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chromosome and chromatids - Biology

Chromosomes

The thread-like structures located inside the nucleus of animal and plant cells are called chromosomes. They were first discovered by Hofmeister and the term was given by Waldeyer. Chromosomes comprise DNA and proteins which bind them. They can be rod-shaped or threadlike. They are deeply sustainable, condensed chromatin fibers, which act as hereditary vehicles as they store and transmit coded hereditary information. Chromosomes appear only during karyokinesis. Chromosomes are present in a fixed number across a species. They appear in a single set in gametophytic or haploid forms and two sets in sporophytic or diploid forms.

The name chromosome indicated that these are colorful bodies visible under a microscope, as chroma means color and soma means body. These strongly stained bodies were first visible to the scientists in the late 1880s. Thomas Hunt Morgan demonstrated the relation of the X chromosome with gender and eye colour in fruit flies in the 1900s. Histone proteins bind the large DNA molecule and condense it inside the unique structures called chromosomes. The large DNA molecule cannot fit inside the small nucleus of a cell, without such packaging.

Each chromosome has two chromatids attached by a narrow area called centromere or primary constriction. The two parts of a chromosome or chromatid on either side of primary constriction are called arms. The two arms are equal in isobrachial chromosomes and unequal in the heterobrachial chromosome. The ratio between two arms of a chromosome is called the centromeric ratio. Based on the position of the centromere, chromosomes are of four types:

  1. Telocentric: Centromere terminal
  2. Acrocentric: Centromere inner to telomere
  3. Submetacentric: Centromere sub-median
  4. Metacentric: Centromere median or in middle

Besides primary constriction or centromere, a chromosome has one or more secondary constrictions present adjacent to the distal part of the arm. The portion of the chromosome present above secondary constriction is called a satellite. The satellite is connected to secondary constriction through chromatin thread.

Chromatids

One-half of a replicated chromosome is called a chromatid. Chromosome copies and makes identical twins before cell divisions; these copies join at their centromeres. The joined strands are called sister chromatids and each strand is called a chromatid. These sister chromatids are genetically identical. They separate from each other during the anaphase of mitosis. At this stage, each separated chromosome is known as the daughter chromosome.

During meiosis and mitosis, chromatids are produced from chromatin fibers. The skeletal proteins and DNA constitute chromatids. It is also known as a nucleosome when wrapped around proteins in sequence. Chromatin fibers are tightly wound nucleosomes. Chromosomes are condensed chromatin fibers that are present inside the nucleus. During cell division, each chromatid replicates to form an X-shaped structure.

In mitosis, each chromosome splits up from the centromere and moves towards the pole. Each daughter cell receives a sister chromatid which is now called a daughter chromosome. This leads to the formation of two distinct but identical cells. In meiosis, homologous pairs of parental chromosomes line up at the metaphase plate. Each pair is called bivalent or tetrad which moves away from each other to form two daughter cells. Exchange of genetic material takes place when sister chromatids are in close proximity to each other. Such a condition is known as sister-chromatid exchange or SCE. A few or more parts of chromatids are broken and rebuilt during SCE.

Sometimes chromatids (in mitosis) or homologous chromosomes (in meiosis) are not able to separate properly during cell division, such a condition is known as nondisjunction. This situation may arise in the anaphase of mitosis. This results in unequal distribution of chromosomes in the daughter cells. It may cause fatal consequences like Down syndrome and Patau syndrome.

Difference between chromosome and chromatids

Chromosome Chromatid
Their primary function is the transfer of genetic material from parents to the offspring. Their primary function is to help the cell by duplicating during cell division.
Chromosomes can be found throughout the lifespan of a cell. Chromatids are visible only at the time of cell division in a cell.
Chromosomes are combined genetic material of both the parents. Each chromosome contains one copy of a gene derived from each parent. Sister chromatids are genetically identical to each other.
They are joined at the center by centromeres. Sister chromatids join at the center via centromere.
DNA present inside the chromosomes can be utilized for transcription and translation DNA material of the chromatids is not used for the synthesis of macromolecules.
It is present permanently in the cell. It is present in the cell for a short period.
Blood moves in a circular motion throughout the body. Lymph moves in a single direction only

 

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