3.GENES AND CELLULAR FUNCTION

• The relationship of a DNA base sequence to peptide structure is represented by the seven base triplets on the template strand of DNA, the corresponding codons of mRNA transcribed from the DNA triplets, the anticodons of tRNA that bind to the mRNA codons, the amino acids carried by those six tRNA molecules, the amino acids linked into a peptide chain, and the UAC, CGC, CCU, UGC, GUA, CUC triplets on the same side and 3 cis = on the same side and 4 trans = across from.
• These compounds are produced by enzymatic reactions, and enzymes are proteins encoded by genes.
• The protein processing and secretion of a protein involves the nucleus, rough ER, cistern, Golgi complex, lysosome, ribosomes, clathrin-coated transport vesicle, and secretory vesicles.
• The genes for hemoglobin and digestive enzymes, for example, are present but inactive in liver cells and skin cells.
• In the case of a woman who has just given birth to her first baby, the hormone prolactin stimulates cells of her mammary glands to synthesize the various components of breast milk, including the protein casein.
• Proteins can be deposited as a structural protein within cells, used in the cytosol as a metabolic enzyme, returned to the nucleus for use in nuclear metabolism, packaged in lysosomes for autophagy, or secreted by exocytosis for extracellular functions.
• Genes can be turned on or off at different times in a person's life, and some genes are permanently turned off in any given cell.
• Some proteins are synthesized by cells but are not proteins, such as glycogen, fat, steroids, phospholipids, pigments, and many other compounds.
• The gene for casein is turned on at this point in her life due to prolactin stimulation.
• There are several ways to turn genes on or off, and an example can convey the general principle.
• The production of testosterone, for example, involves a cell of the testis taking in cholesterol and enzymatically converting it to testosterone.
• Among the multiple phenotypic effects of alkaptonuria are darkening of the skin, darkening and degeneration of the cartilages, and urine that darkens upon standing.
• Some alleles are equally dominant, or codominant, when both of them are present, both are phenotypically expressed.
• Polygenic inheritance is a phenom- enon in which genes at two or more loci, or even on different chromosomes, contribute to a single phenotypic trait.
• Multiple phenotypic effects can result from a single gene mutation.
• Which two alleles one inherits determine the blood type, as follows: Genotype Phenotype I A I A Type A I A i Type A I B I B Type B I B i Type B I A I B Type AB ii Type O.
• Two of the ABO blood type alleles are dominant and symbolized with a capital I (for immunoglobulin) and a super-script: I A and I B.
• For example, there are over 100 alleles responsible for cystic fibrosis, and there are 3 alleles for 17 different blood types.
• A polygenic trait such as eye color is determined by the contributions of genes at multiple loci.
• A Punnett square shows why such a trait can “skip a generation.” Both parents in this case have heterozygous genotypes (Cc) and cleft chins.
• This gives their offspring a one-in-four chance of having an uncleft chin (cc).
• Human eye and skin colors are normal polygenic traits.
• An egg from the mother can carry either allele C or c, as can a sperm from the father.
• Some genes exist in more than two allelic forms—that is, there are multiple alleles within the collective genetic makeup, or gene pool, of the population as a whole.
• There is one recessive allele, symbolized with a lowercase i.
• The more common uncleft chin occurs only when both alleles are recessive.
• Familial hypercholesterolemia is a disease in which people who inherit an abnormal allele from both parents have blood cholesterol levels up to six times higher than in the general population, whereas those who inherit it from only one parent have levels about two or three times normal.
• DNA and RNA are the nucleic acids.
• Some spindle fibers grow toward the chromosomes and become attached to the kinetochore on each side of the centromere.
• The nuclear envelope disintegrates during prophase and releases the chromosomes into the cytosol.
• At the outset of mitosis, the chromosomes shorten and thicken, eventually coiling into compact rods that are easier to distribute to daughter cells than the long, delicate chromatin of interphase.
• Cells divide by two mechanisms called mitosis and meiosis.
• This phase begins with activation of an enzyme that cleaves the two sister chromatids from each other at the centromere.
• One daughter chromosome migrates to each pole of the cell, with its centromere leading the way and the arms trailing behind.
• The four phases of mitosis are recognizable: prophase, metaphase, anaphase, and telophase.
• The centrioles begin to sprout elongated microtubules called spindle fibers, which push the centrioles apart as they grow.
• Mitosis serves all the other functions of cell division: development of an individual, composed of some 50 trillion cells, from a one-celled fertilized egg; growth of all tissues and organs after birth; replacement of cells that die; and repair of damaged tissues.
• Eventually, a pair of centrioles comes to lie at each pole of the cell.
• Each chromatid is now regarded as a separate, single-stranded daughter chromosome.
• The spindle fibers then tug the chromosomes back and forth until they line up along the midline of the cell.