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Telophase I

Chromosomes arrive at the spindle poles and the cytoplasm divides.

Genes come in multiple forms called alleles.

A gene that specifies a charateristic may exist in several forms, called alleles. For example, a gene for coat color in cats may exist as an allele that encodes black fur or as an allele that encodes orange fur.

Chromosomes separate through the processes of mitosis and meiosis.

The process of mitosis and meiosis ensure that a complete set of an organism's chromosomes exists in each cell that results from cell division.

Schleiden and Schwann: Cell theory

The proposal of the cell theory guided scientists to focus on the cell itself as the smallest unit of life and the one that carried the units of inheritance.

Meiosis II

The second division, meiosis II, is sometimes termed the equational division (or the reductional division) because the events in this phase are similar to those of mitosis. However, meiosis II differs from mitosis in that chromosome number has already been halved in meiosis I, and the cell does not begin with the same number of chromosomes as it does in mitosis (FIGURE 2.13).

Random Separation of Homologous Chromosomes

The second process of meiosis that contributes to genetic variation is the random distribution of chromosomes in anaphase I of meiosis following their random alignment during metaphase I. To illustrate this process, consider a cell with three pairs of chromosomes I, II, and III (FIGURE 2.17a). One chromosome of each pair is maternal in origin (Im, IIm, and IIIm); the other is paternal in origin (Ip, IIp, and IIIp). The chromosome pairs line up in the center of the cell in metaphase I and, in anaphase I, the chromosomes of each homologous pair separate. How each pair of homologs aligns and separates is random and independent of how other pairs of chromosomes align and separate (FIGURE 2.17b).

Which theory shares some conceptual overlap with pangenesis?

inheritance of acquired characteristics

What is the process of pairing and separation of chromosomes during the division of sex cells to produce gametes called?

meiosis

A major difference between eukaryotic and prokaryotic cells is the presence or absence of a defined ___, respectively.

nucleus

Molecular genetics

the subfield of biology that studies the molecular structure and function of genes

What is a Genome?

A genome is a complete set of genetic instructions for any organism. It can be either RNA or DNA The entire genome is copied during process of replication

Anaphase

After the spindle-assembly checkpoint has been passed, the connection between sister chromatids breaks down and the sister chromatids separate. This separation marks the beginning of anaphase, during which the chromosomes move toward opposite spindle poles.

Pangenesis

An incorrect idea that acquired characteristics are passed on

M Phase (Mitosis)

Prophase Prometaphase Metaphase Anaphase Telophase

Prophase

As a cell enters prophase, the chromosomes condense, becoming more compact and visible under a light microscope. A group of proteins called condensins bind to the chromosomes and bring about condensation. Because the chromosomes were duplicated in the preceding S phase, each chromosome consists of two sister chomratids attached at the centromere. The mitotic spindle, an organized array of microtubules that move the chromosomes in mitosis, forms. In animal cells, the spindle grows out from a pair of centrosomes that migrate to the opposite sides of the cell.

Eukaryote Characteristics

Both unicellular and multicellular with membrane-bound organelles.Eukaryotes can exist as a single-celled organism (e.g. many protists), a colony (e.g. Volvox) or as a multi-celled organism (e.g. plants, animals, fungi).Cells within multi-celled organisms show specialization to perform specific functions within the organism. Genetic material is surrounded in a nuclear envelope to form a nucleus.Eukaryotic cells have a nuclear envelope, which surrounds the genetic material to form a nucleus and separates the DNA from the other cellular contents. DNA is closely associated with histones to form tightly packed chromosomes.This complex of DNA and histone proteins is termed chromatin, which is the stuff of eukaryotic chromosomes. Histone proteins limit the accessibility of enzymes and other proteins that copy and read the DNA but they enable the DNA to fit into the nucleus. Eukaryotic DNA must separate from the histones before the genetic information in the DNA can be accessed.

A functional chromosome has three essential elements:

Centromere: attachment point for spindle microtubulesSpindle microtubules are the filaments responsible for moving chromosomes during cell division.Appears as a constricted region that often stains less strongly than does the rest of the chromosome.Chromosomes without a centromere cannot be drawn into the newly formed nuclei.On the basis of the location of the centromere, chromosomes are classified into four types (FIGURE 2.8) : metacentric submetacentric acrocentric telocentric Telomeres: tips of a linear chromosome They serve to stabilize the chromosome ends. Provide chromosome stability. May play roles in limiting cell division, the aging process and cancer. Origins of replication: where the DNA synthesis begins

Telophase II

Chromosomes arrive at the spindle poles Nuclear envelope re-forms around the chromosomes Cytoplasm divides Chromosomes relax and are no longer visible

Prophase II

Chromosomes recondense Spindle reforms Nuclear envelope once again breaks down

Prometaphase

Disintegration of the nuclear membrane marks the start of prometaphase. Spindle microtubules, which until now have been outside the nucleus, enter the nuclear region. The spindle microtubules are composed of subunits of a protein called tubulin (FIGURE 2.11). The ends of the microtubules make contact with the chromosome and anchor to the kinetochore of one of the sister chromatids; a microtubule from the opposite centrosome then attaches to the other sister chromatid, and so each chromosome is anchored to both of the centrosomes. The microtubules lengthen and shorten, pushing and pulling the chromosomes about. Some microtubules extend from each centrosome toward the center of the spindle but do not attach to a chromosome.

Drosophila

Drosophila: fruit fly Escherichia coli: Bacterium Caenorhabditis elegans: Nematode Thale-Cress plant House mouse Bakers yeast

Metaphase

During metaphase, the chromosomes arrange themselves in a single plane, the metaphase plate, between the two centrosomes. The centrosomes, now at opposite ends of the cell with microtubules radiating outward and meeting in the middle of the cell, center at the spindle pole. A spindle-assembly checkpoint ensures that each chromosome is aligned on the metaphase plate and attached to spindle microtubules from opposite poles.

Eukaryotic Chromosomes

Each eukaryotic species has a characteristic number of chromosomes per cell: potatoes have 48 chromosomes, fruit flies have 8, and humans have 46. There appears to be no special significance between the complexity of an organism and its number of chromosomes per cell. In most eukaryotic cells, there are two sets of chromosomes.The presence of two sets is a consequence of sexual reproduction; one set is inherited from the male parent and the other from the female parent. Each chromosome in one set has a corresponding chromosome in the other set, together constituting a homologous pair

Darwin: Evolution

Even though Darwin did not understand inheritance correctly (he was a proponent of blending inheritance) his work on evolution by natural selection helped scientists understand the importance of inheritance and how it could change over time.

Evolution is genetic change

Evolution can be viewed as a two-step process: first, genetic variation arises, and second, some genetic variants increase in frequency, whereas other variants decrease in frequency.

Mutations can cause permanent changes in genetic information that can be passed from cell to cell or from parent to offspring.

Gene mutations affect the genetic information of a single gene; chromosome mutations alter the number or the structure of chromosomes and therefore usually affect many genes.

Genetic information is carried in DNA and RNA.Genes are located on chromosomes.

Genetic information is encoded in the molecular structure of nucleic acids, which come in two types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The nitrogenous bases in DNA are of four types: adenine (A), cytosine (C), guanine (G), and the thymine (T). The sequence of these bases encodes genetic information. The vehicles of genetic information within a cell are chromosomes, which consist of DNA and associated proteins. The cells of each species have a characteristic number of chromosomes. For example, bacterial cells normally possess a single chromosome, human cells possess 46, and pigeon cells possess 80. Each chromosome carries a large number of genes.

Gregor Mendel: Principles of heredity

Gregor Mendel provided us the principles of how traits are passed on from parent to offspring. His work will be examined in the next few chapters.

Eukaryotic Chromosomes

Human cells, for example, have 46 chromosomes, comprising 23 homologous pairs. The two chromosomes of a homologous pair are usually alike in structure and size, and each carries genetic information for the same set of hereditary characteristics. (An exception is the sex chromosomes, which will be discussed in Chapter 4.) For example, if a gene on a particular chromosome encodes a characteristic such as hair color, another gene (called an allele) at the same position on that chromosome's homolog also encodes hair color. However, these two alleles need not be identical: one might produce red hair and the other might produce blond hair. Thus, most cells carry two sets of genetic information; these cell sare diploid. But not all eukaryotic cells are diploid: reproductive cells (such as eggs, sperm, and spores) and even nonreproductive cells in some organisms may contain a single set of chromosomes. Cells with a single set of chromosomes are haploid. Haploid cells have only one copy of each gene.

2.3 Sexual Reproduction Produces Genetic Variation Through the Process of Meiosis

If all reproduction were accomplished through the cell cycle, life would be quite dull, because mitosis produces only genetically identical progeny. With only mitosis, you, your children, your parents, your brothers and sisters, your cousins, and many people you didn't even know would be clones (copies of one another). Only the occasional mutation would introduce any genetic variability. This is how all organisms reproduced for the first 2 billion years of Earth's existence (and the way in which some organisms still reproduce today). Then, some 1.5 billion to 2 billion years ago, something remarkable evolved: cells that produce genetically variable offspring through sexual reproduction. The evolution of sexual reproduction is one of the most significant events in the history of life. The pace of evolution depends on the amount of genetic variation present. By shuffling the genetic information from two parents, sexual reproduction greatly increases the amount of genetic variation and allows for accelerated evolution. Most of the tremendous diversity of life on Earth is a direct result of sexual reproduction. Sexual reproduction consists of two processes. The first is meiosis, which leads to gametes in which chromosome number is reduced by half. The second process is fertilization, in which two haploid gametes fuse and restore chromosome number to its original diploid value.

Metaphase I

Initiated when homologous pairs of chromosomes align along the metaphase plate The alignment of homologous pairs of chromosomes is random

Anaphase I

Separation of homologous chromosome pairs, and the random distribution of chromosomes into two newly divided cells― second mechanism of generating genetic variation in the newly formed gametes

Interphase

Interphase is the extended period of growth and development between cell divisions. Although little activity can be observed with a light microscope, the cell is quite busy: DNA is being synthesized, RNA and proteins are being produced, and hundreds of biochemical reactions are taking place. By convention, interphase is divided into three phases: G1 Phase (Gap 1) -Period of growth; proteins necessary for cell division are synthesized -Usually lasts several hoursIf G1/S checkpoint is reached then replication will occur -Cells may exit from the active cell cycle before reaching the G1/S checkpoint in response to regulatory signals and pass into a nondividing phase called G0 S Phase (DNA Synthesis) -Chromosomes duplicate -Following S Phase, each chromosome is composed of two chromatids G2 Phase (Gap 2) -additional biochemical preparation for cell division -G2/M checkpoint: only passed if DNA is completely replicated and undamaged -After G2/M checkpoint the cell moves into M Phase

Anaphase II

Kinetochores of the sister chromatids separate Sister chromatids are pulled to opposite poles (each chromatid is now a distinct chromosome)

Interphase

Like mitosis, meiosis is preceded by an interphase stage that includes G1, S, and G2 phases. This is a time when the cell grows and replicates its DNA in preparation for meiosis in a very similar fashion to autosomal cells.

Eukaryotic Cell Reproduction

Like prokaryotic cell reproduction, eukaryotic cell reproduction requires the processes of DNA replication, copy separation, and division of the cytoplasm. However, the presence of multiple DNA molecules requires a more complex mechanism to ensure that one copy of each molecule ends up in each of the new cells.

Genetic information is transferred from DNA to RNA to protein

Many genes encode characteristics by specifying the structure of proteins. Genetic information is first transcribed from DNA to RNA, and then RNA is translated into the amino acid sequence of a protein.

Many traits are affected by multiple factors.

Many traits are affected by multiple genes that interact in complex ways with one another and with environmental factors. Human height, for example, is affected by may genes as well as by environmental factors such as nutrition.

Which theory has contributed the most to disprove the concept of blending inheritance?

Mendelian inheritance

Which of the following would likely make the best model genetic organism for studying eukaryotic gene regulation in higher animals?

Mus musculus

Crossing Over

Process in which homologous chromosomes exchange portions of their chromatids during meiosis.

Common Characteristics of Model Organisms

Short generation time Production of numerous progeny The ability to carry out controlled genetic crosses The ability to be reared in a laboratory environment The availability of numerous genetic variants An accumulated body of knowledge about their genetic systems

Metaphase II

Similar to metaphase of mitosis Sister chromatids face opposite poles

Cells are of two basic types: Eukaryotic and prokaryotic

Structurally, cells consist of two basic types, although evolutionarily, the story is more complex.

Population genetics

Study of allele frequency distribution and change under the influence of evolutionary processes.

Sutton: Genes are located on chromosomes

Sutton proposed that genes were located on chromosomes and this later led to Thomas Hunt Morgan using fruit flies to map those genes on the chromosome. This was a major shift in genetics once we started to understand the physical nature of genes and chromosomes.

Prophase I

Synapsis: close pairing of homologous chromosomes Tetrad: closely associated four-sister chromatids of two homologous chromosomes Crossing over: crossing over of chromosome segments from the sister chromatid of one chromosome to the sister chromatid of the other synapsed chromsome ― exchange of genetic information, the first mechanism of generating genetic variation in newly formed gametes

Telophase

Telophase is marked by the arrival of the chromosomes at the spindle poles. The nuclear membrane re-forms around each set of chromosomes, producing two separate nuclei within the cell. The chromosomes relax and lengthen, once again disappearing from view. In many cells, division of the cytoplasm (cytokinesis) is simultaneous with telophase.

Transmission genetics

The branch of genetics concerned with the mechanisms by which genes are transferred from parent to offspring.

Eukaryotic Cell Cycle

The cell cycle consists of two major phases. Interphase The period between cell divisions, in which the cell grows, develops, and prepares for cell division. M phase (mitotic phase)The period of active cell division. M phase includes mitosis, the process of nuclear division, and cytokinesis, or cytoplasmic division.

Chromosome structure

The chromosomes of eukaryotic cells are larger and more complex than those found in prokaryotes, but each unreplicated chromosome nevertheless consists of a single molecule of DNA. Although linear, the DNA molecules in eukaryotic chromosomes are highly folded and condensed; if stretched out, some human chromosomes would be several centimeters long (thousands of times longer than the span of a typical nucleus). To package such a tremendous length of DNA into this small volume, each DNA molecule is coiled again and again and tightly packed around histone proteins, forming the rod-shaped chromosomes. Most of the time the chromosomes are thin and difficult to observe but, before cell division, they condense further into thick, readily observed structures; it is at this stage that chromosomes are usually studied

Meiosis I

The first division, meiosis I, is also referred to as the reduction division because the number of chromosomes per cell is reduced by half (FIGURE 2.13). This happens as a result of the separation of homologous chromosome pairs.

Weismann: Germ-plasm theory

The germ-plasm theory holds that the cells in the reproductive organs carry a complete set of genetic information and is passed to the egg and sperm. This was important for many reasons not the least of which was to discredit the idea of inheritance of acquired characteristics.

Interkinesis

The nuclear membrane re-forms around the chromosomes clustered at each pole Spindle fibers break down Chromosomes relax Continues on to Prophase II Some cells do not go through interkinesis but instead move directly from Telophase I to Prophase II

Flemming : Chromosomes

The observation of chromosomes and the detailed description of mitosis helped scientists recognize that the nucleus within the cell contained the genetic material.

Genes confer phenotypes.

Traits are not inherited directly. Rather, genes are inherited, and genes, along with environmental factors, determine the expression of traits. The genetic information that an individual organism possesses is its genotype, the trait is its phenotype. For example, the albinism seen in some Hopis is a phenotype, and the information in OCA2 genes that causes albinism is a genotype.

Prokaryote Characteristics

Unicellular, no membrane-bound organelles.A bacterium does not have many of the organelles we associate with eukaryotes like a golgi apparatus, an endoplasmic reticular system or mitochondria. It does not even contain a membrane-bound nucleus. However, it does have ribosomes that are in close contact with the DNA.Prokaryotes also exist as single-celled individuals and show no degree of specialization in a group. Made up of eubacteria and archaea.Although similar in cell structure, prokaryotes include at least two fundamentally distinct types of bacteria. These distantly related groups are termed eubacteria (the true bacteria) and archaea (ancient bacteria). An examination of equivalent DNA sequences reveals that eubacteria and archaea are as distantly related to one another as they are to the eukaryotes. Although eubacteria and archaea are similar in cell structure, some genetic processes in archaea (such as transcription) are more similar to those in eukaryotes, and the archaea may actually be evolutionarily closer to eukaryotes than to eubacteria. Prokaryotic DNA does not exist in the highly ordered and packed arrangement.Eubacteria do not possess histones, so their DNA does not exist in the highly ordered, tightly packed arrangement found in eukaryotic cells. The copying and reading of DNA are therefore simpler processes in Eubacteria.Archaea do have some histone proteins that complex with DNA, but the structure of their chromatin is different from that found in eukaryotes. Prokaryote DNA is relatively smallGenes of prokaryotic cells are generally on a single, circular molecule of DNA, the chromosome of the prokaryotic cell.

Virus Characteristics

Viruses are relatively simple structures composed of an outer protein coat surrounding nucleic acid (either DNA or RNA). Viruses are neither cells nor primitive forms of lifethey can reproduce only within host cells,which means that they must have evolved after, rather than before, cells. Viruses are not an evolutionarily distinct group but are most closely related to their hostsThe genes of a plant virus are more similar to those in a plant cell than to those in animal viruses, which suggests that viruses evolved from their hosts, rather than from other viruses.The close relationship between the genes of virus and host makes viruses useful for studying the genetics of host organisms.

Genes are the fundamental unit of heredity.

We can think of a gene as a unit of information that encodes a genetic characteristic. We will expand this definition as we learn more about what genes are and how they function.

Genetic consequences of the cell cycle

What are the genetically important results of the cell cycle? From a single cell, the cell cycle produces two cells that contain the same genetic instructions. the resulting daughter cells are genetically identical with each other and with their parent cell because DNA synthesis in the S phase creates an exact copy of each DNA molecule, giving rise to two genetically identical sister chromatids. Mitosis then ensures that one of the two sister chromatids from each replicated chromosome passes into each new cell. Another genetically important result of the cell cycle is that each of the cells produced contains a full complement of chromosomes: there is no net reduction or increase in chromosome number. Each cell also contains approximately half the cytoplasm and organelle content of the original parent cell, but no precise mechanism analogous to mitosis ensures that organelles are evenly divided. Consequently, not all cells resulting from the cell cycle are identical in their cytoplasmic content.

Prokaryotic Cell Reproduction

When prokaryotic cells reproduce, the circular chromosome of the bacterium is replicated (FIGURE 2.5). The two resulting identical copies are attached to the plasma membrane, which grows and gradually separates the two chromosomes. Finally, a new cell wall forms between the two chromosomes, producing two cells, each with an identical copy of the chromosome. Under optimal conditions, some bacterial cells divide every 20 minutes. At this rate, a single bacterial cell could produce a billion descendants in a mere 10 hours.

Thomas Morgan

___ is considered to be a father of modern genetics, and did his work in the first half of the 20th century.

Which theory proposed that germ-line cells in the reproductive organs carry a complete set of genetic information?

germplasm

Which topic would be covered within the subdiscipline of population genetics?

determining the probability that a couple who has one child with a genetic disorder will have another child with the same genetic disorder determining the type of change in DNA that produced a mutation responsible for a genetic disease determining the mechanism of DNA replication in a bacterial species. effect of small numbers of individuals on changes in the frequencies of an allele study of pedigrees to understand the type of inheritance of a trait