[General Instructions: Students need to copy the subject matters and the provided exercise works in their notebooks for record] UNIT 1- BASIC BIOLOGY Structure of chromosomes, Cell Cycle and Cell Division DATE- 28-04-2020 1 WHAT ARE CHROMOSOMES? Chromosomes are thread -like structures in which DNA is tightly packaged within the nucleus. DNA is coiled around proteins called histones, which provide the structural support. Discovery of Chromosomes: ❑ The word chromosome comes as ‘chroma’ meant ‘colour’ and ‘soma’ meant ‘body’ , describing their strong staining by particular dyes. ❑ The term was coined by the German scientist Von Waldeyer-Hartz, referring to the term chromatin, which was itself introduced by Walther Flemming, who discovered cell division. ❑ The German scientists Matthias Jakob Schleiden,Rudolf Virchow and Otto Bütschli were among the first scientists who first recognized the structures now familiar as chromosomes. DATE- 28-04-2020 2 CELL DIVISION:- ❑ Introduction: One of the most fundamental characteristics of life which enables life to perpetuate generation after generation. ❑ Definition: Cell division is the division of a cell into two daughter cells with the same genetic material. ❑ Types of Cell division:- 1. Mitosis 2. Meiosis ❑ The cell cycle is composed of interphase (G₁, S, and G₂ phases), followed by the mitotic phase (mitosis and cytokinesis), and G₀ phase. ❑ Mitosis consists of four basic phases: Prophase, metaphase, anaphase, and telophase. Cytokinesis is the final physical cell division that follows telophase, and is therefore sometimes considered a sixth phase of mitosis. ❑ Meiosis:- • Cell division occurs twice during meiosis, one starting cell can produce four gametes (eggs or sperm). • In each round of division, cells go through four stages: prophase, metapha se, anaphase, and telophase. DATE- 28-04-2020 3 1. Pellicle and Matrix: A membrane which surrounds each chromosome is said as pellicle. A jelly substance present inside the membrane is called as matrix. Presumably the matrix and sheath are considered as non- genetic material. 2. Chromatids, Chromonema and Chromomeres: • A chromatid is one of two strands of a copied chromosome. • Chromatids are produced from chromatin fibers during both meiosis and mitosis. • Chromatin is composed of DNA and skeletal proteins and is called a nucleosome when wrapped around these proteins in sequence. • Chromonema is the spirally coiled central filament of a chromatid along which the chromomeres are aligned. • Chromomere is the small bead-shaped and heavily staining masses of coiled chromatin that are linearly arranged along the chromosome. DATE- 28-04-2020 4 4. Primary and Secondary Constriction: 3. Centromeres (= • Primary Constriction: Chromosomes can bend only at the site of primary constriction during anaphase. Primary constriction): The • Secondary constrictions can be found at constricted non-stained a chromosome in addition to the primary portion of chromosome is constriction at the centromere. known as centromere. it is • Secondary constrictions are useful in identifying a the specific part of the chromosome from a set. There are either 0, 1, 2, 3, chromosome where spindle or 4 secondary constriction sites in a cell. fibres are attached. the • The secondary constriction also contains position of centromere is the genes for rRNA synthesis . constant for a particular • Some parts of these constrictions indicate sites chromosome. of nucleolus formation and are called "nucleolar organizing regions" (NORs). 5. Satellite: 6. Telomere: • It the segment that is separated • A telomere is a region of from the main body of repetitive nucleotide sequences at the chromosome by such a each end of a chromosome, which protects the end of the chromosome secondary constriction. from deterioration or from fusion • The satellite at metaphase appears with neighboring chromosomes. to be attached to the rest • A protein complex known ofthe body of chromosomes by a as shelterin serves to protect the thread of chromatin. ends of telomeres from being recognised as double-strand breaks DATE- 28-04-2020 5 SHAPE OF A CHROMOSOME:- ❑Origin of p and q as chromosome arms: • The symbol "p" was chosen to designate the short arm because "p" stands for "petit", "small" in French. • The letter "q" was selected to signify the long arm merely because "q" is the next letter in the alphabet. • All human chromosomes have 2 arms -- a short arm and a long arm -- that are separated from each other only by the centromere, the point at which the chromosome is attached to the spindle during cell division. By international convention, the short arm is termed the "p arm" while the long arm of the chromosome is termed the "q arm." Depending upon position of centromere and comparative length of two arms of the chromosome, chromosomes may be of following types: • (i) Acrocentric: I-shaped, rod like chromosomes having one arm of very small size. Thus, arms are unequal. Position of centromere is sub-terminal. • (ii) Sub-metacentric: ‘L’ or J-shaped. Centromere is not present in the center of the chromosome. Arms are unequal. • (iii) Metacentric: V shaped. Centromere is present in the center of chromosome. Arms are equal in length. Types of chromosomes: Depending • (iv)Telocentric: I-shaped, single armed. Darlington upon the number of centromeres: (1939) defined telocentric chromosome as chromosome Monocentric: Chromosomes having only one centromere. having a terminal centromere. Dicentric: Chromosomes having two centromeres. Polycentric: Chromosomes having more than two centromeres. Acentric: When the centromere is absent in the chromosome. DATE- 28-04-2020 HUMAN KARYOTYPE: ❑ The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. ❑ Normal karyotypes for females contain two X chromosomes and are denoted 46,XX; males have both an X and a Y chromosome denoted 46,XY. ❑ AUTOSOMES Or Non Sex Chromosomes: ❑ The members of an autosome pair in a diploid cell have the same morphology. ❑ The DNA in autosomes is collectively known as atDNA or auDNA. ❑ ALLOSOMES Or Sex Chromosomes:- ❑ Allosomes are the chromosomes that contain the genes that determine biological sex. ❑ These chromosomes do create pairs, just as autosomes form homologous pairs. ❑ That is, the human X chromosome and the human Y chromosome become physically adjoined during meiosis, the process in which gametes (egg cells in females; sperm cells in males) are formed. DATE- 28-04-2020 8 ASSIGNMENT:- 1. Define the following terms: (a) Pellicle (b) Allosome (c) Centromere (d) Mitosis (e) Interphase 2. Label the diagrams given below: .