Integrated SequencePhysics Chemistry Organic Biology

Web Resources



  click if a link is broken



Special points of emphasis

Proteins

Nucleic Acids

The Eukaryotic Cell

Gene Expression

The nucleus houses the chromosomes which contain the genes, which direct protein synthesis. Eukaryotic chromosomes are composed of chromatin, a composite of protein and DNA. Within each chromosome a single strand of DNA is tightly bound to proteins called histones. The basic central dogma of gene expression begins with the transcription of DNA into mRNA. In eukaryotes the mRNA moves from the nucleus into the cytoplasm where translation occurs either free in the cytoplasm or on the rough ER. A ribosome only binds to the rough ER once it begins to synthesize a protein marked with a signal sequence, an address tag, which destines for sorting within the endomembrane system. The membrane of the rough endoplasmic reticulum is continuous with the outer layer of the nuclear envelope.



Proteins

Biological Membranes

The Eukaryotic Cell

Gene Expression

Whether or not proteins used within the nucleus itself are synthesized on the rough ER is part of an advanced sub-set of questions that could find their way onto the MCAT. The use of signal sequences in protein targeting is an important topic. Although the outer nuclear membrane is continuous with the endoplasmic reticulum (in fact, following cell division, the ER gives rise to the new nuclear membrane), all proteins utilized within the nucleus are synthesized on free ribosomes in the cytosol. These proteins cross the nuclear membrane through nuclear pores. Short peptide signals, called nuclear localization signals, direct these proteins, such as DNA polymerases and RNA polymerases, into the nucleus. This is a similar mechanism to the signal sequences that direct proteins to be sorted, modified, and/or secreted by the cell to the rough ER. In other words, not all signal sequences lead to the rough ER.



The Eukaryotic Cell

Gene Expression

DNA Replication and Cellular Reproduction

Thought previously to be organelles, then nucleoli appear in the nucleus just after cell division. Nucleoli represent a high local concentration of the precursors of ribosomes, associated with certain chromosomal loci. Nucleoli disappear during late prophase of mitosis.







The WikiPremed MCAT Course is a free comprehensive course in the undergraduate level general sciences. Undergraduate level physics, chemistry, organic chemistry and biology are presented by this course as a unified whole within a spiraling curriculum.

Please read our policies on privacy and shipping & returns.  Contact Us.
MCAT is a registered trademark of the Association of American Medical Colleges, which does not endorse the WikiPremed Course.


Creative Commons License
The work of WikiPremed is published under a Creative Commons Attribution Share Alike 3.0 License. There are elements of work here, such as a subset of the images in the archive from WikiPedia, that originated as GNU General Public License works, so take care to follow the unique stipulations of that license in printed reproductions. You can use the resources here for commercial or non-commercial purposes, but please give attribution and a link to the production credits and edit history of the resource. For the works here which began as my individual work, please attribute "John Wetzel, an author at wikipremed.com".