Chromosome Structure and Biology - Class XI

During cell division, spindle fibers attach to primary constriction of the chromosomes. Spindle fibres are mainly composed of microtubules which contract and relax to align the chromosomes at a particular orientation.

Bead-like thickened portions of leptotene chromosomes is chromomeres. During the leptotene stage, those duplicated chromosomes each consisting of two sister chromatids condense from diffuse chromatin into long, thin strands that are more visible within the cytoplasm.

A chromosome is composed of DNA and proteins. DNA are the hereditary units which mainly carries the information from one generation to the other. The functional segments of the DNA are known as genes. Proteins are mainly present as histones which form a complex structure known as a nucleosome.

Chromosomes in humans can be divided into two types, autosomes, and sex chromosome. The sex chromosomes carry the characters linked to a particular trait. The autosomes contain the rest of the genetic hereditary information. Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 per cell.

• Chromosomes are thread-like structures located inside the nucleus of animal and plant cells.
•  Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.

Chromosomes are composed of DNA and proteins. DNA molecules contain the information necessary for constructing and organizing the cells. Functional segments of DNA are called as genes. In a cell, which is not dividing, this DNA is present as part of a chromatin material. Whenever the cell is about to divide, the chromatin material gets organized into chromosomes and transferred  to the next generation.

The nucleus of a cell contains chromosomes, which carry genetic information in a long molecule called as DNA. Genes are regions of DNA that carry the code to control a particular cell activity or the production of a particular protein.

Sex determination method is well understood in animals but not in plants. Plants may be monoecious or dioecious but the mechanism of sex differentiation was not well understood. It now appears that at least higher or flowering plants may be having sex determination mechanism based on sex chromosomes like animals, as humans which have XX and XY system of sex determination.

Let's take Human somatic cell as an example for the typical eukaryotic cell.

Centromere is a region of chromosomes which holds sister chromatids together and the area where chromosome are attached to spindle fibres during cell division.

At first time Tizo and Levan in 1956 enabled the human geneticists to as certain the correct chromosome number (46 pairs).

The chromosomes are thread-like structures which can be seen enclosed in a cell nucleus. These chromosomes contain genes which are responsible for the transfer of characteristics from parents to the children.

The chromosomes are the condensed chromatin. They are made up of DNA and proteins Those sequences of DNA that code for proteins are called the genes. 

The chemical composition of a chromosome is DNA and proteins(histone). Because chromosome is a supercoiled structure made up by coiling of DNA on the proteins like histone

The single set of chromosome consists of half the total number of chromosomes, also called a haploid set. Here, a total of 156 chromosomes is there so one set will have 78 chromosomes.

Chromosomes contain the genetic material that is conserved and modified over evolutionary time. By comparing chromosome structures and gene sequences, scientists can determine evolutionary relationships (phylogeny) and classify organisms (taxonomy).

Walther Flemming is credited with the discovery of chromatin and the observation of mitosis. He proposed that chromosomes were the carriers of heritable elements during cell division.

August Weismann proposed the germ plasm theory and hypothesized that the number of chromosomes is constant for individuals of the same species, which is a fundamental concept in genetics.

Certain chromosomes possess a secondary constriction at a constant specific location, which is non-staining and gives the appearance of a small chromosomal fragment separated from the main body. This distinct morphological feature is known as a satellite, and chromosomes possessing it are called satellite chromosomes.

Not all chromosomes have a secondary constriction; only specific chromosomes (SAT-chromosomes) possess them. The other statements correctly describe chromatin, DNA length, and composition.

Root tip cells are diploid (2n), so 2n = 42, which means the haploid number n = 21. Aleurone cells in endosperm are triploid (3n) because they are formed by triple fusion. Therefore, the number of chromosomes in aleurone cells is 3 * 21 = 63.

Chromosomes are made up of DNA and protein. Histone and non-histones proteins are associated with chromosomal DNA. Histone participate as structural components and provide strength and stability and protect DNA from shearing and breakage while the non-histones act as functional or regulatory components either in activation or repression of genes.

Chromosomes were discovered by Hofmeister (1848), studied by Strasburger (1875) and given the present name by Waldeyer (1888) after their staining by dyes like Janus Green. Chromosomes are a thread like structure. It is composed of DNA and protein and it is considered as a carrier of genetic characters from parents to offspring.

Chromosomes are composed of DNA, RNA, histones, non-histones. A chromosome is a packaged and organized chromatin, a complex of macromolecules found in cells, consisting of DNA and protein. The functional segments of DNA are known as genes. The DNA-bound macromolecules are proteins, which serve to package the DNA and control its functions.

The telomeres do not have any overhanging ends of nucleotides so it is known as non sticky chromosome ends called telomere. A telomere is a region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. 

Chromonemata are embedded in the matrix. Chromonemata are the spirally coiled central filament of a chromatid along which the chromomeres are aligned. A chromomere is one of the serially aligned beads or granules of a eukaryotic chromosome, resulting from local coiling of a continuous DNA thread. It is visible on a chromosome during the prophase of meiosis and mitosis. 

A chromomere  is one of the serially aligned beads or granules of a eukaryotic chromosome, resulting from local coiling of a continuous DNA thread. It is visible on a chromosome during the prophase of meiosis and mitosis. The chromomeres are present during leptotene phase of prophase I during meiosis. During zygotene phase of prophase I, the chromomeres of homologs align with each other to form a homologous rough pairing. These chromomeres help provide a unique identity for each homologous pairs.

All chromosomes mentioned are large chromosomes. These chromosomes are producing large amounts of RNA for the oocyte, and most of the genes present in the DNA loops are being actively expressed. Each lateral loop contains one or several transcription units with polarized RNP-matrix coating the DNA axis of the loop.The majority of the DNA, however, is not in loops but remains highly condensed in the chromomeres on the axis, where genes are generally not expressed. This is about lampbrush chromosomes, the polytene and mega chromosomes are also larger than the other normal chromosomes.

Deoxyribonucleic acid (DNA) is the information-carrying molecule found in all living organisms. In most animal, plant and fungal cells, DNA is stored in the nucleus coiled up in thread-like structures called chromosomes.

Chromosomes are thread like structures located inside the nucleus of animal and plant cells. Each chromosome is made of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.

B chromosomes are called supernumerary or accessory chromosomes. It is because these chromosomes are not considered as essential for being normal. These are not found in every individual. During alloploids, it suppresses homologous pairing which reduces multiple pairing between homologous chromosomes in allopolyploids.  

• A telocentric chromosome is a chromosome in which the centromere is located at the terminal end of the chromosome and their shape is similar to the letter i.
  
• These chromosomes have a repetitive nucleotide sequence at each end of chromosomes. 
• It seals and protects each end of the chromosomes from deterioration or fusion with the neighbouring chromosomes during cell division.
• So, the correct answer is 'Seal ends of chromosomes'.

• The centromere is the lighter staining region on the chromosome of a cell which appears as a constriction.
  
• They are also known as a primary constriction or kinetochore.
• It is a specialized DNA sequence that links a pair of sister chromatids where the spindle fibres are attached during cell division.
• It is responsible for the transfer of replicated chromosomes into the two daughter cells during the cell division.
  
• So, the correct answer is 'Centromere'

• Satellite chromosomes or SAT chromosomes are those chromosomes which have secondary constrictions other than a centromere.
• The terminal end of the chromosome present beyond the secondary constriction is called satellite which is attached to the chromosome by a thin thread called chromatin.
  
• So, the correct answer is 'Terminal part of the chromosome beyond the secondary constriction'.
  

• The centromere is the lighter staining region present on the chromosome of a cell which appears as a constriction.
  
• They are also known as a primary constriction or kinetochore.
• It is a specialized DNA sequence that links a pair of sister chromatids where the spindle fibres are attached during cell division.
• So, the correct answer is 'Centromere'.

A duplicated chromosome consists of two identical sister chromatids joined at the centromere.

• The centromere is the lighter staining region on the chromosome of a cell which appears as a constriction.
  
• They are also known as a primary constriction or kinetochore.
• The kinetochore is a proteinaceous structure that assembles at the surface of the centromere which forms the site for binding of spindle fibres during cell division.
  
• So, the correct answer is 'Surface of centromere'.

The term "chromosome" (comes from the Greek word "chroma" meaning colour and "soma" meaning "body", describing their strong staining by particular dyes) was first suggested in 1888 by the German anatomist Heinrich Wilhelm Gottfried von Waldeyer-Hartz.

Chromosome theory of inheritance or Boveri–Sutton chromosome theory is a fundamental unifying theory of genetics which identifies chromosomes as the carriers of genetic material

1) Chicken has a diploid number of 78 (2n = 78).

Wilhelm Friedrich Benedikt Hofmeister observed chromosomes in a dividing cell nucleus in 1848.

In eukaryotic chromatin, the mass of histone proteins is roughly equal to the mass of DNA, making the protein content approximately the same as the DNA content.

A pericentric inversion involves a chromosome segment that undergoes a 180-degree reversal and includes the centromere, thus affecting both arms of the chromosome. In contrast, paracentric inversions occur entirely within a single arm and exclude the centromere.

Lampbrush chromosomes are giant chromosomes found in the oocytes of certain vertebrates. They are characterized by extensive loops that are sites of very high transcriptional activity (RNA and protein synthesis), meaning they are not compactly packed with high inactive DNA content as claimed by the incorrect statement.

In standard cytogenetic chromosome nomenclature, a band designation such as 6p23 is read as chromosome 6, short arm (p), region 2, band 3. Thus, the digit 3 represents the band number in region 2 of the short arm.

Intercalary deficiency involves deletion (4), Duplication involves change in gene order/dosage (3), Paracentric inversion leads to bridge/fragment (2), and Translocation involves ring formation (1).

Chromatin fibers are made from coiling of a long polynucleotide strand of a DNA to fit into the cell. Chromatids are made up of chromatin. The two sister chromatids are present in a chromosome.

The chromosomes are the threadlike structures of nucleic acids found in the nucleus of most of the living organisms, carrying the genetic material in the form of genes. Thus DNA is concentrated in the chromosomes.

• Prophase is the third stage of the mitosis where the chromosomes which were lined up at the metaphase plate in the metaphase are to be pulled towards the pole of the dividing cells.
• The centrioles which are present at the pole of the dividing cells produce spindle fibers which are will help pull the chromosomes towards the pole .
• These spindle fiber will attach to the kinetochore which is present with the centromere in the middle of the chromosome,
• The kinetochore also produces microtubles which will attach will the incoming spindle fibers forming a bond so that they can be pulled towards the pole.
• After reaching the pole the fibers will distangle from the kinetochore and disappear leaving the chromosomes to decondense and formation of the nuclear membrane around them .
• Therefore the answer option 'kinetochore microtubules' is correct.
  

• The kinetochore is the region which is present in the center of the chromosome near the centromere or the primary constriction.
• The spindle fibers which are formed from the centrioles are required to pulled the chromosome towards the poles of the dividing daughter cells.
• The spindle fiber of the centrioles are attached to the kinetochore which is a protein ring or made up of protein which forms bonds with tubulin protein of the spindle fiber, so that the chromosomes can be pulled apart towards the poles. 
• Therefore the answer option 'kinetochores' is correct.
  

Enzymes are present inside cells for catalyzing chemical reactions. While genetic material is present inside the nucleus as storage of information for all the cellular activities in the form of genes. Genetic material replicates during cell division. It can also undergo mutation during replication. 

Chromosomes are heavily coiled DNA, a DNA molecule is negatively charged and has to be therefore stained with basic dyes like Basic fuchsin, Methylene green and carmine.while safranin is used to stain cell walls of plants and bacteria (counter stain)

There is a single, long, double-stranded, linear DNA molecule in a chromosome. The DNA contains biological and genetic information. It is highly coiled and folded, however this packing varies during the cell cycle.

The nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

An inactivated X-chromosomes : female have XX chromosomes and male have XY chromosomes ,for balance Xlinked gene between male and female,one X chromosome undergo inactivated during the female embryo development this is termed as X chromosomes inactivation

(a) Basic Fuchsin-  It is a green crystalline dye. It includes staining of tissues sections.

Chromosome condensation is the dramatic re-organization of the long thin chromatin strands into the compact short chromosomes that occur in mitosis and meiosis. Here, the unfolding of proteins takes place, the DNA coils, dehydration takes place. However, there is no addition of water molecule or hydration that takes place during condensation.

Centromere provides structure to the cell. The sister chromatids are linked together at the centromere. During mitosis, spindle fibers attach to the centromere via kinetochore. Their physical role is to act as the site of assembly of the kinetochore, a highly complex multiprotein structure that is responsible for the actual events of chromosome segregation, i.e., binding microtubules and send signals to the cell cycle machinery.

Chromosomes are the carrier of genetic information in form of DNA. The secondary constrictions are used as a point of identification of chromosomes. These are always constant in their positions and hence, can be used as markers that identify particular chromosomes. A satellite chromosome or SAT chromosome has a chromosome segment that is separated from the main body of the chromosome by such a secondary constriction.

Chromosomes are the genetic material present in the nucleus of a cell. It is composed of DNA and protein. The functional segment of DNA is known as a gene. The size of chromosomes varies from as small as 0.1$\mu$m to as big as 15$\mu$m in length.

The largest normal metaphasic chromosome has the size of 30nm with the base of the nucleosome core particle consists of the 146 base pairs of DNA. During metaphasic state, when a chromosome in its most condensed state can range in size between 130,000 base pairs in the endosymbiotic bacteria, then the chromatin can be further condensed by coiling into 30nm in fibres.
So, the correct answer is option D.

• Stains are used in the microscopic studies of different components and organelles within a cell.

Chromonema is spirally coiled central filament that extends the entire length of a chromosome and on which the genes are located. During mitotic prophase, the chromosome material appeared as thin filaments which are known as chromonema. This term is introduced by Vijdovsky in 1912. 

• A telomere is the end portion of a chromosome.
• It is a region of repetitive nucleotide sequences present at each end of a chromosome that helps the chromosome from deterioration or fusion with other chromosomes. 
  
• So, the correct answer is 'Telomere'.

Parascaris, first studied by Boveri (1887), has polycentric chromosomes, each of which consists of six segments, each segment having one centrally located centromere.
So, the correct answer is option C.

Every chromosome has a primary constriction or the centromere on the sides of which kinetochores are present. Based on the position of centromere, the chromosomes are of 4 types:

Q band is a typical pattern which is obtained by using Quinacrine mustard dye. Nucleus located in the centre of each cell has chromosomes coiled in the form of chromatin network. Chromosomes are gene carriers. They are made up of 2 chromatids, that are joined to each other through a centromere. 

The snaptinemal complex was first described by Montrose J. Moses and D. Fawcett in 1956. Moses discovered in primary spermatocyte of crayfish and Fawcett discovered in spermatocytes of pigeon, cat, and man etc. It is protein structure that forms between homologous chromosomes during meiosis and mediates chromosome pairing.
So the correct option is A

Anaphase is a stage where the chromosomes split from the middle and two chromatin get separate and both the chromatids move toward opposite poles due to a repulsive force which is known as an aphasic moment.