Review of Key Concepts

The Nucleic Acids (p. 130)

  1. The chromatin in a cell nucleus is composed of DNA and protein. The chromatin is elaborately coiled to prevent damage to the DNA.
  2. Nucleic acids are polymers of nucleotides. A nucleotide is composed of a sugar, a phosphate group, and a nitrogenous base.
  3. Cytosine (C), thymine (T), and uracil (U) are single-ringed nitrogenous bases called pyrimidines. Adenine (A) and guanine (G) are double-ringed bases called purines.
  4. The DNA molecule is like a twisted ladder, with backbones of sugar (deoxyribose) and phosphate, and "rungs" of paired bases in the middle. A base pair is always A-T or C-G (DNA contains no uracil).
  5. DNA codes for the amino acid sequences of polypeptides. A gene is a segment of DNA that codes for one polypeptide. All the genes in one person are the genome.
  6. Three types of RNA—mRNA, rRNA, and tRNA—carry out protein synthesis.
  7. RNA is much smaller than dNa and consists of just one nucleotide chain. Except in some regions of tRNA, its bases are unpaired. RNA contains ribose in place of deoxyribose, and uracil in place of thymine.

Protein Synthesis and Secretion (p. 134)

  1. DNA directly controls polypeptide structure and indirectly controls the synthesis of other molecules by coding for the enzymes that make them.
  2. Each sequence of three bases in DNA is represented by a complementary three-base codon in mRNA. The codons include 61 that code for amino acids and 3 stop codons that code for the end of a gene. The genetic code is the correspondence between the mRNA codons and the 20 amino acids that they represent.
  3. Protein synthesis begins with transcription, in which DNA uncoils at the site of a gene and RNA polymerase makes an mRNA mirror-image copy of the gene. mRNA usually leaves the nucleus and binds to a ribosome in the cytoplasm.
  4. Protein synthesis continues with translation, in which a ribosome binds mRNA, reads the coded message, and assembles the corresponding polypeptide.
  5. In the ribosome, rRNA reads the code. tRNA molecules transport amino acids to the ribosome and

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Chapter 4 Genetics and Cellular Function 153

contribute them to the growing peptide chain.

  1. Older proteins called chaperones often bind new proteins, guide their folding into correct secondary and tertiary structure or their conjugation with nonprotein moieties, and escort them to their destinations in a cell.
  2. Proteins destined for use in the cytosol are usually made by free ribosomes in the cytoplasm. Proteins destined to be packaged in lysosomes or secretory vesicles enter the rough ER and are modified here and in the Golgi complex. Such alterations are called posttranslational modification.

DNA Replication and the Cell

Cycle (p. 139)

  1. Since every cell division divides a cell's DNA between two daughter cells, the DNA must be replicated before the next division.
  2. The enzyme DNA helicase prepares DNA for replication by opening up the double helix at several points and exposing the nitrogenous bases.
  3. DNA polymerase reads the base sequence on each chain of DNA and synthesizes the complementary chain. Thus, the two helices of DNA separate from each other and each acquires a new, complementary helix to become a new, double-helical DNA molecule.
  4. Most replication errors are detected and corrected by a second "proofreading" molecule of DNA polymerase. Undetected errors persist as mutations in the genome. Some mutations are harmless but others can cause cell death or diseases such as cancer.
  5. Cells have a life cycle, from division to division, of four phases: G1 S, G2, and M. G1 through G2 are collectively called interphase (the period between cell divisions) and M is mitosis.
  6. Mitosis is responsible for embryonic development, tissue growth, and replacement of old, injured, or dead cells. It occurs in four stages— prophase, metaphase, anaphase, and telophase—followed by cytokinesis, the division of the cytoplasm into two cells.
  7. Normal tissue structure depends on a balance between cell division and cell death. Cell division is stimulated by growth factors and suppressed by contact inhibition.

Chromosomes and Heredity (p. 145)

  1. Heredity is the transmission of genetic characteristics from parent to offspring.
  2. Germ cells are developing and mature eggs and sperm. They have 23 unpaired chromosomes and are thus called haploid cells.
  3. All other cells of the body are called somatic cells and are diploid, having 46 paired chromosomes. These pairs are shown in the karyotype, a chart of metaphase chromosomes arranged in pairs and by size.
  4. Many of the fundamental terms and concepts of heredity are defined and summarized in table 4.4.
  5. Traits controlled by recessive alleles can "skip a generation" if masked by a dominant allele. Thus, a heterozygous person may lack a certain trait (including some genetic diseases) and yet be a carrier who passes it on to future generations.
  6. All traits result from a combination of genetic and environmental influences, so the environment affects whether a given genotype is expressed.
  7. Whether an allele is dominant or recessive has no relationship to whether it is more or less common in the population.

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