Bases of genetic

Nucleus & Related Structures A nucleus is present in all eukaryotic cells that divide. If a cell is cut in half, the anucleate portion eventually dies without dividing. The nucleus is made up in large part of the chromosomes, the structures in the nucleus that carry a complete blueprint for all the heritable species and individual characteristics of the animal. A nucleus is present in all eukaryotic cells that divide. If a cell is cut in half, the anucleate portion eventually dies without dividing. The nucleus is made up in large part of the chromosomes, the structures in the nucleus that carry a complete blueprint for all the heritable species and individual characteristics of the animal. Except in germ cells, the chromosomes occur in pairs, one originally from each parent. Each chromosome is made up of a giant molecule of deoxyribonucleic acid (DNA). Except in germ cells, the chromosomes occur in pairs, one originally from each parent. Each chromosome is made up of a giant molecule of deoxyribonucleic acid (DNA).

Endoplasmic Reticulum The endoplasmic reticulum is a complex series of tubules in the cytoplasm of the cell. The outer limb of its membrane is continuous with a segment of the nuclear membrane, so in effect this part of the nuclear membrane is a cistern of the endoplasmic reticulum. The endoplasmic reticulum is a complex series of tubules in the cytoplasm of the cell. The outer limb of its membrane is continuous with a segment of the nuclear membrane, so in effect this part of the nuclear membrane is a cistern of the endoplasmic reticulum. The tubule walls are made up of membrane. In rough or granular endoplasmic reticulum, granules called ribosomes are attached to the cytoplasmic side of the membrane, whereas in smooth or agranular endoplasmic reticulum, the granules are absent. The tubule walls are made up of membrane. In rough or granular endoplasmic reticulum, granules called ribosomes are attached to the cytoplasmic side of the membrane, whereas in smooth or agranular endoplasmic reticulum, the granules are absent.

RNA The nucleus of most cells contains a nucleolus, a patchwork of granules rich in ribonucleic acid (RNA). In some cells, the nucleus contains several of these structures. The nucleus of most cells contains a nucleolus, a patchwork of granules rich in ribonucleic acid (RNA). In some cells, the nucleus contains several of these structures. Nucleoli are most prominent and numerous in growing cells. They are the site of synthesis of ribosomes, the structures in the cytoplasm in which proteins are synthesized Nucleoli are most prominent and numerous in growing cells. They are the site of synthesis of ribosomes, the structures in the cytoplasm in which proteins are synthesized

DNA3 DNA4 DNA5 DNA6 ribosomes

Ribosomes The ribosomes that become attached to the endoplasmic reticulum synthesize all transmembrane proteins, most secreted proteins, and most proteins that are stored in the Golgi apparatus, lysosomes, and endosomes. All these proteins have a hydrophobic signal peptide at one end. The ribosomes that become attached to the endoplasmic reticulum synthesize all transmembrane proteins, most secreted proteins, and most proteins that are stored in the Golgi apparatus, lysosomes, and endosomes. All these proteins have a hydrophobic signal peptide at one end. The Golgi apparatus, which is involved in processing proteins formed in the ribosomes, and secretory granules, vesicles, and endosomes are discussed below in the context of protein synthesis and secretion. The Golgi apparatus, which is involved in processing proteins formed in the ribosomes, and secretory granules, vesicles, and endosomes are discussed below in the context of protein synthesis and secretion.

The Genome DNA is found in bacteria, in the nuclei of eukaryotic cells, and in mitochondria. It is made up of two extremely long nucleotide chains containing the bases adenine (A), guanine (G), thymine (T), and cytosine (C). DNA is found in bacteria, in the nuclei of eukaryotic cells, and in mitochondria. It is made up of two extremely long nucleotide chains containing the bases adenine (A), guanine (G), thymine (T), and cytosine (C). The chains are bound together by hydrogen bonding between the bases, with adenine bonding to thymine and guanine to cytosine. An indication of the complexity of the molecule is the fact that the DNA in the human haploid genome (the total genetic message) is made up of 3 × 109 base pairs. The chains are bound together by hydrogen bonding between the bases, with adenine bonding to thymine and guanine to cytosine. An indication of the complexity of the molecule is the fact that the DNA in the human haploid genome (the total genetic message) is made up of 3 × 109 base pairs.

Formation of mRNA by Transcription of DNA Formation of mRNA by Transcription of DNA A segment of the DNA molecule is opened, and RNA polymerase (an enzyme that is not shown) assembles nucleotides into mRNA according to the base-pair combinations shown in the inset. A segment of the DNA molecule is opened, and RNA polymerase (an enzyme that is not shown) assembles nucleotides into mRNA according to the base-pair combinations shown in the inset. Thus the sequence of nucleotides in DNA determines the sequence of nucleotides in mRNA. As nucleotides are added, an mRNA molecule is formed. Thus the sequence of nucleotides in DNA determines the sequence of nucleotides in mRNA. As nucleotides are added, an mRNA molecule is formed.

The Human Genome When the human genome was finally mapped several years ago, there was considerable surprise that it contained only about 30,000 genes and not the 50,000 or more that had been expected. Yet humans differ quite markedly from their nearest simian relatives. When the human genome was finally mapped several years ago, there was considerable surprise that it contained only about 30,000 genes and not the 50,000 or more that had been expected. Yet humans differ quite markedly from their nearest simian relatives. The explanation appears to be that rather than a greater number of genes in humans, there is a greater number of mRNAs—perhaps as many as 85,000. The implications of this increase are discussed below. The explanation appears to be that rather than a greater number of genes in humans, there is a greater number of mRNAs—perhaps as many as 85,000. The implications of this increase are discussed below.

Transcription & Translation The strands of the DNA double helix not only replicate themselves, but also serve as templates by lining up complementary bases for the formation in the nucleus of messenger RNA (mRNA), transfer RNA (tRNA), the RNA in the ribosomes (rRNA), and various other RNAs. The strands of the DNA double helix not only replicate themselves, but also serve as templates by lining up complementary bases for the formation in the nucleus of messenger RNA (mRNA), transfer RNA (tRNA), the RNA in the ribosomes (rRNA), and various other RNAs. The formation of mRNA is called transcription and is catalyzed by various forms of RNA polymerase. Usually after some posttranscriptional processing (see below), mRNA moves to the cytoplasm and dictates the formation of the polypeptide chain of a protein (translation). This process occurs in the ribosomes. The formation of mRNA is called transcription and is catalyzed by various forms of RNA polymerase. Usually after some posttranscriptional processing (see below), mRNA moves to the cytoplasm and dictates the formation of the polypeptide chain of a protein (translation). This process occurs in the ribosomes.

Posttranscriptional Change in mRNA Posttranscriptional Change in mRNA An intron is cleaved from between two exons and is discarded. The exons are spliced together by spliceosomes to make the functional mRNA. An intron is cleaved from between two exons and is discarded. The exons are spliced together by spliceosomes to make the functional mRNA.

Translation of mRNA to Produce a Protein To start protein synthesis a ribosome binds to mRNA. The ribosome also has two binding sites for tRNA, one of which is occupied by a tRNA with its amino acid. Note that the codon of mRNA and the anticodon of tRNA are aligned and joined. The To start protein synthesis a ribosome binds to mRNA. The ribosome also has two binding sites for tRNA, one of which is occupied by a tRNA with its amino acid. Note that the codon of mRNA and the anticodon of tRNA are aligned and joined. The other tRNA binding site is open. other tRNA binding site is open.

By occupying the open tRNA binding site the next tRNA is properly aligned with mRNA and with the other tRNA. By occupying the open tRNA binding site the next tRNA is properly aligned with mRNA and with the other tRNA. Translation of mRNA to Produce a Protein

An enzyme within the ribosome catalyzes a synthesis reaction to form a peptide bond between the amino acids. Note that the amino acids are now associated with only one of the tRNAs. An enzyme within the ribosome catalyzes a synthesis reaction to form a peptide bond between the amino acids. Note that the amino acids are now associated with only one of the tRNAs. Translation of mRNA to Produce a Protein

The ribosome shifts position by three nucleotides. The ribosome shifts position by three nucleotides. The tRNA without the amino acid is released from the ribosome, and the tRNA with the amino acids takes its position. A tRNA binding site is left open by the shift. The tRNA without the amino acid is released from the ribosome, and the tRNA with the amino acids takes its position. A tRNA binding site is left open by the shift. Additional amino acids can be added by repeating steps 2 through 4. Eventually a stop codon in the mRNA ends the production of the protein, which is released from the ribosome. Additional amino acids can be added by repeating steps 2 through 4. Eventually a stop codon in the mRNA ends the production of the protein, which is released from the ribosome. Translation of mRNA to Produce a Protein

Multiple ribosomes attach to a single mRNA. Multiple ribosomes attach to a single mRNA. As the ribosomes move down the mRNA, proteins attached to the ribosomes lengthen and eventually detach from the mRNA. As the ribosomes move down the mRNA, proteins attached to the ribosomes lengthen and eventually detach from the mRNA. Translation of mRNA to Produce a Protein

Cell Cycle The cell cycle is divided into interphase and mitosis. Interphase is divided into G1, S, and G2 subphases. During G1 and G2, the cell carries out routine metabolic activities. During the S phase DNA is replicated. (a) Following mitosis, two cells are formed by the process of cytokinesis. The cell cycle is divided into interphase and mitosis. Interphase is divided into G1, S, and G2 subphases. During G1 and G2, the cell carries out routine metabolic activities. During the S phase DNA is replicated. (a) Following mitosis, two cells are formed by the process of cytokinesis. Each new cell begins a new cell cycle. (b) Many cells exit the cell cycle and enter the G0 phase, where they remain until stimulated to divide, at which point they reenter the cell cycle. Each new cell begins a new cell cycle. (b) Many cells exit the cell cycle and enter the G0 phase, where they remain until stimulated to divide, at which point they reenter the cell cycle.

Cell Cycle Obviously, the initiation of mitosis and normal cell division depends on the orderly occurrence of events during what has come to be called the cell cycle. A diagram of these events is shown in. Obviously, the initiation of mitosis and normal cell division depends on the orderly occurrence of events during what has come to be called the cell cycle. A diagram of these events is shown in. There is intense interest in the biochemical machinery that produces mitosis, in part because of the obvious possibility of its relation to cancer. When DNA is damaged, entry into mitosis is inhibited, giving the cell time to repair the DNA; failure to repair damaged DNA leads to cancer. There is intense interest in the biochemical machinery that produces mitosis, in part because of the obvious possibility of its relation to cancer. When DNA is damaged, entry into mitosis is inhibited, giving the cell time to repair the DNA; failure to repair damaged DNA leads to cancer.

Mitosis Interphase. DNA, which is dispersed as chromatin, replicates. The two strands of each DNA molecule separate, and a copy of each strand is made. Consequently, Interphase. DNA, which is dispersed as chromatin, replicates. The two strands of each DNA molecule separate, and a copy of each strand is made. Consequently, two identical DNA molecules are produced. The pair of centrioles replicates to produce two pairs of centrioles two identical DNA molecules are produced. The pair of centrioles replicates to produce two pairs of centrioles

Prophase. Chromatin strands condense to form chromosomes. Each chromosome is composed of two identical strands of chromatin called chromatids, which are joined together at one point by a specialized region called the centromere. Each chromatid contains one of the DNA molecules replicated during interphase. Prophase. Chromatin strands condense to form chromosomes. Each chromosome is composed of two identical strands of chromatin called chromatids, which are joined together at one point by a specialized region called the centromere. Each chromatid contains one of the DNA molecules replicated during interphase. Mitosis

Metaphase. Metaphase. The chromosomes align along the equator with spindle fibers from each pair of centrioles, located at opposite poles of the cell, attached to their centromeres. The chromosomes align along the equator with spindle fibers from each pair of centrioles, located at opposite poles of the cell, attached to their centromeres. Mitosis

Anaphase. The centromeres separate, and each chromatid is then referred to as a chromosome. Thus, when the centromeres divide, the chromosome number doubles, and there are two identical sets of chromosomes. Anaphase. The centromeres separate, and each chromatid is then referred to as a chromosome. Thus, when the centromeres divide, the chromosome number doubles, and there are two identical sets of chromosomes. The two sets of chromosomes are pulled by the spindle fibers toward the poles of the cell. The two sets of chromosomes are pulled by the spindle fibers toward the poles of the cell. Mitosis

Telophase. The migration of each set of chromosomes is complete. A new nuclear envelope develops from the endoplasmic reticulum, and the nucleoli reappear. Telophase. The migration of each set of chromosomes is complete. A new nuclear envelope develops from the endoplasmic reticulum, and the nucleoli reappear. During the latter portion of telophase the spindle fibers disappear, and the chromosomes unravel to become less distinct chromatin threads. During the latter portion of telophase the spindle fibers disappear, and the chromosomes unravel to become less distinct chromatin threads. Mitosis

Interphase. Cytokinesis, which continued from anaphase through telophase, becomes complete when the plasma membranes move close enough together at the equator of the cell to fuse, completely separating the two new daughter cells, each of which now has a complete set of chromosomes (a diploid number of chromosomes) identical to the parent cell. Interphase. Cytokinesis, which continued from anaphase through telophase, becomes complete when the plasma membranes move close enough together at the equator of the cell to fuse, completely separating the two new daughter cells, each of which now has a complete set of chromosomes (a diploid number of chromosomes) identical to the parent cell. Mitosis

Meiosis In germ cells, reduction division (meiosis) takes place during maturation. The net result is that one of each pair of chromosomes ends up in each mature germ cell; consequently, each mature germ cell contains half the amount of chromosomal material found in somatic cells. Therefore, when a sperm unites with an ovum, the resulting zygote has the full complement of DNA, half of which came from the father and half from the mother. In germ cells, reduction division (meiosis) takes place during maturation. The net result is that one of each pair of chromosomes ends up in each mature germ cell; consequently, each mature germ cell contains half the amount of chromosomal material found in somatic cells. Therefore, when a sperm unites with an ovum, the resulting zygote has the full complement of DNA, half of which came from the father and half from the mother. The term "ploidy" is sometimes used to refer to the number of chromosomes in cells. Normal resting diploid cells are euploid and become tetraploid just before division. Aneuploidy is the condition in which a cell contains other than the haploid number of chromosomes or an exact multiple of it, and this condition is common in cancerous cells. The term "ploidy" is sometimes used to refer to the number of chromosomes in cells. Normal resting diploid cells are euploid and become tetraploid just before division. Aneuploidy is the condition in which a cell contains other than the haploid number of chromosomes or an exact multiple of it, and this condition is common in cancerous cells.