Biology Today — May 2017

• It controls the synthesis of RNAs (mRNA, rRNA, tRNA), enzymes and structural proteins.

• It directs cell differentiation, nucleus replication and cell division.

CHROMOSOMES

• They are rod-shaped or thread-like condensed chromatin fibres which appear only during
karyokinesis (late prophase and metaphase).

• Their number is fixed in all individuals of same species.

• Shape and size of chromosomes varies in individuals. The shape is more clearly visible in late
prophase and metaphase when primary constriction becomes distinct.

• Two chromatids are attached by centromere to form a single chromosome.

• Depending upon the position of centromere, chromosomes are of four types:

• Telocentric - Terminal centromere


• Acrocentric - Centromere is situated close to end


• Sub-metacentric - Centromere occurs slightly away from the middle of chromosome


• Metacentric - Centromere present in the middle of chromosome.

Fig.: Types of chromosome on the basis of position of centromere

Telocentric Acrocentric Sub-

metacentric

Metacentric

Satellite

Short Arm

Centromere

Long Arm

Secondary

constriction

Giant Chromosomes

Polytene chromosomes

•    First reported by E.G. Balbiani in 1881.

•    Commonly found in salivary glands of insects and thus, also

known as salivary chromosomes. Also present in antipodal

cells, endosperm cells, etc.

•    Length can be 2000 μm (1000 – 16000 times larger than

ordinary somatic chromosomes).

•    Formed by somatic paring between homologous chromosomes

and repeated replication of their chromonemata.

•    Conspicuous swellings on polytene chromsomes during

certain developmental stages are called puffs. Larger

swellings are called Balbiani rings.

•    In region of puffs or Balbiani rings, DNA strands uncoil,

become active and produce number of copies of messenger

or mRNA.

Fig.: Polytene chromosome showing puff

Puff KnobDark bands

Interbands

Lampbrush chromosomes

•    First seen by Flemming (1882) but described by Ruckert (1892).

•    Highly elongated special kind of diplotene chromosome bivalents.

•    Occur in diplotene stage of animal oocytes, spermatocytes, etc.

•    Total length may be up to 5900 μm or three times the aggregate length

of total polytene chromosomes.

•    Occur in pairs, consist of homologous chromosomes.

•    Chromomeres give out lateral projections or loops, which provide

lampbrush-like appearance to the chromosome pair. Loops are uncoiled

parts of chromomere with one to several transcriptional units. Loops are

region for rapid transcription of mRNA meant for synthesis of yolk and

other substances required for growth and development of meiocytes.

Matrix

(Protein +RNA)

Matrix

(Protein +RNA)

Chromomere

pair

Chromosome

Filament (DNA) axis

Main axis of chromosome

Loops

Fig.: Lampbrush chromosome

Scaffold

Long arm

Short arm

Two identical

chromatids

Centromere

Chromatin loops

(chromonema)

Fig.: Structure of chromosome