Human Physiology   7/e   Vander/Sherman/Luciano
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Chapter 5: Genetic Information and Protein Synthesis


Study Outline

Chapter 5: Genetic Information and Protein Synthesis

  1. GENETIC CODE

    1. Genetic information is coded in the nucleotide sequences of DNA molecules. A single gene contains either (a) the information that, via mRNA, determines the amino acid sequence in a specific protein, or (b) the information for forming rRNA or tRNA, which assists in protein assembly.
    2. Genetic information is transferred from DNA to mRNA in the nucleus (transcription), and then mRNA passes to the cytoplasm, where its information is used to synthesize protein (translation).
    3. The words in the DNA genetic code consist of a sequence of three nucleotide bases that specify a single amino acid. The sequence of triplet code words along a gene determines the sequence of amino acids in a protein. More than one code word can specify a given amino acid.

  2. PROTEIN SYNTHESIS, DEGRADATION, AND SECRETION

    1. The steps leading from DNA to protein synthesis are a series of events which are collectively called transcription and translation.
    2. Transcription involves the formation of a primary RNA transcript by base pairing with the template strand of DNA containing a single gene and the removal of intron-derived segments by spliceosomes to form mRNA which moves to the cytoplasm.
    3. Translation of mRNA occurs on the ribosomes in the cytoplasm by base pairing between the anticodons in tRNAs linked to single amino acids with the corresponding codons in mRNA.
    4. Chaperones help fold large proteins into their proper conformation as they are released from ribosomes.
    5. Protein transcription factors activate or repress the transcription of specific genes by binding to regions of DNA that interact with the promoter region of a gene.
    6. The concentration of a particular protein in a cell depends on (1) the rate of its gene's transcription, (2) the rate of initiating protein assembly on a ribosome, (3) the rate at which mRNA is degraded, (4) the rate of protein digestion by enzymes associated with proteosomes, and (5) the rate of secretion, if any, of the protein from the cell.
    7. Proteins secreted by cells pass through the sequence of steps including synthesis, transport, modification, packaging, and export. Targeting of a protein to the secretory pathway depends on the signal sequence of amino acids that first emerges from a ribosome during synthesis.

  3. REPLICATION AND EXPRESSION OF GENETIC INFORMATION

    1. Human cells contain 46 chromosomes, consisting of 44 autosomes and 2 sex chromosomes. At conception, 22 homologous chromosomes and one sex chromosome are supplied to the fertilized egg by each parent.
    2. When a cell divides, the DNA molecule in each of the 46 chromosomes is replicated, one copy passing to each daughter cell, so that both receive the same complete set of genetic instructions.
    3. DNA replication involves base pairing of the exposed bases in each of the two unwound strands of DNA with free deoxyribonucleotide triphosphate bases. DNA polymerase joins the nucleotides together, forming two molecules of DNA, one from each of the original DNA strands.
    4. Telomeres are added to the ends of replicating DNA in some cells. In the absence of telomeres, the length of DNA decreases with each replication.
    5. Proofreading mechanisms help prevent the introduction of errors during DNA replication.
    6. Cell division, consisting of nuclear division (mitosis) and cytoplasmic division (cytokinesis) lasts about 1 h. The period between divisions, known as interphase, is divided into 3 phases (G, S and G).
    7. During the S phase of interphase, DNA replicates, forming two identical sister chromatids joined by a centromere.
    8. In mitosis:

      1. The chromatids condense into highlycoiled chromosomes.
      2. The centromeres of each chromosome become attached to spindle fibers extending from the centrioles, which have migrated to opposite poles of the nucleus.
      3. The two chromatids of each chromosome separate and move toward opposite poles of the cell as the cell divides into two daughter cells. Following cell division, the condensed chromatids uncoil into their extended interphase form.

    9. Entry into the S and M phases of the cell cycle is controlled by cell division cycle kinases. These enzymes are activated by a rising concentration of cyclin proteins, which are then rapidly destroyed as the cell passes through each of these checkpoints.
    10. Extracellular growth factors act on cells to produce intracellular signals that regulate the rate of cell proliferation.
    11. Mutagens alter DNA molecules, resulting in the addition or deletion of nucleotides or segments of DNA. The result is an altered DNA sequence known as a mutation.

      1. A mutation may (a) cause no noticeable change in cell function, (b) modify cell function but still be compatible with cell growth and replication, or (c) lead to the death of the cell.
      2. Mutations occurring in egg or sperm cells are passed on to all the cells of the new individual and possibly to some individuals in future generations.

    12. DNA repair mechanisms are important in preventing the accumulation of mutations, particularly in long-lived cells that do not divide.
    13. A number of modified forms of each gene, alleles, exist in the population. An individual who inherits homologous genes of the same allele is homozygous, and heterozygous if the two alleles are different.
    14. The phenotypic traits produced by the expression of genes are described as dominant if only one altered allele is required to express the trait and recessive if both homologous alleles must be altered.
    15. Genetic diseases are the result of inherited mutated genes. They can result from single gene mutations, for example, losses or additions of chromosomal segments, or be polygenic when more than one mutated gene is required for the disease to be expressed.

  4. CANCER

    1. Cancer cells are characterized by their capacity for unlimited multiplication and their ability to metastasize to other parts of the body, forming multiple tumor sites.

      1. Mutations in dominant proto-oncogenes and recessive tumor suppressor genes can lead to cancer. In their unmutated state, these genes code for proteins that perform functions at various stages of the control systems that regulate cell replication.
      2. More than one mutation is necessary to cause the transformation of a normal cell into a cancer cell.

  5. GENETIC ENGINEERING

    1. With the use of bacterial restriction nucleases, segments of DNA can be cut from the DNA of one cell and inserted into the DNA of another cell (transfection) forming a transgenic organism.
    2. Transfection of human genes into bacteria provides a mechanism of producing large quantities of the expressed protein, which can be isolated and used to treat disease, as, for example, the production of insulin.
    3. Analysis of the pattern of tissue DNA fragments formed by nuclease digestion is the basis of DNA fingerprinting used to identify a specific individual.
    4. Experimental techniques that lead to the selective removal or inactivation of a specific gene produce a knockout organism that can be used to study the functional consequences of the loss of the gene's activity. for a given genetic trait and demonstrate how the students can prepare one of their own.

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