Skip to Main Content




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.


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.

Graphic Jump Location

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


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 ...

Want remote access to your institution's subscription?

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.


Create a Free MyAccess Profile

* Required Fields

Note: If you have registered for a MyAccess profile on any of the Access sites, you can use the same MyAccess login credentials across all sites.

Passwords must be between 6 and 40 characters long (no whitespace), cannot contain characters #, &, and must contain:
  • at least one lowercase letter
  • at least one uppercase letter
  • at least one digit

Benefits of a MyAccess Profile:

  • Remote access to the site off-campus on any device
  • Notification of new content via custom alerts
  • Bookmark your favorite content such as chapters, figures, tables, videos, cases and more
  • Save and download images to PowerPoint
  • Self-Assessment quizzes saved for quick review
  • Custom Curriculum access for both instructors and learners

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.