BIOLOGY 03050, INVERTEBRATE ZOOLOGY, WEEK 14 SUMMARY:
THE CELL CYCLE; GENETICS 11/29/99
Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu)
Text (7th ed.): Ch. 2, pp. 35-38; Ch. 3 (aspects, e.g., meiosis, basic genetics)
possible web site: http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html
(University of Arizona site highlighting cell cycle and mitosis issues)
1. Nucleic acids: DNA and RNA are deoxyribonucleic acid and ribonucleic acid, respectively. DNA can replicate itself and is present in the nucleus of a cell, and RNA is a carrier of information between DNA and the sites of protein synthesis. The two strands of DNA can separate and self-replicate. In the process of transcription in the nucleus, the DNA can be read into messenger RNA (mRNA). Later, at the ribosomes in the cytoplasm, the mRNA can form proteins for all the metabolic functions of the cell, in the process of translation [via transfer RNA (tRNA) attached to amino acids].
2. The cell can reproduce itself by the process of cell division, which is controlled by the DNA in the cell. When cells divide (except in meiosis), they are genetic copies of each other, as the DNA is copied faithfully from one generation to the next. Cell division occurs in two basic stages:1
mitosis, which is the division of the nucleus.
cytokinesis, or division of the cytoplasm.
3. Cell division is actually one small time of the cell cycle. The cell must grow between divisions and carry out its metabolic functions. We therefore have a prolonged interphase condition between divisions (90-95% of the time the cell is in interphase1). We can highlight a general trend of:
G1à Sà G2à M
Overall, interphase consists of G1, S, and G2. The first gap phase represents the early growth phase of the cell, the synthesis phase involves DNA replication, the second gap phase gets the cell ready for division with production of such things as spindle proteins, and the mitotic phase is the actual time of mitosis and cytokinesis.
The DNA during interphase is in association with nuclear proteins in an uncondensed fashion which we call chromatin. When the cell duplicates its DNA in the S phase, and gets ready for division in G2, we end up with chromosomes (thickened, threadlike complexes of DNA and proteins) containing sister chromatids (copy of a chromosome produced by replication) attached at a site called a centromere. This doubled chromosome is now ready to be divided by mitosis. Mitosis has several phases associated with it: prophase, metaphase, anaphase, and telophase.
4. Phases of mitosis. These phases represent a continuum without distinct stop and start points, but they are useful in highlighting different features of the division process.
4a. prophase: the chromosomes condense and the mitotic apparatus of microtubules is set up with the help of the centrioles.
4b. metaphase: the chromosomes are lined up in the middle of nuclear remains.
4c. anaphase: the chromosomes are pulled apart at the centromeres, so that sister chromatid copies of the chromosomes end up at opposite poles of the cell.
4d. telophase: once the chromosomes are at those opposite ends, the mitotic apparatus is disassembled, nuclear envelopes reform around the two sets of chromosomes, and cytokinesis, or division of the cell, follows.
There are universal genes that control the timing of this cell cycle for all living organisms, e.g., rapid dividing of cells in embryo development and cancer.
5. Humans and most animals do not simply divide in half to reproduce. Most of the organisms we talked about this semester undergo sexual reproduction, where egg and sperm fuse to form a zygote. Now to keep the same number of chromosomes in the zygote as there are in the adult, one must halve the chromosome in both the egg and the sperm. The process by which this occurs is called meiosis.
6. Meiosis occurs in the ovaries and testes to reduce the chromosome number. Instead of doubled chromosomes being separated as in mitosis, in meiosis, the doubled chromsomes cross-over (or exchange DNA) and then separate first as whole chromosomes.
The way this plays out is as follows:
In prophase I, the crossing-over occurs.
In metaphase I, pairs of chromosomes (each with sister chromatids) line up.
In anaphase I and telophase I, these pairs of chromosomes are pulled apart, hence reducing the chromosome number.
In prophase II à telophase II, things proceed a bit more typically, so as to end up with 4 cells that have ½ of the genetic information in each of them.
7. The major advantage of this crossing-over is that it is a form of genetic recombination that is a major source of variation in the populations of given species. Sexual reproduction allows for this variation from parent to offspring, and after all, variation is the foundation for evolution. With sex, you can not clone yourself, but you offer the opportunity for the next generation to be different, and therefore possibly better adapted to a changing environment.
8. Genetics is the science of heredity or natural selection. Offspring are not always the"average" of the parents' features. How does this occur? Gregor Mendel, the Austrian monk who started the science in the 1860's, looked at Pisum sativum or pea plants. Pea is self-pollinating, and the offspring of tall plants are always tall and the offspring of short plants always small. When he hand-crossed a tall plant and a short plant, he discovered all of the offspring were tall, not intermediate height. If he let those offspring self-pollinate and produce offspring of their own, then he discovered a ratio of 3 tall to 1 short. Ultimately, he tried 7 different characteristics to highlight similar relationships. We now call these the P (parental) generation [tall and short parents], the F1 (first filial) [the tall offspring] and the F2 (the second filial) [the grandchildren in the 3:1 ratio] generations. We also call the offspring from crossed purebred lines hybrids. He called the factors which control the chosen characteristics genes (remember, 100 years before molecular biology!), and decided that they must come in pairs (well, actually they can be in great number, but that's another story!). The alternative forms of genes are called alleles. In our example, since the tall factor"covered" the short factor in the F1, the tall allele is dominant and the short allele is considered recessive.
With dominant and recessive genes, the appearance and the gene makeup do not always correspond exactly. Since a tall/tall and a tall/short plant look alike, we say they have similar phenotypes, but different genotypes. We CAPITALIZE the dominant allele of a gene, and keep the small case for the recessive. A homozygous plant has gene alleles that are alike, while a heterozygous plant has gene alleles that vary.
As you might imagine, these genes (usually) code for different protein products!1
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Please send comments and questions to: cronewil@hvcc.edu
Copyright 1999 by Wilson Crone
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This page updated on November 22, 1999