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INTRODUCTION

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The human genome is comprised of nuclear DNA sequences, tightly packed into distinct subunits called chromosomes and hundreds to thousands of circular DNA molecules within the mitochondrion. DNA sequences that encode for proteins account for few percent of the total genome, and the rest of the genome is involved in coding for various RNA molecules that do not code for protein (noncoding RNA) as well as regulatory function. DNA packed into chromosomes is coated with histone and nonhistone proteins, and these proteins play an important role in the regulation of gene expression. There are a total of 22 autosomal chromosomes (1–22) and the sex-determining X and Y chromosomes (Figure 2-1). The gametes, eggs and sperm, each contain a haploid set of 23 different chromosomes that upon fertilization gives rise to the diploid set, 46 chromosomes. In a normal diploid human cell, 23 chromosome pairs are present: 22 pairs of autosomes and two sex chromosomes—XX in females and XY in males. In each individual, each member of a pair is derived from either the father or the mother.

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FIGURE 2-1.

A human normal male karyotype. Homologous chromosomes (homologues), the two chromosomes in a pair of autosomes, are composed of similar (but not identical) DNA sequences. Each homologue encodes the same set of genes in the same order, but may contain different variant form of the same gene (allele), as well as variable noncoding DNA (introns). Centromeres are indicated by arrows, separating the short and long arms.

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Chromosomes must be replicated at high fidelity and separated into their daughter cells with each cell division. Centromeres and telomeres are important specialized DNA structures on chromosomes. DNA replication during the interphase stage of mitosis produces two copies of a chromosome (sister chromatids) that are connected at the centromere (Figures 2-1 and 2-2). The kinetochore, a complex of proteins that interact with the centromere, serves as the attachment point for the spindle fibers during cell division and enable separation of sister chromatids into individual cells. Chromosome fragments that lack a centromere (acentric fragments) do not become attached to the spindle, and fail to be included in the nuclei of the daughter cells. Telomeres are located at the end of each chromosome, are composed of DNA and protein complexes and serve as caps to maintain chromosome integrity and to prevent chromosomes from fusing and from degradation. The telomere consists of a simple 5′-TTAGGG-3′ sequence that is repeated thousands of times. With each round of DNA replication in a somatic cell, telomeres are shortened and finally lost, providing a mechanism for natural aging and death.1

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FIGURE 2-2.

The cell cycle. For most cells, interphase accounts for about 90% of the cell cycle in which the cell grows, matures, and carries out its life function. Interphase has three stages: G1, S, and G2. During ...

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