Plasma Membrane and Associated Structures 50
I. The structure of the cell
(plasma) membrane is described by a fluid-mosaic model.
A. The membrane is composed predominately of a double layer of phospholipids.
B. The membrane also contains proteins, most of which span its entire width.
C. Exocytosis, the reverse of endocytosis, is a process that allows the cell to secrete its products.
IV. Ribosomes are small protein factories composed of ribosomal RNA and protein arranged into two subunits.
V. The endoplasmic reticulum is a system of membranous tubules in the cell.
VI. The Golgi complex is a series of membranous sacs that receive products from the endoplasmic reticulum, modify those products, and release the products within vesicles.
II. Genetic expression occurs in two stages: transcription (RNA synthesis) and translation (protein synthesis).
A. The DNA in the nucleus is combined with proteins to form the threadlike material known as chromatin.
B. In chromatin, DNA is wound around regulatory proteins known as histones to form particles called nucleosomes.
C. Chromatin that is active in directing RNA synthesis is euchromatin; the highly condensed, inactive chromatin is heterochromatin.
IV. Active euchromatin directs the synthesis of RNA in a process called transcription.
A. The enzyme RNA polymerase causes separation of the two strands of DNA along the region of the DNA that constitutes a gene.
B. One of the two separated strands of DNA serves as a template for the production of RNA. This occurs by complementary base pairing between the DNA bases and ribonucleotide bases.
Protein Synthesis and Secretion 65
I. Messenger RNA leaves the nucleus and attaches to the ribosomes.
II. Each transfer RNA, with a specific base triplet in its anticodon, binds to a specific amino acid.
A. As the mRNA moves through the ribosomes, complementary base pairing between tRNA anticodons and mRNA codons occurs.
III. Proteins destined for secretion are produced in ribosomes located on the granular endoplasmic reticulum and enter the cisternae of this organelle.
IV. Secretory proteins move from the granular endoplasmic reticulum to the Golgi complex.
A. The Golgi complex modifies the proteins it contains, separates different proteins, and packages them in vesicles.
B. Secretory vesicles from the Golgi complex fuse with the plasma membrane and release their products by exocytosis.
DNA Synthesis and Cell Division 69
A. The strands of the original DNA molecule gradually separate along their entire length and, through complementary base pairing, form a new complementary strand.
B. In this way, each DNA molecule consists of one old and one new strand.
II. During the Gj phase of the cell cycle, the DNA directs the synthesis of RNA, and hence that of proteins.
III. During the S phase of the cycle, DNA directs the synthesis of new DNA and replicates itself.
Cell Structure and Genetic Control
IV. After a brief time gap (G2), the cell begins mitosis (the M stage of the cycle).
C. This results in the production of two daughter cells, each containing forty-six chromosomes, just like the parent cell.
V. Meiosis is a special type of cell division that results in the production of gametes in the gonads. A. The homologous chromosomes line up side by side, so that only one of each pair is pulled to each pole.
This results in the production of two daughter cells, each containing only twenty-three chromosomes, which are duplicated.
The duplicate chromatids are separated into two new daughter cells during the second meiotic cell division.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.