Chromosomes Genes GENOME The nucleus of a human

Chromosomes & Genes

GENOME The nucleus of a human cell contains between 30 000 and 40 000 genes. This complete set of genes is called the GENOME.

The structure of the genome

The structure of the genome • Tightly coiled threads of deoxyribonucleic acid (DNA) and associated protein molecules. • Organized into structures called chromosomes.

CHROMOSOMES

• Rod-shaped dark stained nucleoprotein bodies seen during mitosis metaphase. • The name chromosome was given by Waldeyer in 1888. • They can be clearly visible during cell division. • Composed of thin chromatin threads called chromonema.

What are CHROMOSOMES? • A packet of coiled up DNA. • Humans have 23 pairs of chromosomes. • They are in the nucleus of cells.

What are CHROMOSOMES? • Chromosomes contain a single, long piece of DNA. • A chromosome is about 0. 004 mm long. • The DNA is about 4 cm long. • This is about 10 000 times longer than the chromosome. – So it has to twist and coil to fit inside.

What are CHROMOSOMES? • There are 23 pairs of chromosomes in the nucleus of any one of your cells. • Each chromosome has a single strand of DNA (deoxyribonucleic acid) which carries the code for a couple of thousand genes.

CHROMOSOME NUMBER • Two main types: 1. Somatic chromosome number (2 n). It is the number of chromosome found in somatic or meristematic cell of a species. 2. Gametic chromosome number (n). It is one half of the somatic number. Haploid number.

CHROMOSOME SIZE • It shows variation depending upon the stage of the cell. • Longest & thinnest chromosome seen during interphase. • In prophase, decrease in their length with an increase in thickness. • Smallest chromosome seen during anaphase.

• Plants possess longer chromosome than animals. • Smallest chromosome is about 1/80, 000 in length.

CHROMOSOME MORPHOLOGY 1. Chromatid 2. Centromere 3. Telomere 4. Nucleolus organizer region 5. Chromomere

CHROMATID • Structural & functional unit of chromosome. • A chromatid contains 8 sets of the replicated DNA of each individual chromosome, which are joined by a centromere, for the process of cell division(mitosis or meiosis).

• A chromatid is "one-half of two [normally] identical copies of a replicated chromosome“. • The two copies may have slight differences due to mutations. • A chromatid is simply a copied chromosome which is paired with a (normally) identical chromosome at the centromere.

CENTROMERE • The centromere is the part of a chromosome that links sister chromatids. • Classified as: 1. Localized centromere 2. Neo centromere (False) 3. Non-localized centromere Ø Polycentric (Diffused having more than two centromeres) Ø Holocentric (Entire length acts as centromere)

• According to the position, centromere can be classified as: 1. Metacentric (V-shaped, middle) 2. Submetacentric (Arm’s length unequal) 3. Telocentric (Terminal) 4. Subtelocentric (Centromere located closer to its end than center) 5. Acrocentric (p arm is so short that

TELOMERE • Two ends of chromosome • Highly stable part • Made up of loops (300 A) chromatin • Maintenance of structural fibers & integrity

NOR • Also called secondary chromosome. • Nucleolus formed during telophse. • Chromosomal site of ribosomal RNA synthesis. • It is the region b/w secondary constriction & nearest to telomere called satellite. • Satellites are attached to short arm of nucleolus organizer.

CHROMOMERE • • Bead-like structure Formed in prophase of meiosis. Clearly visible They represent units of DNA replication or RNA synthesis. • One chromosome represent single gene

KARYOTYPE • A karyotype (Greek karyon = kernel, seed or nucleus) is the number and appearance of chromosomes in the nucleus of a eukaryotic cell. • The term is also used for the complete set of chromosomes in a species, or an individual organism.

• Karyotypes describe the number of chromosomes, and what they look like under a light microscope. • Attention is paid to their length, the position of the centromeres, banding pattern, any differences between the sex chromosomes, and any other physical characteristics.

• The study of whole sets of chromosomes is sometimes known as karyology. • The chromosomes are depicted (by rearranging a microphotograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size.

GENES

What is a GENE? • A code made up of pairs of bases carried on the DNA molecule. • Each DNA molecule contains many genes. • The basic physical and functional units of heredity.

What is a GENE? • A specific sequence of bases. – Sequences carry the information needed for constructing proteins. • Proteins provide the structural components of cells and tissues as well as enzymes for essential biochemical reactions. • The human genome is estimated to be made of more than 30, 000 genes.

What is a GENE? • Each chromosome carries a couple of thousand genes. • Many of these are common to all human beings. • So, 99. 9% of your DNA is identical to everyone else's.

What is a GENE? • The remaining 0. 1% influences the differences between us – height, hair color and susceptibility to a particular disease • Environmental factors, such as lifestyle (for example, smoking and nutrition) also influence the way we look and our susceptibility to disease

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