Page 22, paragraph 3, line 10 should read, "...no human chromosomes are telocentric."
I. The cellular basis of life.
A. All living forms are composed of cells.
B. All cells arise from pre-existing cells, not by spontaneous generation.
C. All cells are bounded by a cell membrane (plasma membrane).
II. All organisms can be classified into either of two types.
A. Eukaryotes (all higher organisms, yeast, fungi, etc.) are composed of cells that have nuclei (s. nucleus) bounded by a double membrane.
1. A nucleus is an example of a cell organelle, i.e. a structure within a cell.
2. The nucleus stains readily, the substance that stains being known as chromatin.
3. Nuclei often have more heavily stained bodies called nucleoli (s. nucleolus).
4. The part of the cell within the cell membrane but excluding the nucleus is called cytoplasm.
B. Prokaryotes (bacteria, some algae) do not have nuclei.
C. Viruses are not considered to be living organisms, since they do not have a cellular structure and can grow and reproduce only inside cells of organisms.
III. Some attributes of cells and cell organelles.
A. Cells occur in a wide range of sizes, even within a single organism.
B. Mammals have several hundred different types of cells, all of which descended from a single cell (a fertilized egg or zygote).
C. The plasma membrane has an important role in regulating the passage of substances to and from the cytoplasm.
1. The plasma membrane consists of a double layer of molecules that are hydrophilic at one end and hydrophobic at the other. In an aqueous medium, the hydrophobic ends attract each other and form the interior of the membrane.
2. Imbedded within the membrane are numerous protein complexes that regulate passage through the membrane.
D. Mitochondria are structures with double membranes that use oxygen to burn certain cell components to produce energy. The energy is in the form of a very reactive chemical structure called adenosine triphosphate (ATP), which serves as the universal energy currency in all organisms.
E. The endoplasmic reticulum is a membraneous structure that fills most of the cytoplasm. It is the site of protein synthesis.
F. Ribosomes are small structures often found in association with the endoplasmic reticulum. They are important in protein synthesis.
G. Lysosomes are single membrane structures that are important in the breakdown of substances imported into the cell as well as substances made in the cell.
H. The cytoskeleton is a network of needle-like structures in the cytoplasm that help maintain cell form.
I. Golgi complexes (Golgi bodies) are important in processing proteins and in cell secretions.
IV. Chromosomes are the thread-like structures in the nucleus that are the primary repository of genetic information.
A. Each species has a characteristic number of chromosomes.
B. With rare exceptions, every cell in an individual has the same chromosome complement.
C. In many organisms, including humans, there is one pair of sex chromosomes (XX in females and XY in males). The remaining chromosomes also occur in pairs and are called autosomes.
D. Members of a pair of homologous chromosomes have the same array of genes.
E. A complete set of unpaired chromosomes is a haploid set, symbolized by n. In fertilization, haploid cells from each parent fuse to form a zygote that is diploid.
F. In humans, the diploid (2n) number of chromosomes is 46 (22 pairs of autosomes and one pair of sex chromosomes).
G. Each chromosome pair can be distinguished from other chromosomes by the position of the centromere and by the banding patterns produced by certain staining procedures.
1. The centromere is important in cell division and is the last part of a chromosome to divide.
2. In metacentric chromosomes, the centromere is in the middle.
3. In acrocentric chromosomes, the centromere is near one end.
4. In telocentric chromosomes, the centromere is at one end. No human chromosomes are telocentric.
5. A centromere divides the chromosome into two arms, the shorter being labeled p and the longer q.