1. The nucleus is surrounded by a nuclear membrane with passages in it, called nuclear pores. The material within the nucleus is nucleoplasm.

2. Nucleoplasm contains one or more masses of RNA, called nucleoli. DNA and associated proteins make up the chromatin in the nucleus.

1. Chromatin looks like a granular thread with each granule (nucleosome) consisting of a cluster of proteins called histones, and DNA wound around each cluster.

2. Histones serve as spools to organize the DNA. Other proteins (nonhistones) provide structural support for the chromatin and regulate gene activity.

1. Nucleic acids are made of monomers called nucleotides. Each nucleotide is made up of a monosaccharide, a phosphate group, and a nitrogenous base.

2. Cytosine, thymine, and uracil are the pyrimidine bases because of their single carbon-nitrogen ring.

3. Adenine and guanine are the purines with their double rings.

1. The ladder-like structure of DNA is arranged with a monosaccharide-phosphate backbone (the uprights), and base-pairing as the rungs of the ladder. The "ladder" is twisted, making the DNA molecule a double helix.

2. In the base pairs, adenine always pairs with guanine, and cytosine with thymine. This arrangement is called the law of complimentary base pairing.

1. DNA serves as a code for the structure of the proteins manufactured within the cell.

2. A gene is a sequence of DNA nucleotides that codes for a specific protein.

3. The human genome contains 100,000 genes, accounting for only 2-4% of the DNA. The rest is "junk".

1. RNA is a single strand of nucleotides, with the base uracil instead of thymine. It also contains ribose, instead of the deoxyribose found in DNA.

2. The primary functions of RNA are to carry the instructions from DNA to the cytoplasm, and to direct the synthesis of proteins.

1. DNA contains the genetic code that specifies which proteins a cell can make.

2. Although each cell (except sex cells) has the same DNA, different genes are activated in different cells.

3. Messenger RNA copies the DNA template and carries it to the cytoplasm to a ribosome, where the genetic code is "read".

4. Transfer RNA molecules deliver the correct amino acids to the ribosome.

1. The human genome codes for 100,000 proteins, and the genes are made of the same four nucleotides. The genetic code is a system in which four nucleotides code for the amino acid sequences of all proteins.

2. A sequence of three DNA nucleotides that stands for one amino acid is called a base triplet; the mirror image sequence in the mRNA is called a codon. Start and stop codons signal the mRNA where to begin and end.

1. DNA is too large to leave the nucleus.

2. Transcription is the process whereby mRNA makes a copy of the DNA with the aid of RNA polymerase.

3. mRNA can then leave the nucleus to carry as the next step in protein synthesis.

1. Before mRNA leaves the nucleus, it is called pre-mRNA, and requires posttranscriptional modification.

2. DNA contains actual information (exons) and nonsense portions (introns). Introns must be excised before mRNA carries its message to the cytoplasm.

1. Genetic translation translates the message in the mRNA into a sequence of amino acids that will comprise a protein.

2. The mRNA molecule begins with a leader sequence that serves as a binding site on the ribosome.

3. Cloverleaf-shaped transfer RNA (tRNA) contains an anticodon complimentary to the mRNA codon. tRNA carries the correct amino acid into position along the mRNA.

4. One mRNA holds several ribosomes together in a cluster called a polyribosome. Each mRNA is translated by several ribosomes at once; a cell can make 100,000 protein molecules per second.

1. After translation, chaperone proteins prevent the new protein from folding prematurely, and assist it in its proper folding once the amino acid sequence is complete.

1. Once it is formed, a protein may be altered by rough ER. These changes are called posttranslational modification.

1. When the finished protein enters the Golgi complex, it may become encased in Golgi or secretory vesicles.

1. The process of DNA replication is semiconservative; each daughter DNA double helix consists of a parental strand plus a newly-formed strand.

2. The double helix unwinds from the histones in each nucleosome, and DNA helicase "unzips" a short segment of the DNA, exposing its bases.

3. DNA polymerase "reads" the exposed strand, and assembles complimentary bases across from them at a rate of 100 base pairs per second.

4. DNA ligase splices the short segments of DNA together.

5. Each helix winds around newly synthesized histones.

1. DNA polymerase is accurate but occasionally makes mistakes. Changes in DNA structure are called mutations.

2. Mutations can have no effect, or seriously detrimental effects.

1. During the first growth phase, G₁, the cell synthesizes new proteins and grows in size. At the end of this phase, centrioles replicate.

2. During the S, or synthesis phase, the cell undergoes semiconservative replication of DNA.

3. G₂, the second growth phase, is a brief period in which the cell makes the enzymes needed for cell division. G₁, S, and G₂ are collectively known as interphase.

4. During the M, or mitotic, phase, the nucleus replicates its contents.

1. The functions of mitosis are for growth of the embryo, growth of tissue, replacement of old tissue, and tissue repair. Mitosis occurs in four phases.

2. During prophase, the chromatin condenses into chromosomes that are duplicated DNA (sister chromatids) held together at the centromere at this point. The nuclear envelope disappears.

3. During metaphase, the chromosomes are aligned across the equator of the cell. Microtubules reorganize to become the mitotic spindle.

4. In anaphase, each centromere is pulled apart, and the daughter chromosomes are pulled toward the poles of the cell. The kinetochore directs the movements of the mitotic spindle.

5. During telophase, the rough ER forms a new nuclear membrane around each set of chromosomes, and the mitotic spindle disappears.

6. At the end of mitosis, the splitting of the cytoplasm, cytokinesis, occurs as a cleavage furrow forms.

7. Timing of Cell Division (p. 156)

1. New growths, or neoplasms, may be benign or malignant. Malignant growths are deadly because of their tendency to metastasize.

2. Causes of Cancer (p. 157)

1. Heredity is the transmission of genetic traits from one generation to the next. A karyotype is a chart of the chromosomes, arrested in metaphase, and arranged in homologous pairs.

2. There are 2 sex chromosomes, X and Y, and 22 pairs of chromosomes called autosomes.

3. In the normal, paired state, chromosomes are diploid. They become haploid during meiosis.

1. A gene is located at a locus on a chromosome. Two pairs of alternate genes at the same locus are alleles.

2. A person can be homozygous or heterozygous for a given pair of alleles. Their actual genes make up their genotype; the outward expression of the genes is the phenotype.

3. If one of the alleles is expressed while the other is not, we say it is dominant; the hidden gene is recessive. An individual with a dominant phenotype, but having the recessive gene, is a carrier.

1. Some genes have more than two alleles in the population (an individual can have only two). This is a case of multiple alleles, such as the human ABO blood grouping.

2. When two alleles are equally dominant, they are said to be codominant.

3. Incomplete dominance indicates an intermediate condition when two different alleles are both expressed in the individual.

1. In polygenic inheritance, more than one pair of genes controls a trait (human skin and eye color).

2. Pleiotropy is a phenomenon in which one gene produces multiple effects (i.e., sickle-cell disease).

1. Certain genes are located on the sex chromosomes and code for sex-linked traits. Red-green colorblindness is an example of a trait carried on the X chromosome.

1. Penetrance is the percentage of the population that actually exhibit the predicted phenotype.

2. Sometimes environmental factors (such as nutrition) play a role in the extent to which a gene is expressed.

1. Dominant and recessive alleles may be present in a given population in unexpected percentages.