Cell Cycle and Chromosomes

  Maximum growth takes place in G1 phase.

Interphase occurs just prior to cell division. Prepubertal male primordial germ cells are reactivated from interphase at puberty and enter rounds of mitosis followed by meiosis 1 and 2 to produce unique haploid sperm.

In the absence of a Y chromosome, the fetus develops ovaries and female phenotypes. Hence, SRY switches on the male sex determination.

Ans - 2

30,000genes

30,000 genes

The primary oocyte completes its first meiotic division within the secondary follicle (also known as the Graafian follicle) just before ovulation. The primary follicle contains only the primary oocyte arrested in prophase I.

Lampbrush chromosomes are a special type of chromosomes found in amphibian oocytes. They are found during the prolonged diplotene stage of first meiotic division in primary oocytes of amphibians. 

Turner’s syndrome is a human adnormality in which the affected individuals are pheotypically females but have rudimentary sex organs and mammary glands is caused by the absence of one X-chromosome. Turner’s syndrome is characterised by monosomy of XO type. 

In Klinefelter's syndrome individual has 47 chromosomes (44+ XXY). Such persons are sterile males with undeveloped testes, sparse body hair, mental retardation, long limbs and some female characters.

On the basis of morphology, 23 chromosomes in human beings are arranged in 7 groups.Tijo and Levan in 1956 demonstrated that human karyotype consists of 23 pairs (total 46) chromosomes. 

The theory where ratio between the number of X-chromosomes and number of complete sets of autosomes will determine the sex is known as genic balance theory of sex determination. The X-chromosome is believed to carry female tendency genes, while autosomes carry male tendency genes.

The condition of chromosomal change which involve the loss or gain of single chromosome is called aneuploidy. Genotype 22 pairs + XXY males shows the condition of Klinefelter's syndrome. Klinefelter's syndrome was studied in 1942 by an American Dr. H.F. Klinefelter. The patient of this syndrome have an extra X chromosome, hence the genotype is XXY instead of normal XY and patient has 47 chromosomes instead of 46. 

The carrier of heredietary metarial is gene. It is a functional unit of heredietary material. The term gene was proposed in 1909 for the first time by Johannsen. Gene's is also the smallest unit that could undergo a mutational change or could take part in recombination. 

In recent past human chromosome have been studied by a techinque using specific, often flouroscent dyes, known as karyotyping technique. The term karyotype is given to the group of characteristics that identifies a particular chromosome set and is usually represented by a diagram called idiogram.