Cells are the basic living units in the bodies of living things on this earth. Each cell is a marvel of structural miniaturization. The tiny organelles within cells accomplish the activities called "life processes" through a division of labor: each organelle performs functions different from other organelles. Taken all together, they get the tasks of living accomplished. The life processes, or functions, are based on the precise structure of the organelle that carries out that function. So structure permits function, and function generates the energy and metabolic products (building blocks) necessary for building cellular structure. We see that structure and function (anatomy and physiology) always go hand-in-hand and that cells, the fundamental units of life, demonstrate this structure-function relationship. As cells associate to build tissues, and as groups of tissues form organs, and so on through organ systems to organisms, this structure-function relationship holds true.
You studied many organelles in Lab2, Exercise 2. Today the nucleus occupies our attention. Usually deep within the cell, the nucleus is bounded by a double-layered nuclear membrane. The chromosomes lie within the nucleus. These thread-like structures, most visible when a cell is dividing, are composed of DNA and protein. The nucleus also contains one or more deeply staining structures called nucleoli that are rich in RNA, a nucleic acid related to DNA. Some details of DNA-RNA interactions are found in Lab5, Exercise 2.
DNA (deoxyribonucleic acid) is
the extraordinary molecule that carries the genetic code. This is life's fundamental
code that is translated into the cellular building of proteins. This is an important
point. DNA's code, sometimes called the code of life on earth, is not a code
for how to build carbohydrates like sugar or starch; it is also not a code for
how to build lipids like animal fats or plant oils. DNA codes for the building
(or synthesizing) of proteins. Proteins serve cells in both structural and functional
capacities: proteins can help hold a fragile, little cell together in a structural
role, and proteins can serve as enzymes to catalyze chemical reactions. These
reactions would proceed too slowly to sustain life if occurring in the absence
of enzymes. So proteins seving as enzymes speed up the "chemistry of life
processes," also known as metabolism. We can say that metabolism is possible
only because of enzymes coded for by DNA. So think! DNA does not code directly
for the synthesis of carbohydrates and lipids, but DNA codes for their synthesis
(as well as their breakdown and chemical transformations) indirectly
through DNA-encoded enzymes.
The portion of a DNA molecule that codes for the synthesis of some particular protein, is called a gene. Each cell's DNA codes for, in total, tens of thousands of different proteins, each of which takes part in the cell's metabolism.
Specific portions of DNA molecules are called genes. So some genes code for structural proteins that can be used to build more cells and cell parts. Other genes code for enzymatic proteins that catalyze cellular chemistry (metabolism).
Cellular chemistry, or metabolism, encompasses many areas of study. Examples are:
DNA's genetic code is responsible for coding
all the enzymes required for all of metabolism. DNA and protein make up the
structure of the chromosomes. DNA, along with protein, makes up the structure
of chromosomes. All living things, with the exception of bacteria, have a unique
number of chromosomes per cell. For example, corn has 16, some fruit flies have
four, gorillas have 48 and humans have 46 chromosomes per cell.
The bacteria are exceptional in that their DNA
is located in the cytoplasm (bacterial cells have no nucleus) and consists of
one, single, circular DNA molecule per cell. Since bacterial DNA is not combined
with protein, technically, bacteria do not have any "chromosomes."
However, some authors still speak of bacterial cells possessing a single "bacterial
chromosome."
Using humans as an example, most human cells contain 46 chromosomes. This is known as the diploid number. The genes carried by these chromosomes code for the structural and enzymatic proteins that are responsible for the normal anatomy and physiology of cells.
But not all cells have 46 chromosomes. A woman's eggs and a man's sperm (sex cells), also called gametes, contain one-half the diploid number of chromosomes. This number is known as the haploid number of chromosomes and in humans it is 23 chromosomes per gamete. During fertilization, each egg and sperm contributes 23 chromosomes. This fusion restores the diploid number with the new individual possessing one-half the nuclear genetic content from each parent.
Some cells in humans function without chromosomes.
Red blood cells which constitute approximately 25% of all the cells in a person,
have no nucleus and therefore no chromosomes. [So where do red blood cells come
from if cells come from pre-existing cells and these have no nucleus?] Some
liver cells with extremely high metabolic activity possess twice the diploid
number of chromosomes (i.e., 92).
Some individuals have an abnormal number of chromsomes per cell. For example, individuals with Down's syndrome possess 47 chromosomes per cell. They have an extra copy of chromosome #21. Women with Triple-X syndrome (so-called superfemales) have am extra X chromosome which gives them 47 chromosomes per cell. So-called supermales possess an extra Y chromosome giving them 47 chromosomes per cell. This general type of chromosomal abnormality will be considered in this exercise.