomere represents a unit of DNA replication, chromosome coiling, RNA synthesis and RNA processing.

Chromonema:

   Under light microscope, thread like coiled structures is found in the chromosomes and chromatids which are called chromonema (plural chromonemata). Chromonema is considered to be associated with three main functions. It controls size of chromosomes, results in duplication of chromosomes and is the gene bearing portion of chromosomes.

Matrix:

   A mass of aromatic material in which chromonemata are embedded is called matrix.

Pellicle:

   Matrix is enclosed in a sheath which is known as pellicle. Both matrix and pellicle are non-genetic materials.

Euchromatin:

   The region of the chromosome that shows relatively uncoiled chromonema, which takes up little stain during interphase, is called Euchromatin. It is the active region of the chromosome, involved in transcription.

Heterochromatin:

   The region of the chromosome that shows highly condensed portions, which takes up deep stain during interphase and prophase, is called heterochromatin. It is classified into two types,

• Constitutive Heterochromatin – The regions centromere and telomere of the chromosome remain permanently in the heterochromatin stage. i.e., it does not revert to euchromatic stage.

• Facultative Heterochromatin – It is the region of the chromosome which undergo euchromatin stage.

Difference between euchromatin and heterochromatin:

Karyotype:

   Karyotype is a phenotypic appearance of chromosomes of a particular species. In the study of karyotype, various features of chromosomes are taken into account viz.,

• Number

• Position of centromere

• Size

• Possibility of satellite

• Degree and distribution of meter chromatin etc.

   It is represented by arranging the somatic chromosome complements according to the Somatic chromosome complements according to their length keeping their centromeres in a straight line. Thus, the longest chromosome is placed in the extreme left and smallest in the extreme right.

Idiogram:

   Diagrammatic representation of morphological feature of haploid chromosome complements of a species is known as idiogram.