The basic plan for the cell contained in the genome ... work[s] so well that each human develops with few defects from a single fertilized egg into a complicated ensemble of trillions of specialized cells that function harmoniously for decades in an ever-changing environment.
—Pollard & Earnshaw, 2002
Individuals, or organisms, are built of organs, which are made of tissues, which are comprised of cells. Humans (and everything but bacteria and Archaea) are made of eukaryotic cells that contain nuclei (sing. nucleus). All types of cells fit into two main categories: somatic cells and gametes. Cells are separated into these two groups according to their different functions, their different modes of cell replication, and their different amounts of genetic material in the nucleus.
The nucleus of a cell contains the genetic material in the form of chromosomes, which are very condensed, long strands of DNA (deoxyribonucleic acid). Each nucleus in a human cell contains 23 pairs of chromosomes. There are 22 pairs of autosomal chromosomes
(44 chromosomes) and one pair of sex chromosomes (2 chromosomes) for a total of 23 pairs (46 chromosomes). Females have two X sex chromosomes (XX) and males have one X and one Y (XY). Chimpanzees have 24 pairs of chromosomes and humans have one pair less because two of them fused during evolution.
Gametes contain exactly half the complement of chromosomes in somatic cells. That is, they contain one of each of the 23 chromosomes, or 23 unpaired chromosomes. During fertilization, when the sperm and the egg unite and form a zygote, each gamete contributes half the necessary DNA to produces cells with the complete set of all the pairs of chromosomes, enabling it to develop normally into an embryo and beyond. When the offspring develops further it will eventually produce its own gametes that contain half of its genetic material to be passed on when it reproduces. Then the cycle of life continues.
Somatic cells need to divide and duplicate to grow tissues during development and to maintain the tissues when old or damaged cells die. Somatic cell division, or mitosis, is constantly occurring throughout the human body at the estimated rate of 300 million cell duplications per day, which slows down with age. Mitosis results in two copies of the parent cell.
Gametes divide to make the creation of offspring possible. Meiosis, the process of gamete division that occurs in the ovaries in females ("oogenesis") and the testes in males ("spermatogenesis"), results in four daughter cells. Each daughter cell inherits only half the chromosomes of the parent cell. For female humans, meiosis occurs in the developing fetus and by the time they are born little girls possess all the eggs they will have for the rest of their lives. Contrarily in males, meiosis continuously replenishes sperm throughout most of the life span. All eggs carry the X chromosome (since the female parent cell has two Xs) but sperm can carry either the Y or the X. It is therefore the male contribution to the zygote that determines the sex of the offspring, since if a sperm carrying a Y chromosome fertilizes the egg, the baby will be male (XY) and if it is an X chromosome the baby will be female (XX).
Crossing-over of the chromosomes is a phenomenon that only occurs during meiosis. Parental DNA gets swapped and the resulting chromosomes contain a mosaic of genes from the mother and the father. As a consequence, a chromosome can have one paternal end and one maternal end. The offspring has new combinations, or a recombination, of genes from both the mother and the father that comprise a brand new combination that does not exist in either of the parents. Crossing-over and recombination are crucial for maintaining genetic variability in successive generations because offspring are far from being clones of their parents.
DNAis commonly called the "genetic code" or the "genetic sequence." The DNA molecule is comprised of two strands of DNA sequences twisted together like a spiral staircase (called a "double helix") with steps made ofnucleotides. For simplicity, these nucleotides are symbolized by letters that "spell out" the DNAsequence: A (Adenine), C (Cytosine), T (Thymine), and G (Guanine). These nucleotides or "base pairs" bond and hold together the double-stranded DNA molecule. Adenine only binds to thymine (A-T) and cytosine only binds to guanine (C-G). For example, the sequence of ATAG binds to TATC in the other strand.
Genes are segments of DNA, or pieces of the code, that are spelled out by these letters. Genes code for amino acids that build proteins. Proteins make up everything in the body, including structures, fluids, hormones, and even enzymes that catalyze, or enable, reactions to build other proteins. The chromosomal loci (sing. locus) of many genes are known and when one person has a different arrangement of nucleotides for that gene than another person they are said to have different alleles, or variants, of a gene. For instance, there are two alleles for type of earwax in humans, one dry and one wet, but there is only one gene for earwax (ABCC11). Some genes have more than two alleles in a population, like the gene for blood type which has alleles A, B, and O.
An individual can only carry two alleles for any given gene; the one on the chromosome they inherited from their mother and the one on the chromosome they inherited from their father. In many cases, but not all, one allele will be dominant (A) with respect to the other that is recessive (a). The dominant allele (A) will be expressed in homozygous, (AA) or heterozygous (Aa) individuals where it masks the expression of the recessive allele. The recessive allele will be expressed only in homozygous individuals for that allele (aa) because there is no dominant allele to mask it. In Mendel's experiments, the allele for yellow was dominant and was expressed in both "AA" and "Aa" individuals and the allele for green was recessive so it was expressed less frequently because it could only occur in "aa" plants. Beyond Mendel we know that alleles can be codominant (e.g., Sickle Cell Trait, Chapter 4) and they can also vary in expression on an individual basis depending on the presence of other linked alleles or because of alleles that regulate their expression levels.
For genes to be properly expressed, DNA must get read and copied properly in cell division and must get read and translated properly during protein construction. If any nucleotide is read incorrectly in the sequence, the wrong amino acid can be called for, which could build a totally different protein. Fortunately there are checks and balances built into the system to prevent such reading errors from causing disaster. For instance, for any single amino acid, there are up to four different (albeit very similar) nucleotide arrangements that call for it. This redundancy in the code, or error tolerance, is how many reading and translating errors (i.e., mutations) are prevented from being harmful or lethal. Code redundancy is also how mutations in the sequence can remain neutral and cause no harm to an individual, since a new arrangement of nucleotides does not always express a different protein.
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