Chapter 13 Text

The transmission of traits from one generation to the next is called __, or __.
Differences between members of the same species: __.
Along with inherited similarity, there is also __.
__ is the scientific study of heredity and hereditary variation.

Parents endow their offspring with coded info in the form of hereditary units called __. Our __ program the specific traits that emerge as we develop from fertilized eggs to adults.

The genetic program is written in the language of __, the polymer of four different nucleotides. Inherited info is passed on in the form of each gene's specific sequence of __nucleotides. Most genes program cells to synthesize specific enzymes and other proteins, whose cumulative action produces an organism's inherited traits. The programming of these traits in the form of __ is one of the unifying themes of biology.

The transmission of hereditary traits has its molecular basis in the precise replication of DNA, which produces copies of genes that can be passed along from parents to offspring. In animals and plants, reproductive cells called __ are the vehicles that transmit genes from one generation to the next. During __, male and female __ unite, thereby passing on genes of both parents to their offspring.

The DNA of a eukaryotic cell is packaged into __ within the nucleus, except for small amts. of DNA in mito. and chloro. One __ consists of a single long DNA molecule elaborately coiled in association with various proteins. One includes several hundred to a few thousand genes, each of which is a specific sequence of nucleotides within the DNA molecule. A gene's specific location along the length of a chromosome is called the gene's __.

Only organisms that reproduce __ produce offspring that are exact copies of themselves. In __, a single individual is the sole parent and passes copies of all its genes to its offspring. The __of the offspring are virtually exact copies of the parent's __. Some multiceullular organisms are also capable of reproducing __. Because the cells of the offspring are derived by mitosis in the parent, the "chip off the old block" is usually genetically identical to its parent. AN individual that reproduces asexually gives rise to a __, a group of genetically identical individuals. Genetic differences occasionally arise in asexually reproducing organisms as a result of changes in the DNA called __.

In __, two parents give rise to offspring that have unique combinations of genes inherited from the two parents. In contrast to a __, offspring of __ vary genetically from their siblings and both parents: They are variations on a common theme of family resemblance, not exact replicas. Genetic variation is an important consequence of __.

A __ is the generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.

In humans, each __- any cell other than those involved in gamete formation- has __ chromosomes.
Because chromosomes differ in size, in the positions of their __, and in the pattern of colored bands produced by certain stains, they can be distinguished from one another by microscopic examination when sufficiently condensed.

Careful examination of a micrograph of the 46 human chromosomes from a single cell in mitosis reveals that there are two chromosomes of each of __ types. This becomes clear when images of the chromosomes are arranged in pairs, starting with the longest. The resulting ordered display is called a __. The two chromosomes composing a pair have the same length, centromoere position, and staining pattern: These are called __. Both chromosomes of each pair carry genes controlling the same inherited characters.

The two distinct chromosomes referred to as _ and _ are an important exception to the general pattern of __ in human __ cells. Human females have a homologous pair of __ chromosomes (XX), but males have one _ and _ chromosome (XY). Only small parts of the X and Y are homologous. MOst of the genes carried on the X chromosome do not have counterparts on the tiny Y, and the Y chromosome has genes lacking on the X. Because they determine an individual's sex, the X and Y chromosomes are called __. The other chromosomes are called __.

Any cell with two chromosome sets is called a __, and has a diploid number of chromosomes. For humans, it is 46.

In a cell in which DNA synthesis has occurred, all the chromosomes are replicated, and therefore each consists of two identical __, associated closely at the __ and along the __.

Unlike somatic cells, __ contain a single chromosome set. SUch cells are called __ , and each has a haploid number of chromosomes. For humans, it is 23. The set of 23 consists of the __ plus a single __. AN unfertilized __ contains an X chromosome, but a __ may contain an X or a Y chromosome.

The human life cycle begins when a __ from the father fuses with a __ from the mother. This union of gametes, culminating in fusion of their nuclei, is called __. The resulting fertilized egg, or __, is __ because it contains two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines. As a human dvelops into a sexually mature adult, mitosis of the __ and its descendants generates all the __cells of the body. Both chromosome sets in the __ and all the genes they carry are passed with precision to the __ cells.

THe only cells of the human body not produced by mitosis are the __, which develop from specialized cells called __in the gonads- testes and ovaries.
In sexually reproducing organisms, the gametes are formed by a modified type of cell division called __. This type of cell division reduces the number of sets of chromosomes from two to one in the gametes, counterbalancing the doubling that occurs at __. In animals, __ occurs only in the ovaries or testes. As a result of meiosis, each human sperm and egg is __. __ restores the __ condition by combining two __ sets of chromosomes and the human life cycle is repeated, generation after generation.

In general, the steps of the human life cycle are typical of many sexually reproducing animals. Indeed, the processes of __ and __ are the unique trademarks of __, in plants as well as animals. __ and __ alternate in sexual life cycles, maintaining a constant number of chromosomes in each species from one generation to the next.

Although the alternation of __ and __ is common to all organisms that reproduce sexually, the timing of these two events in the life cycle varies, depending on the species. These variations can be grouped into __ main types of life cycles. In the type that occurs in humans and most other animals, __ are the only haploid cells. Meiosis occurs in germ cells during the prodcution of __, which undergo no further cell division prior to __. After fertilization, the __ divides by mitosis, producing a multicellular organism that is __.

Plants and some species of algae exhibit a second type of life cycle called __. This type includes both __ and __ stages that are multicellular. The multicellular diploid stage is called the __. Meiosis in the sporophyte produces haploid cells called __. Unlike a gamete, a haploid spore doesn't fuse with another cell but divides mitotically, generating a multicellular haploid stage called the __. Cells of the __ give rise to gametes by mitosis. Fusion of two haploid gametes at fertilization results in a diploid zygote, which develops into the next __ generation. Therefore, in this type of life cycle, the __ produces a __ as its offspring, and the __ generation produces the next __. Clearly, the term __ is a fitting name for this type of life cycle.

A third type of life cycle occurs in most fungi and some protists, including some algae. After __ fuse and form a diploid __, meiosis occurs without a multicellular diploid offspring developing. Meiosis produces not __ but __ cells that then divide by mitosis and give rise to either unicellular descendants or a __ multicellular adult organism. Subsequently, the __ organism carries out further mitoses, producing the cells that develop into __. The only diploid stage found in these species is the single-celled __.

Note that either __ or __ cells can divide by mitosis, depending on the type of life cycle. Only __ cells, however, can undergo meiosis because __ cells have a single set of chromosomes that cannot be further reduced. Though the three types of sexual life cycles differ in the timing of meiosis and fertilization, they share a fundamental result: __ among offspring.

Meiosis, like mitosis, is preceded by the __.
However, this singlie replication is followed by not one but two consecutive cell divisions, called __ and __. These two divisions result in __ daughter cells (rather than the __ daughter cells of mitosis), each with only half as many chromosomes as the parent cell.

Sister chromatids are two copies of __ chromosome, closely associated all along their lengths; this asociation is called __. Together, the sister chromatids make up one replicated chromosome. In contrast, the two chromosomes of a __ are individual chromosomes that were inherited from different parents.

__ appear like in the microscople, but they have different versions of genes, called __, at corresponding __. __ are not associated with each other except during meiosis.

__ halves the total number of chromosomes in a very specific way, reducing the number of sets from two to one, with each daughter cell receiving one set of chromosomes.

What is the difference between mitosis and meiosis?

Three events unique to meiosis occur during meiosis I: __, __, __

__: During prophase I, replicated homologs pair up and become physically connected along their lengths by a zipper-like protein structure, the __; this process is called __. Genetic rearrangement between nonsister chromatids, known as __, is completed during this stage. Following disassembly of the __ in late prophase, the two homologs pulla part slightly but remain connected by at lease one X-shaped region called a __, the physical manifestation of crossing over; it appears as a cross because sister chromatid cohesion still holds the two original sister chromatids together, even in regions where one of them is now part of the other homolog. __ and __ usually do not occur during mitosis.

__: At metaphase I of meiosis, chromosomes are positioned on the metaphase plate as pairs of homologs, rather than individual chromosomes, as in metaphse of mitosis.

__: At anaphase I of meiosis, the replicated chromosomes of each homologous pair move tward opposite poles, but the sister chromatids of each replicated chromosome remain attached. In anaphase of mitosis, by contrast, sister chromatids separate.

Sister chromatids are attached along their lengths by protein complexes called __. In mitosis, this attachment lasts until the end of __, when enzymes cleave the __, freeing the sister chromatids to move to opposite poles of the cell. In meiosis, __ is released in two steps. In metaphase I, __ are held together by cohesion between sister chromatid arms in regions where DNA has been exchanged. At __, cohesins are cleaved along the arms, allowing homologs to separate. At __, cohesins are cleaved at the centromeres, allowing chromatids to separate.

Meiosis I is called the __ beacuse it halves the number of chromosome sets per cell- a reduction from two sets (the __) to one set (the __). During the second meitoic division, meiosis II( sometimes called the __), the sister chromatids separate, producing haploid daughter cells. The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis.

__ are the original source of genetic diversity. These changes in an organism's DNA create the different versions of genes known as __. Once these differences arise, reshuffling of the __ during sexual reproduction produces the variation that results in each member of a species having its own unique combo of traits.

What are three mechanisms that contribute to the genetic variation arising from sexual reproduction?

One aspect of sexual reproduction that generates genetic variation is the what?
At __, the homologous pairs, each consisting of one maternal and one paternal chromosome, are situated on the __. Each pair may orient with either its __ or __ closer to a given pole- its orientation is as random as a flip coin. Thus, there is a __ chance that a particular daughter cell of __ will get the maternal chromosome of a certain homologous pair and a __ chance that it will get the paternal chromosome.

Because each homologous pair of chromosomes is positioned independently of the other pairs at __, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair. This is called __. Each daughter cell represents one outcome of all possible combinations of maternal and paternal chromosomes.

The number of combinations possible for daughter cells formed by meiosis of a __ cell with two homologous pairs of chromosomes is __. Only two of the four combos of daughter cells would result from __ of a single diploid cell, because a single parent cell would have one or the other possible chromosomal arrangement at __, but not both. However, the pop. of daughter cells resulting from meiosis of a large # of dipolid cells contains all four types in approx. equal numbers.

Crossing over produces __, individual chromosomes that carry genes derived from two different parents. In meiosis of humans, an avg. of one to three __ events occur per chromosome pair, depending on the size of the chromosomes and the position of their __.

Crossing over begins very early in __,as __ chromosomes pair loosely along their lengths. Each gene on one __ is aligned precisely with the corresponding gene on the other __. In a single crossover event, specific proteins orchestrate an exchange of corresponding segments of two __ chromatids- one matermal and one paternal chromatid of a homologous pair. In this way, crossing over produces chromosomes with new combos of maternal and paternal __.

In humans and most other organisms, crossing over also plays an essential role in what?
A __ forms as the result of a crossover occuring while sister chromatid cohesion is present along the arms.
__ hold homologs together as the spindle forms for the first meiotic division. During __, the release of cohesion along sister chromatid arms allows homologs to separate. During __, the release of sister chromatid cohesion at the __ allows the sister chromatids to separate.

At __, chromosomes that contain one or more __ can be oriented in two alternative, nonequivalent ways with respect to other chromosomes, because their sister chromatids are no longer identical. The dif. possible arrangements of nonidentical sister chromatids during __ further increases the # of genetic types of daughter cells that can result from meiosis.

TRUE OR FALSE:
The important point is that crossing over, by combining DNA inherited from two parents into a single chromosome, is an important source of genetic variation in sexual life cycles.

The random nature of __ adds to the genetic variation arising from meiosis.

Dif. combos of __ may work better than those that previosuly prevailed. __ are the original source of dif. __, which are then mixed and matched during meiosis.