1 Packaging of chromosomal By: Nouf alyami

Contents I.Nucleic acids II.The humane genome III.Chromosomes IV.Genes V.Chromatin

I.Nucleic acids {deoxyribonunuclic acid (DNA) and ribonucleic acid (RNA)} consist of nucleotides which are composed of

A. nitrogenous bases 1. Purines Adenine (A) Guanine (G) 2. Pyrimidines Cytosine (C) Thymine (T) Uracil (U) B. Sugars Deoxyribose Ribose, which is found in RNA C. Phosphate

II.The human genome is all of the genetic information stored in the chromosomes of a human. It contains approximately 3*10 9 nucleotide pairs organized as 23 chromosomes.

II.Chromosomes contain: A.Specialized nucleotide sequences 1.The centromere is nucleotide sequence that binds to the mitotic spindle during cell division. 2.The telomere is a nucleotide sequence located at the end of the chromosome. It allows replication of full length of linear DNA (i.e., the DNA dose not shorten with each replication cycle) 3. replication origins are nucleotide sequence that act as origination sites, for replication. A human chromosome contains numerous replication origins.

B.Bands. bands contain genes and are identified with fluorescent dyes and Giemsa stain. 1.G bands are rich in A-T nucleotide pairs, they stain dark. 2.R bands are rich in G-C nucleotide pairs, they stain light.

telomeres

fluorescent dyes and Giemsa stain

IV.Genes. A gene is region of DNA that produces a functional RNA molecule. A.Gene size varies. For example: 1. the insulin gene has 1.7 *10 3 (approximately 1700) nucleotides. 2. the low density lipoprotein (LDL) receptor gene has 24 * 10 3 (approximately 45,000) nucleotides. 3. the dystrophin gene has 200 *10 3 (approximately 200,000,000) nucleotides. B.Gene regions 1.Noncoding regions (introns “interventing sequences”) make up majority of nucleotide sequences of gene. 2.Coding regions (exons ”expression sequences”) make up minority of the nucleotide sequences of gene.

V. Chromatin is a complex of histone and nonhistone protein that is bound to DNA. A.H2A, H2B, H3, and H4 histone protein 1.Histone protein are small and contain a high proportion of Iysine and arginine amino acids. Lysine and arginine give the a positive charge that enhances binding to negative charge DNA. 2.Histone protein bind to DNA regions that are rich in A-T ) nucleotide pairs. 3. histone protein bind to DNA as two full turns of DNA wind around a histone octamer (two each of H2A, H2B, H3, and H4 histone protein) forming a nucleosome (the fundamental unit of chromatin packing).

Introns and exons

chromatin

B.H1 histone protein joins nucleosomes to form a 30-nm fiber. C.Type of chromatin. Heterochromatin and euchromatin (active and inactive) are packed in the cell nucleus. 1.10% of the chromatin is highly condensed, transcriptioally inactive heterochromatin % of the chromatin is less condensed, 10% is transcriptioally active euchromatin and 90% is transcriptioally inactive euchromatin.

D. Degree of compaction 1.Human chromosome 1 contains approximately 260,000,000 base pairs. 2.The distance between each base pair is 0.34 nm. 3.The DNA in chromosome 1 is 88,000,000 nm or 88,000 µm long (260,000,000 * 0,34nm = 88,400,000 nm). 4. during metaphase, the chromosomes condense, and the 88,00 µm of DNA is reduced 10 µm, an 8800-fold compaction.

Heterochromatin and euchromatin euchromatin

Histone octomer