Cell Reproduction and Meiosis

• DNA replication occurs during the S phase of interphase.
• Mitosis results in the production of two diploid cells.
• DNA damage is assessed at the G2 checkpoint in the cell cycle.
• Programmed cell death is also known as apoptosis.
• Euchromatin is described as nonstaining dark and active.
• Chromosomes become visible during prophase of mitosis.
• In animal cells, a cleavage furrow forms as the plasma membrane indents during cytokinesis.
• Cell division in prokaryotes occurs through binary fission.

Meiosis and the Human Life Cycle

• Meiosis I separates homologous pairs of chromosomes.
• Crossing over occurs during prophase I of meiosis.
• Independent assortment occurs during metaphase I.
• Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate during meiosis.
• Meiosis results in the production of 4 haploid cells.
• Compared to mitosis, meiosis involves two nuclear divisions.
• Spermatogenesis occurs in the testes.
• Oogenesis results in one functional egg and polar bodies.

Animal Development

• The acrosome is the cap on a sperm cell that releases enzymes to digest the egg's outer layer.
• A solid ball of cells resulting from cleavage is called a morula.
• The inner cell mass of a blastula develops into a baby.
• The mesoderm is the middle germ layer of the gastrula.
• The nervous system forms from the ectoderm.
• The vertebral column develops from the notochord, which induces the formation of the nervous system.
• Cellular differentiation is the specialization of cells due to the activation of different genes.
• Programmed cell suicide is necessary for the normal development of phalanges.

Patterns of Gene Inheritance

• Gregor Mendel is known as the "Father of Genetics".
• Mendel's Law of Dominance states that one trait can mask another.
• Alternate forms of a gene are called alleles.
• An individual with two identical alleles for a particular trait is said to be homozygous.
• Mendel's Law of Independent Assortment is best explained by events that occur during metaphase I.
• To determine if a tall plant with a dominant phenotype is homozygous (TT) or heterozygous (Tt), it should be crossed with a homozygous recessive short plant.
• In an autosomal recessive disorder, most affected children have unaffected parents.
• When a heterozygote has a phenotype intermediate between that of either homozygote, this is called incomplete dominance.
• When both dominant alleles are expressed in a heterozygote, this is called codominance.
• Genes that are located on the X chromosome and are not related to sex determination are X-linked genes.
• A trait governed by two or more sets of alleles is an example of polygenic inheritance.
• Genes located on the same chromosome are considered linked.
• The cell cycle is controlled by internal and external signals.
• Growth factors are an example of a molecule that can stimulate a metabolic event.
• If a cell's DNA is damaged and cannot be repaired at a checkpoint, the cell may undergo apoptosis.
• The synthesis of proteins necessary for cell division occurs primarily during the G2 phase of interphase.
• A cell that is not actively dividing and has exited the cell cycle is likely in the G0 phase.

Maintaining Chromosome Number

• During interphase, after the S phase, a chromosome consists of two sister chromatids.
• DNA winds around histone proteins to form nucleosomes.
• Somatic cells in humans typically have a chromosome number that is diploid (2n).
• The function of mitosis in somatic cells is primarily for growth and repair.
• In plant cells, a cell plate forms during cytokinesis because the cell walls do not permit furrowing.
• Prokaryotic cells have a chromosome that is typically circular and located in the nucleoid region.
• During binary fission, after the chromosome replicates, the cell elongates and builds a cell membrane and wall between the two chromosomes.

Reducing Chromosome Number

• Cells undergoing meiosis initially are diploid.
• Alternative forms of a gene are called alleles.
• Meiosis ultimately generates 4 haploid cells.
• Homologous pairs of chromosomes separate during anaphase I.
• Sister chromatids separate during anaphase II.
• The resting stage that may occur between meiosis I and meiosis II is called interkinesis.
• A diploid somatic cell formed from the fusion of two gametes is a zygote.
• Genetic recombination during meiosis leads to new combinations of genes not found in either parent.
• The exchange of portions of chromatids between homologous chromosomes during prophase I is called crossing over.
• The random alignment of maternal and paternal homologous chromosomes during metaphase I leads to independent assortment.
• Down syndrome (Trisomy 21) is an example of a condition caused by nondisjunction.

Comparison of Meiosis and Mitosis

• Mitosis involves one nuclear division, while meiosis involves two nuclear divisions.
• Daughter cells produced by mitosis are genetically diploid and identical; daughter cells produced by meiosis are genetically haploid and not identical.
• Meiosis occurs in reproductive organs.
• Spermatogenesis results in the production of four haploid sperm.
• In oogenesis, cytokinesis occurs unevenly to ensure that the egg cell retains most of the nutrients.
• Polar bodies produced during oogenesis typically deteriorate.

Principles of Animal Development

• The enzymes released by the acrosome of a sperm help to digest the egg's zona pellucida (jelly coat).
• The hollow ball of cells with an inner cell mass formed during early development is the blastula.
• The cavity within the blastula is called the blastocoel.
• The outer layer of the gastrula, the ectoderm, will give rise to the nervous system.
• The inner cavity of the gastrula that will eventually become the digestive tract is the archenteron.
• The opening into the archenteron is the blastopore.
• The process by which one tissue influences the development of another is called induction.
• The movement of cells that changes the shape and form of body parts during development is morphogenesis.
• Cellular differentiation is the specialization of cells due to the activation of different genes.

Mendel's Laws and Beyond

• Mendel's work on inheritance was based on his studies using garden peas.
• According to the Law of Segregation, each gamete contains only one factor from each pair of factors.
• The physical expression of a genotype is the phenotype.
• A cross between two individuals heterozygous for one trait (e.g., Tt x Tt) is called a monohybrid cross.
• To determine the genotype of an individual with a dominant phenotype, a testcross is performed.
• When a gene has more than two possible alleles, it is referred to as multiple allelic traits.
• The AB blood type in humans, where both A and B antigens are expressed, is an example of codominance.
• A pink flower resulting from a cross between a red flower and a white flower is an example of incomplete dominance.
• Genes that are carried on the X chromosome are referred to as X-linked genes.
• A trait controlled by multiple genes, such as skin color in humans, is an example of polygenic inheritance.
• Genes located on the same chromosome that tend to be inherited together are called linked genes.