bio chap 19
Genes occur in variant versions, called _______
alleles
The gene dictating pea color, for ex., exists in two "flavors"- one that directs the production of yellow peas and one that directs production of green peas. Such alternative versions of a gene are now called ______, and the whole collection of alleles possessed by an individual-its genetic makeup- is called its _______
alleles genotype
_______ genetic info through sexual reproduction can help a species survive in an unpredictably variable environment
reshuffling
By holding homologous chromsomes together during prophase I, the chiasmata help ensure that the maternal and parernal homologs will _______ from one another correctly at the first meiotic division
segregate
The most notable diff. between the maternal and paternal chromosomes set is the ___ chromosomes, which in some species, distinguish males from females.
sex
_______ ________ is favored because males can serve as a genetic filtering device: the males who succeed in mating allow the best, and only the best, collections of genes to be passed on, whereas males who fail to mate serve as a genetic "_____ ____"-a way of discarding bad collections of alleles from the population.
sexual reproduction trash can
Crossing-over occurs between the duplicated maternal and paternal chromosomes in each _____
bivalent
Meiosis requires the _____ of duplicated homologous chromosomes
pairing
each diploid cell that divides by mitosis produces _ diploid cells. Although mitosis and meiosis II are usually accomplished within hours, meiosis I can last days, months, or even years, because of the long time spent in ______ _
2 prophase I
Each diploid cell, therefore carries _ copies of every gene(except for those found on the ___ chromosomes, which may be present in only one copy). The specialized cells that perform the central process in sexual reproduction-the ___ cells, or gametes-are _____: they each contain only one set of chromosomes.
2 sex germ haploid
Between _ and _ crossovers occur on average between each pair of human homologs, generating new chromosomes with novel assortments of maternal and paternal alleles. Because crossing-over occurs at more or less random sites along the length of a chromosome, each meiosis will produce _ sets of entirely novel chromosomes
2 and 3 4
When this abnormal gamete fuses with a normal gamete at fertilization, the resulting embryo contained _ copies of choromosome 21 instead of _. This chromrosome inbalance produces and extra dose of the _______ encoded by chromosome 21 and thereby interferes with the proper development of the embryo and normal functions in the adult.
3 2 proteins
About __% of all human cancers contain such dominant, gain-of-function mutations in the Ras gene.
30
Mendel chose to follow _ traits- such as flower color and pea shape- that were distinct, easily observable, and most importantly, inherited in a discrete fashion: for ex., the plants have either purple flowers or white flowers- nothing in between.
7
Mendel tried his seven pea characters in various pairwise combinations and always observed a characteristic _____ phenotypic ratio in the F2 generation. The independent segregation of each pair of alleles during gamete formation is Mendels second law- the law of _______ ________
9:3:3:1 independent assortment
homologous recombination is a type of repair that uses info from an intact ___ ____ ____ to restore the correct nucleotide sequence to a damaged , newly duplicated _____. A similar process takes place when homologous chromosomes pair during the long ______ of the first meiotic division
DNA double helix homolog prophase
The maternal and paternal homologs will separate during meiotic division _, and the individual sister chromatids will separate during meiotic division _.
I II
at _______ of meiosis I, the cohesins holding the arms of the sister chromatids together are suddenly degreaded, allowing the homologs to be separated. Cohesins in the centromere continue to hold the sister chromatids together as the ______ are pulled apart.
anaphase homologs
at the start of ______ _, the cohesion proteins that hold the chromosome arms together are suddenly degraded. This release allows the arms to separate and the recombined homologs to be pulled apart. This release is necessary because if the arms did not come apart, the duplicated maternal and paternal homologs would remain tethered to one another by the homologous ___ ______ they had exchanged
anaphase I DNA segments
. In meiosis, however, the recombination occurs between the non-sister chromatids in each ______, rather than between the identical sister chromatids within each duplicated chromosome. the maternal and paternal homologs end up physically swapping homologous chromosomal segments in a complex, multistep process called _____-_____
bivalent crossing-over
each gamete contains the maternal versions of some chromosomes and the paternal versions of others. This random assortment depends solely on the way each ______ happens to be positioned when it lines up on the spindle during ________ of meiosis I.
bivalent metaphase
Meiosis involves one round of DNA replication followed by 2 rounds of ____ ______
cell division
Most bivalents contain more than one ______, indicating that multiple crossovers occur between homologous chromosomes.
chiasma
mitosis and meiosis both begin with a round of _______ ________
chromosome duplication
MITOSIS 2N (diploid somatic cell) -> _______ ________ -> 4N -> ______ ______ -> 2N 2N (2 diploid cells)
chromosome duplication cell division
the independent assortment of the maternal and paternal homologs during meiosis produces 2^n different haploid gametes for and organism with n ________
chromosomes
. In addition to the chiasmata, which hold the maternal and paternal homologs together, _____ ______ keep the sister chromatids glued together along their entire length at meiosis I
cohesion proteins
If 2 genes are far enough away from each other on the same chromosome, they will also sort independently, because of the _____-_____ that occurs during meiosis.
crossing-over
because several ________ events occur randomly along each chromosome during prophase of meiosis I, 2 genes on the same chromosome will obey mendels law of independent assortment if they are far enough apart.
crossover
Mendel deliberately simplified the problem of heredity by starting with breeding experiments that focused on the inheritance of one trait at a time, called monohybrid crosses. In the simplest situation, a ______cross, mendel followed the inheritance of 2 traits at once: for ex., pea color and pea shape.
dihybrid
For almost all multicellular animals, including vertebrates, practically the whole life cycle is spent in the ______ state.
diploid
In mitosis, this duplication is followed by a single round of cell division to yield 2 _____ cells
diploid
in meiosis, 2 cell divisions are required after chromosome duplication to produce haploid cells. Each ______ cell that enters meiosis therefore produces 4 haploid cells
diploid
Sexual reproduction involves both _____ and _____ cells
diploid and haploid
MEIOSIS 2N (______ ____-____ cell) -> chromosome duplication -> 4N -> cell division -> 2N 2N -> cell division -> N N N N (4 ____ cells)
diploid germ-line cell haploid
What makes individuals within a species genetically unique is the inheritance of different combinations of alleles. And with its cycles of ______, meiosis, haploidy, and cell _____, sex breaks up old combinations of alleles and generates new ones.
diploidy fusion
The synaptonemal complex also helps space out the crossover events that take place along each chromosome. By the time prophase I ends, the synaptonemal complex has ________, allowing the homologs to separate along most of their length. But each bivalent remains held together by at least one ______, a structure named after the Greek letter chi, X, which is shaped like a cross
disassembled chiasma
what is the moving apart of bivalent chromsomes at the first anaphase of meiosis.
disjunction
gain-of-function mutations are usually ______. For ex., certain mutations in the Ras gene generate a form of the protein that is always active. Because the normal Ras protein is involved in controlling cell proliferation, the mutant protein drives cells to multiply inappropriately, even in the absence of signals that are normally required to stimulate cell division-thereby promoting the development of cancer
dominant
______ _______- for ex.- a disorder associated with cognitive disability and characteristic physical abnormalities- is caused by an extra copy of chromosome __. This error results from nondisjunction of a chromosome 21 pair during _______ _, giving rise to a gamete that contains 2 copies of that chromosome instead of one
down syndrome 21 meiosis I
In the first step of meiosis, all of these chromosomes are ________, and the resulting copies remain closely attached to each other, as they would during an ordinary mitosis.
duplicated
Before a diploid cell divides by mitosis, it ________ its 2 sets of chromosomes. This duplication allows a full set of chromosomes-including a complete maternal set plus a complete paternal set-to be transmitted to each daughter cell.
duplicates
. If 2 parents produce many offspring with a wide variety of gene combination, they increase the odds that at least one of their progeny will have a combination of features necessary for survival in a variety of __________ conditions. They are more likely, for ex., to survive infections by bacteria, viruses, and parasites, which themselves continually change in a never-ending evolutionary battle. This genetic gamble may explain why even unicellular organisms, such as yeasts, intermittently indulge in a simple form of sexual reproduction. Typically, they switch on this behavior as an alternative to ordinary cell division when times are hard and starvation looms.
environmental
Yeasts with a genetic defect that makes them unable to reproduce sexually show a reduced ability to _____ and adapt when they are subjected to harsh conditions. in any population, new mutations continually occur, giving rise to new ______- and many of these new mutations may be harmful.
evolve alleles
in division I of meiosis, duplicated maternal and paternal homologs pair long before lining up at the metaphase plate. The maternal and paternal homologs separate during the ____ meiotic division, and the sister chromatids separate during ____ __
first meiosis II
Mutations that increase the activity of a gene or its product, or result in the gene being expressed in inappropriate circumstances, are called ___-__-_____ mutations
gain-of-function
Sexual reproduction also generates genetic diversity through a second mechanism called _______ ________. This process scrambles the genetic information on each chromosome during ______.
genetic recombination meiosis
In sexually reproducing animals, diploid ____-____ cells, which are specified early in development, give rise to haploid _______ by meiosis.
germ-line gametes
Pairing ensures that the homologs will segregate properly during the 2 subsequent cell divisions and that each of the final gametes will receive a complete _______ set of chromosomes
haploid
Thanks to this random reassortment of maternal and paternal homologs, as individual could in principle produce 2n genetically different gametes, where n is the ______ number of chromosomes. With __ chromosomes to choose from, each human, for ex., could in theory produce 2^23- or 8.4 x 10^6- genetically distinct gametes
haploid 23
In meiosis, chromosome duplication in a diploid germ-line cell is followed by 2 rounds of cell division, without further DNA replication, to produce 4 ______ cells. N represents the number of ________ in the haploid cell.
haploid chromosome
Together, the 2 successive meiotic cell divisions, called meiotic division I (meiosis I) and meiotic division II (meiosis II), parcel out one complete set of chromosomes to each of the 4 _______ cells produced. Because the assignment of each homolog to the haploid daughter cells is random, each of the resulting gametes will receive a different mixture of maternal and paternal chromosomes. Thus, meiosis produces 4 cells that are genetically dissimilar and that contain exactly ____ as much chromosomes as the original parent germ-line cell. Mitosis, in contrast, produces _ genetically identical daughter cells.
haploid half 2
in meiosis, however, the need to halve the number of chromosomes introduces an extra demand on the cell-division machinery. To ensure that each of the 4 haploid cells produced by meiosis will receive a single sister chromatid from each chromosome set, a germ-line cell must keep track of both the maternal and paternal _______ _________(homologs). It does so by _______ the duplicated homologs before they line up at the metaphase plate. Each pairing forms a structure called a _______, in which all 4 sister chromatids stick together until the cell is ready to divide
homologous chromosomes pairing bivalent
In sexually reproducing organisms, the pairing of the maternal and paternal chromosomes is accompanied by ______ ________, a process in which 2 identical or very similar nucleotide sequences exchange genetic information.
homologous recombination
The maternal and paternal versions of every chromosome-called the maternal and paternal _______-carry the same set of genes
homologs
Two kinds of genetic reassortment generate new chromosome combinations during meiosis.
independent assortment crossing-over
genes that lie far enough apart on the same chromosome will segregate _________
independently
When the F1 plants were then allowed to self-fertilize, the results were clear:the 2 alleles for seed color segregated _______ from the 2 alleles for seed shape, producing 4 different pea _______: yellow-round, yellow-wrinkled, green-round, and green wrinklied
independently phenotypes
This precursor cell lineage, which is dedicated solely to the production of germ cells, is called the germ ____. The cells forming the rest of the animals body-the ______ cells-ultimately leave no progeny of their own. They exist, in effect, only to help the cells of the germ line survive and propagate.
line somatic
Mutations that reduce or eliminate the activity of a gene are called ____-___-_______ mutations
loss-of-function
crossing-over during _____ ______ _ exchanges segments of DNA between homologous chromosomes and thereby reasserts genes on each individual chromosome. Both independent chromosome assortment and crossing-over occur during every meiosis.
meiotic prophase I
during meiosis, duplicated homologous chromosomes pair before lining up on the _____ ______
meiotic spindle
In mitosis, the duplicated chromosomes line up, single file, at the ________ plate. As mitosis continues, the sister chromatids separate and are segregated into one or other of the 2 daughter cells.
metaphase
in _______ of meiosis I, chiasmata created by crossing-over hold the maternal and paternal homologs together. At this stage, cohesion proteins keep the sister chromatids glued together along their entire length. The _________ of sister chromatids function as a single unit in meiosis I, and microtubules that attach to them point toward the same spindle pole
metaphase kinetochores
In mitosis, the individual duplicated maternal (M) and paternal (P) chromosomes line up independently at the _______ _____; each consists of a pair of ______ _______, which will separate just before the cell divides
metaphase plate sister chromatids
whether the maternal or paternal homolog is captured by the microtubules from one pole or the other depends on which way the bivalent is facing when the ________ connect to its kinetochore.
microtubules
Each chiasma corresponds to a crossover between 2 ___-_____ _____
non-sister chromatids
_______ : homologs fail to separate properly. As a result, some of the haploid cells that are produced lack a particular chromosome, while others have more than one copy of it. Upon fertilization, such gametes form abnormal______, most of which die. Some survive, however.
nondisjunction embryos
meiosis generates 4 _________ haploid cells, whereas mitosis produces 2 ________ diploid cells
nonidentical identical
For most organisms, the males and females produce diff. types of gametes. In animals, one is large and _______ and is referred to as the egg; the other is a small and motile and is referred to as the _____. These 2 dissimilar haploid gametes join together to regenerate a diploid cell, called the fertilized egg, or _____, which has chromosomes from both the mother and father.
nonmotile sperm zygote
In human _____- the cells that give rise to the egg- and average of 2 or 3 crossover events occur within each bivalent.
oocytes
the frequency of chromosome mis-segregation during the production of human gametes is remarkably high, particularly in females: nondisjunction occurs in about 10% of the meiosis in human _______, giving rise to eggs that contain the wrong number of chromosomes (a condition called _______)
oocytes aneuploidy
Meiosis begins in specialized diploid germ-line cells that reside in the ______ or ______. Like somatic cells, these germ-line cells are _____; each contains 2 copies of every chromosome-a paternal homolog, inherited from the organisms father, and a maternal homolog, inherited from its mother
ovaries testes diploid
The next phase of the process, however, is unique to meiosis. Each duplicated paternal chromosome first locates and then attaches itself to the corresponding duplicated maternal homolog, a process called ______.
pairing
Mendel chose to study ___ _____ because they are easy to cultivate in large numbers and could be raised in a small space
pea plants
Geneticists that follow the inheritance of specific traits in humans get around this problem by working with large numbers of families- or with several generations of a few large families- and preparing ______ that show the phenotype of each family member for the relevant trait
pedigrees
An organism in which both alleles of a gene bear loss-of-function mutations will generally display an abnormal ______-one that differs from the most commonly occurring phenotype
phenotype
The appearance, or ______, of the organism depends on which versions of each allele in inherits.
phenotype
before anaphase I, the 2 ____ of the spindle pull on the duplicated homologs in opposite directions, and the chiasmata resist this pulling. In so doing, the chiasmata help to position and stabilize bivalents at the _____ ____
poles metaphase plate
These haploid gametes are generated from diploid ______ cells by a specialized form of reductive division called ______.
precursor meiosis
genetic differences are initiated long before sperm meets egg, when meiosis I produces 2 kinds of randomizing genetic _________
reassortment
Before a eukaryotic cell divides-by either meiosis or mitosis- it first duplicates all of its chromosomes. The twin copies of each duplicated chromosome, called ____ ________, at first remain tightly linked along their length.
sister chromatids
______ cells form the body of the org. and are therefore necessary to support sexual reproduction, but they themselves leave no progeny
somatic
somatic cells+germ-line cells v meiosis v haploid gametes (____ and _____) v fertilization v _____ _____ v mitosis v diploid organism
sperm and egg diploid zygote
The chiasmata created by crossover events keep the maternal and paternal homologs bundled together until the ______ seperates them during meiotic anaphase I.
spindle
Crossing-over is facilitated by the formation of a ________ complex. As the duplicated homologs pair, this elaborate protein complex helps to hold the bivalent together and align the ______ so that strand exchange can readily occur between the non-sister chromatids. Each of the chromatids in a duplicated homolog can form a crossover with either (or both) of the chromatids from the other chromosome in the bivalent.
synaptonemal homologs
Not all genes segregate independently as per mendels second law. If genes lie close together on a chromosome, they are likely to be inherited as a ___. These measurements of genetic linkage have been used to map the relative positions of the genes on each chromosome of many organisms. Such _____ ____ have been crucial for isolating and characterizing mutant genes responsible for human genetic diseases such as cystic fibrosis.
unit genetic maps
