Chromosome, chromatid, chromatin reticulam, DNA molecule, gene often these terms create confusions. But to understand, cytology or genetics, we need to have a good concept about these terms. Otherwise we may mix up with these terms while learning different process like mitosis, meiosis, cell cycle etc. in this post, let us try to understand these terms and concepts behind these terms with clarity.
First and foremost, Chromosome, chromatid, chromatin reticulam, DNA molecule, gene All these terms denote the same thing the genetic material DNA with its frame work proteins called histones.
Now the story of nucleus, as you know nucleus is the site where DNA the genetic material resides. DNA is seen associated with proteins called histones. This nucleoprotein complex is called chromatin. The condensation of chromatin differ in differ in different stages of cell cycle. We use different terms to designate the same thing at different stages of cell cycle.
The lowest level of chromatin organisation is called nucleosome.
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Chromatin reticulam or chromonemata refers to network of thin chromatin filaments observed during prophase.
Chromosomes are highly condensed chromatin visible during metaphase.
In eukaryotes, there are two setas of chromosmes, one inherited from father and other from mother.
Each chromosome in one set has a corresponding pair in other set, forming a homologous pair.
Within our cells, we have 23 homologous pairs thus 46 chromosomes
The two chromosomes in this pair are almost alike in structure and size and each carry carry genetic information for sma set of characters (except sex chromosome).
For example if a gene on a particular chromosome encodes for a character like eye colour, then the gene on the other chromosome also codes for the same trait. We call the term allele. Alternate forms of a gene. These alleles or genes in the homologous spar need not be identical. One may produce ble eye colour or other may be black. Thus each organisms carries two sets of genetic information we call the condition diploid.
Now we need to resoleve one more thing, the terms like chromosome chromatid, sister chromatid etc during mitosis or meiosis.
A functional chromosome consists of a centromere, a pair of telomeres ( stable chromosome ends) and an origin of replication
Chromosome has 1 chromatid or DNA molecule before S phase or DNA synthesis phases after S phase chromosome has 2 chromatids. Sister chromatids are copies of a chromosome held together at the centromere. Functional chromosomes contain centromeres, telomeres and origin of replication. The kinetochore is the point of attachment for the spindle microtubules; telomeres are the stabilizing ends of chromosome; origin of replication are sites where DNA synthesis begins.
Chromosome consists of very long DNA molecule that is packaged by condensation.
Gene is a segment of DNA molecule coding for a functional protein, ribozyme or RNAs. The number of genes in each chromosome may vary. According to HGP, we have 20000 genes distributed in 46 chromosomes.
Chromatin and Chromosomes
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| Chromatin and Chromosomes |
Chromatin
- It is uncondensed part of nucleoprotein complex.
- Chromatin is observable in the interphase nucleus.
- It is active in controlling metabolism and other activities of the cell.
- Chromosomes are condensed parts of the nucleoprotein complex.
- Chromosomes are observable during M-phase or nuclear division.
- Chromosomes are mainly meant for distribution of genetic informations to the daughter cells.