92. The cell membrane’s resting potential (about −70 mV inside with respect to the
outside) is due mainly to which of the following mechanisms?
A. The sodium potassium pump.
B. The diffusion of cations and anions through the membrane along their con-
centration gradients.
C. The diffusion of sodium and potassium across the cell membrane.
D. The presence inside the cell of anions too large to passively cross the cell
membrane.
Answer is A: The ATPase pump shifts 3 Na
+ out of the cell and 2 K
+ into the
cell. This disparity in positive charge is the major infl uence on the resting
potential.
2.2 Cell Cycle (Mitosis and Protein Synthesis)
The cell nucleus contains chromosomes which are composed of molecules of
DNA. DNA is composed of units called nucleotides which consist of a sugar (deoxy-
ribose) attached to a phosphoric acid group (PO
3 OH) and one of four bases.
Chromosomes contain the code for the sequence of amino acids used to construct
different proteins. Each amino acid is coded for by a particular sequence of three of
the four bases (adenine, guanine, cytosine & thymine). This sequence is called a
“codon”. mRNA “transcribes” this code then moves from the nucleus to a ribosome
in the cytoplasm where it is “translated” and the protein is assembled by joining the
required amino acids in the appropriate sequence.
Mitosis is the process by which a somatic cell divides to produce two cells with
identical DNA. In this way an organism can grow. Before mitosis, the DNA must be
duplicated. Hence the chromosomes (consisting of one strand or “chromatid”) dou-
ble up by becoming two chromatids. Then during mitosis the two chromatids sepa-
rate and move into the two daughter cells.
Meiosis occurs only in the gonads. This process results in four daughter cells.
Human cells have two copies of each of 23 chromosomes, one copy being inherited
from the father and the other copy from the mother. The gametes need to have only
one copy of each of the 23 chromosomes, so that when the sperm fuses with the
ovum, the “diploid” number of 46 (two copies of each chromosome) is restored.
Meiosis is the process by which cells reduce their number of chromosomes from 46
to 23 different chromosomes. Of the 23 chromosomes in a sperm (or ovum), some
(between 0 and 23) will have come from the sperm owner’s mother and the rest from
the sperm owner’s father. The same can be said of the 23 chromosomes in the ovum.
In this way the resulting children will be genetically different from each of their
parents (and siblings) as each sperm/ovum will have a different assortment of the 23
available chromosomes.
