Question

what is a locus on a chromosome? ​

Answer 1

Explanation:

A locus is the specific physical location of a gene or other DNA sequence on a chromosome, like a genetic street address.

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Answer 2

Answer:

Locus" is a term that we use to tell us where on a chromosome a specific gene is. So it's really the physical location of a gene or of a DNA polymorphism on a chromosome. And it's sort of like a street address for people. And one of the things that we think about when we're thinking about genes and chromosomes is we may think of the chromosome as a country, and then a region of a chromosome would maybe be the city, and then we'll get down to a very specific area, which is the locus, and that would be equivalent to, say, a person's street address. And that's the street address of that gene.

Explanation:

$\underline\bold{{LET'S \: EXPLORE \: MORE}}$

What are Chromosomes?

Chromosomes are thread-like structures present in the nucleus, which carries genetic information from one generation to another. They play a vital role in cell division, heredity, variation, mutation, repair and regeneration.

In Eukaryotic cells, genetic material is present in the nucleus in chromosomes, which is made up of highly organized DNA molecules with histone proteins supporting its structure.

Chromosome means ‘coloured body’, that refers to its staining ability by certain dyes.

Karl Nägeli in 1842, first observed the rod-like structure present in the nucleus of the plant cell.

W. Waldeyer in 1888 coined the term ‘chromosome’.

Walter Sutton and Theodor Boveri in 1902 suggested that chromosomes are the physical carrier of genes in the eukaryotic cells.

$\bold \pink{Chromosome Structure}$

Each cell has a pair of each kind of chromosome known as a homologous chromosome. Chromosomes are made up of chromatin, which contains a single molecule of DNA and associated proteins. Each chromosome contains hundreds and thousands of genes that can precisely code for several proteins in the cell. Structure of a chromosome can be best seen during cell division.

Main parts of chromosomes are:

$\blue{ \sf \:Chromatid: }$

• Each chromosome has two symmetrical structures called chromatids or sister chromatids which is visible in mitotic metaphase.

• Each chromatid contains a single DNA molecule

• At the anaphase of mitotic cell division, sister chromatids separate and migrate to opposite poles

$\blue{ \sf \:Centromere \: and \: kinetochore: }$

• Sister chromatids are joined by the centromere.

• Spindle fibres during cell division are attached at the centromere

• The number and position of the centromere differs in different chromosomes

• The centromere is called primary constriction

• Centromere divides the chromosome into two parts, the shorter arm is known as ‘p’ arm and the longer arm is known as ‘q’ arm.

• The centromere contains a disc-shaped kinetochore, which has specific DNA sequence with special proteins bound to them

• The kinetochore provides the centre for polymerisation of tubulin proteins and assembly of microtubules

$\blue{ \sf \: Secondary \: constriction \: and \: nucleolar \: organisers}$

• Other than centromere, chromosomes possess secondary constrictions.

• Secondary constrictions can be identified from centromere at anaphase because there is bending only at the centromere (primary constriction)

• Secondary constrictions, which contain genes to form nucleoli are known as the nucleolar organiser

$\sf \:Telomere:$

• Terminal part of a chromosome is known as a telomere.

• Telomeres are polar, which prevents the fusion of chromosomal segments

$\sf \: Satellite:$

• It is an elongated segment that is sometimes present on a chromosome at the secondary constriction.

• The chromosomes with satellite are known as sat-chromosome

$\sf \: Chromatin:$

• Chromosome is made up of chromatin. Chromatin is made up of DNA, RNA and proteins. At interphase, chromosomes are visible as thin chromatin fibres present in the nucleoplasm. During cell division, the chromatin fibres condense and chromosomes are visible with distinct features.

• The darkly stained, condensed region of chromatin is known as heterochromatin. It contains tightly packed DNA, which is genetically inactive

$\blue{ \sf \: Structural \: Organisation \: of \: Chromatin}$

• Chromatin consists of DNA and associated proteins. DNA is packaged in a highly organised manner in chromosomes

1. Nucleosomes are the basic unit of chromatin. It is 10 nm in the diameter

• DNA packing is facilitated by proteins called histones. DNA is wound around histone proteins to form a nucleosome

• There are 5 types of histone proteins in the eukaryotic chromosomes, namely H1, H2A, H2B, H3 and H4

• Nucleosomes prevent DNA from getting tangled

• 2.Linker DNA and the fifth histone (H1) pack adjacent nucleosomes to a 30 nm compact chromatin fibre

• 3.These fibres form a large coiled loop held together by non-histone proteins (actin, and tubulin, myosin) called scaffolding proteins to form extended chromatin which is 300 nm in diameter

• 4.Chromatin further condenses with the help of protein known as condensin, it binds to DNA and wraps it into coiled loops and we get the compacted chromosome