Frank Brothers Solutions for ICSE Class 10 Biology Chapter 2 Structure Of Chromosomes

Chromatin is an extremely thin, long nuclear fibre occurring in the nucleus before cell division. Think of it like thin threads of wool tangled up inside a ball. It contains all the genetic information needed to make a new cell.

In interphase, the chromatin appears as a network of long, extremely thin fibres. Think of it like very fine threads spread out inside the cell. These fibres are so thin that they look like a tangled web under the microscope.

During prophase of mitosis, the chromatin fibres shorten and become thick to form chromosomes. Think of it like loose thread being twisted into a tight rope. During metaphase, the chromosomes appear more distinct and clear and each consists of two parallel strands called chromatids joined by a centromere.

Chromosomes are the thread like structures or chromatin material present inside the nucleus. Think of them like tiny strings that carry all the instructions for making you who you are. Chromosomes are so called because they take up certain basic dye and stain very rapidly. The word chromosome comes from two words "chromos" meaning color and "soma" meaning body.

(i) Strasburger – He observed thread like structures during cell division.(ii) Balbiani – Described rod like structures in nucleus before cell division. Think of these structures like tiny strings inside each cell that carry all the information about how you look and grow. (iii) Waldeyer – Coined the term 'chromosomes'.(iv) Sutton and Boveri – They described chromosomes as physical structures and transmitters of hereditary traits.

Chromosomes are the thread like structures present in the nucleus of the cell. They are covered with a sheath made up of proteins and filled with granular matter called matrix. Inside the matrix, there are two threads called chromonemata. Think of it like a rope that has two smaller threads twisted together inside. The chromosome consists of two symmetrical strands called chromatids. Each chromosome consists of a distinct constriction called centromere which gets attached to the spindle network.

(i) Sheath – It is a proteinaeous covering present around the chromosomes.(ii) Matrix – It is a granular matter present inside the sheath.(iii) Chromonemata – They are the subunits of chromatids. Think of chromonemata like the threads in a rope that make up the whole rope.(iv) Centromere – A constriction in the chromosome is called centromere.(v) Secondary Constriction – A constriction other than primary constriction is called a secondary constriction.(vi) Telomere – The ends of the chromosomes is termed as telomere.

Chromosomes are best observed at metaphase. During this stage, the chromosomes line up neatly in the middle of the cell like students standing in a straight line during assembly.

The chromosomes other than sex chromosomes present in the human body are called autosomes. These autosomes carry genes for traits like height, hair color, and blood type. 22 pairs of autosomes are present in man.

The chromosomes which determine the sex of an individual are called sex chromosomes. In humans, girls have XX chromosomes and boys have XY chromosomes. These special chromosomes decide whether a baby will be born as a boy or girl.

Sex chromosomes are significant as they determine the sex of an individual. In humans, girls have XX chromosomes and boys have XY chromosomes. This is why some families have more boys or more girls - it depends on which chromosome the father passes on.

In man, there is a pair of sex chromosomes. In males, it is X and Y while in female, the sexchromosomes are X and X which are identical to each other. Think of it like this - boys get one chromosome from mom (X) and one from dad (Y), while girls get X from both parents.

Chromosomes are the main source of chemical information which determines that the cell should become like its parent cell. This is why you look like your parents - you got chromosomes from both of them. During the developmental stage they also determine that the cells of the organism will give the animal or the plant, the characteristic features of its species.

DNA and RNA are the two nucleic acids. DNA is like a recipe book that tells your body how to make you look like your parents. Frederick Miescher discovered DNA

Nucleic acids are made up of three types of molecules: a pentose sugar, nitrogenous bases and a phosphate. Think of these three parts like the ingredients in your favorite laddu - you need flour, sugar, and ghee to make it complete. Each part has a special job in making the nucleic acid work properly.

The chemical composition of chromosome consists of: Approximately 40% of DNA 50% of Histone proteins. Think of DNA as the instruction book and histone proteins as the packaging material that keeps it organized. 8.5% Non histone proteins Metallic ions like magnesium and calcium in traces.

Two nitrogenous bases present in DNA are: Purines – Adenine and Guanine. Think of these bases like the letters in our alphabet that make words. Pyrimidines – Cytosine and Thymine.

DNA (Deoxyribonucleic acid) is a double-stranded molecule that stores genetic information in the nucleus of cells, while RNA (Ribonucleic acid) is usually single-stranded and helps in protein synthesis. Think of DNA like a master recipe book that stays safe in the kitchen, while RNA is like a photocopy of one recipe that goes to the cooking area. DNA contains the sugar deoxyribose and the bases A, T, G, C, whereas RNA contains ribose sugar and A, U, G, C bases. DNA is found mainly in the nucleus and is responsible for heredity, while RNA is found in both nucleus and cytoplasm and carries out various functions like protein synthesis and gene regulation.

Features of DNA: DNA consists of two strands which are spirally arranged around an axis. This is called a double helical arrangement. Each DNA strand is made up of nitrogenous bases, pentose sugar and phosphate. Sugars are pentose type; bases are adenine, guanine, cytosine and thymine. Think of DNA like a twisted ladder where the sides are made of sugar and phosphate, and the steps are the bases. Both strands are connected with hydrogen bonds. There are two bonds between adenine and thymine and three bonds between guanine and cytosine. In the DNA, bases are inside and sugars are outside and two sugars are connected with phosphoric acid. Each DNA strand replicates and from each replicated DNA a new DNA is formed. This is called replication of DNA.

Watson and Crick described the detailed structure of DNA. They found that DNA looks like a twisted ladder with two sides. This discovery happened in 1953 and changed how we understand life itself.

DNA is most important because it is the hereditary material. It acts as the director of protein synthesis. Think of DNA like a recipe book that tells your body how to make everything from your eye color to your height. Some DNA of chromosomes forms nucleolus. DNA in presence of enzymes forms mRNA which acts as messenger.

The rungs of "DNA ladder" are made up of two types of nitrogenous bases: Purines: Adenine and Guanine. Think of these bases like puzzle pieces that only fit with their perfect partner. Pyrimidines: Cytosine and Thymine.

Repeating components of each DNA strand length wise are pentose sugar and phosphate group. Think of it like a chain where the same two parts keep repeating over and over again. These two components form the backbone of DNA that holds everything together.

(i) Interphase (ii) histone proteins (iii) RNA, DNA. Think of DNA like a recipe book in your kitchen - it contains all the instructions needed to make different dishes. Just like how you need the right ingredients to follow a recipe, cells need DNA to make proteins. (iv) Protein (v) Nitrogen

(i) False (ii) False - DNA chains are like two ribbons twisted together but running in opposite directions, not side by side. (iii) True (iv) True

(i) This diagram represents the structure of chromosome.(ii) 'd' is centromere which gets attached to the spindle fibres.(iii) 'b' is heterochromatin which is darkly stained region when stained with acetocarmine and 'c' is euchromatin which when stained with acetocarmine or felugen gets lightly stained. Think of it like a dark blue shirt and a light blue shirt - both are blue but one appears darker than the other under the same light.(iv) Waldeyer(v) Matrix(vi) No, secondary constriction or 'e' is not present in all chromosomes.