Reading Essentials for Biology Glencoe (794133), страница 28
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In a diploidcell the two chromosomes of each pair are called homologous(hoh MAH luh gus) chromosomes. Each chromosome of thepair has genes for the same traits in the same order.Because there are different alleles for the same gene, it ispossible that two chromosomes of a homologous pair will not beidentical to each other. For example, garden peas have 14 chromosomes, or seven pairs.
The pairs are numbered 1 through 7.Chromosome 4 has genes for three of the traits Mendel studied.Every pea plant has two copies of chromosome 4, one from eachparent. Chromosome 4 contains the allele for height. As you knowfrom the previous section, it is possible for one chromosome tocontain T (tall), and the other chromosome to contain t (short).The two chromosomes in a homologous pair might not be identical.How can an organism pass on half its chromosomes? It does soin a process of cell division called meiosis.
Meiosis (mi OH sus)produces gametes containing half the number of chromosomes asthe parent’s body cell.Meiosis is actually two separate divisions. The first division ismeiosis I, and the second is meiosis II. The whole process beginswith one diploid cell and ends with four haploid cells. The haploidcells are gametes, or sex cells. Male gametes are called sperm.Female gametes are called eggs. When a sperm unites with, orfertilizes an egg, the resulting zygote has a diploid number ofchromosomes. Then, using mitosis, which you studied earlier, thezygote becomes a multicellular organism. This way of reproducing, which involves producing and then uniting haploid sex cells,is called sexual reproduction.Before meiosis begins, thechromosomes in a cell arereplicated, or copied.
Eachchromosome consists of twosister chromatids connectedby a centromere. A cell inprophase I of meiosis actssimilarly to a cell in prophaseof mitosis. The DNA of theREADING ESSENTIALSProphase IMetaphase I________________________________________________________________________Anaphase ITelophase IMeiosis IThe Phasesof Meiosis3. What does meiosisproduce?Meiosis IICopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.How does a diploid cell become a haploid cell?Prophase IIMetaphase IIAnaphase IITelophase IIChapter 10109NameDateClassSectionMeiosis, continued4.
What happens duringcrossing over?________________________________________________________________________chromosomes coils up and a spindle forms. The chromosomespair up with their homologous chromosomes near the middle ofthe cell. This pairing brings the two chromatids of each chromosome close together, making what is called a tetrad.
The homologous chromosomes pair so tightly that sometimes a piece of achromatid can break off. The piece changes places with a piece ofthe chromatid from the other homologous chromosome of thetetrad. This exchange is called crossing over. Crossing over canoccur at several places at the same time.During metaphase I of meiosis, the centromere of each chromosome becomes attached to the spindle fiber. The spindle fiberspull the chromosomes into the center of the cell. The spindlelines up the tetrads. If the cell were a globe, it would be as if onepair of sister chromatids were lying horizontally above the equator, and the other set of sister chromatids were lying horizontallybelow the equator.
Let’s stick with our globe illustration. Duringanaphase I the chromosomes begin to move apart from eachother. It is as if the chromosomes above the equator move to theNorth Pole, and the chromosomes below the equator move to theSouth Pole. During telophase I of meiosis, the spindle is brokendown and the chromosomes uncoil. The cytoplasm divides intotwo new cells.How are haploid cells formed?Each cell has half the genetic information that the original cellhad because it has only one chromosome from each homologouspair. But remember that the chromosomes copied themselves atthe beginning of the process. The new cells are going to have todivide in order to create haploid cells.
This second division iscalled meiosis II. During prophase II, a spindle forms in each ofthe new cells. The spindle fibers attach to the centromeres of thechromosomes. The chromosomes, still made up of sister chromatids, are pulled to the center of the cell where they line up randomly during metaphase II. Anaphase II begins as the centromereof each chromosome splits. This allows the sister chromatids toseparate and move to opposite poles of the cell. Nuclei re-form,the spindle breaks down, and the cytoplasm divides duringtelophase II.
There are four haploid cells, each cell containingone chromosome from each homologous pair. These haploid cellsbecome gametes, passing on the genes they contain to offspring.The events of meiosis II are the same as those you studiedfor mitosis, except the chromosomes do not replicate beforethey divide.110Chapter 10READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.10.2NameDateClassSection10.2Meiosis, continuedMeiosis Provides for Genetic VariationAs you can see, the cell division that happens during meiosiscreates many possible gene combinations. When crossing overoccurs, even more variations are possible. The reassortment ofgenetic information that occurs during meiosis is called geneticrecombination.
It is a major source of variation among organisms. Meiosis also provides the physical basis for explainingMendel’s results. Mendel’s laws and meiosis provide the foundation for heredity.5. How does meiosis providefor genetic variation?________________________________________________________________________Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.NondisjunctionMeiosis usually happens without any problems, but sometimeschromosomes do not separate correctly. When this happens, thegametes that form will either have too many chromosomes or notenough chromosomes.
The failure of chromosomes to separatecorrectly is called nondisjunction.An organism with extra chromosomes may survive, but anorganism that is missing one or more chromosomes does not usually develop. Surprisingly, in plants, extra chromosomes can actuallybe helpful. Often the flowers and fruits are larger, and the plant ishealthier. Because of this, plant breeders have learned to causenondisjunction by using chemicals.6.
What is nondisjunction?________________________________________________________________________Gene Linkage and MapsGenes on the same chromosome are usually linked and inherited together instead of independently. It is the chromosomes thatfollow Mendel’s law of independent assortment, not the genes.Linked genes can separate as a result of crossing over. Scientistshave found that genes that are farther apart on a chromosometend to cross over more often than genes that are close together.Using this information, scientists can make chromosome mapsthat show the sequence of genes on a chromosome.READING ESSENTIALSChapter 10111NameDateClassSection10.2Meiosis, continued◗ After You ReadMini Glossarycrossing over: pieces of homologous chromosomes change places during prophase I ofmeiosis; results in new allele combinationson a chromosomemeiosis (mi OH sus): type of cell division whereone specialized body cell produces fourgametes, each containing half the numberof chromosomes as a parent’s body celldiploid: cell with two of each kind of chromosomenondisjunction: failure of homologous chromosomes to separate correctly during meiosis;results in gametes with too many or too fewchromosomesegg: haploid female sex cell produced by meiosisgenetic recombination: reassortment of geneticinformation during meiosis that createsmany possible gene combinationshaploid: cell with one of each kind of chromosomehomologous (hoh MAH luh gus) chromosome:each chromosome pair contains genes forthe same traits arranged in the same ordersexual reproduction: pattern of reproductionthat involves producing and subsequentuniting of haploid sex cellssperm: haploid male sex cell produced by meiosis1.
Review the terms and their definitions in the Mini Glossary above. Then list the four termsthat have to do with genetic variation on the lines below.____________________________________________________________________________________2. Fill in the boxes below explaining how each of the topics relates to meiosis.MeiosisGenetic VariationNondisjunction112Chapter 10SexualReproductionMendel's ResultsCell DivisionREADING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.____________________________________________________________________________________NameDateClassSection10.2Meiosis, continued3. In column 1 are some new concepts you learned about in this section. Column 2 gives oneexample of each concept. Put the letter of the example on the line next to the concept thatit matches.New ConceptExample_____1.
meiosis_____2. nondisjunction_____3. crossing overc. one diploid cell produces four haploid cells_____4. Mendel’s resultsd. the failure of chromosomes to separate properlyduring meiosisa. explained by the distribution of chromosomesduring meiosisb. exchange of genetic materialCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Visit the Glencoe Science Web site at science.glencoe.comto find your biology book and learn more about meiosis.READING ESSENTIALSChapter 10113NameDateClassSection11.1 DNA: The Molecule of HereditySC.F.2.4.2 The student knows that every cell contains a “blueprint” coded in DNA molecules that specify how proteinsare assembled to regulate cells.
Also covers SC.F.1.4.1, SC.F.1.4.5, SC.G.1.4.3◗ Before You ReadHave you ever used a combination to open a lock? Most combination locks have the numbers1 through 50. With those numbers there are many possible combinations. Imagine a combination lockwith just the numbers 1 through 4 on it. On the lines below write as many combinations of those fournumbers as you can think of. This will help you understand how DNA can carry so much information.______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnIdentifyDetails Highlight each question head.
Then use anothercolor to highlight the answerto the question.1. What three items form anucleotide?________________________________________________________________________114Chapter 11Living things contain proteins. Your skin, muscles, and bonescontain protein. All the actions you perform such as eating, running, and thinking depend on proteins called enzymes.
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