Questions and Answers on DNA and Chromosomes

CHAPTER 13 OBJECTIVES The Basis of Heredity
1. Explain in general terms how traits are transmitted from parents to offspring.
An organism's traits and appearance are controlled by specific forms of proteins contained in DNA. When gametes are formed, the male and female parents pass along their DNA into their respective gametes. Fertilization fuses the gametes and combines the genetic contribution of each parent into the new offspring. 2. Distinguish between asexual and sexual reproduction.
Asexual Reproduction-A form of reproduction which does not involve meiosis or fertilization without fusion of gametes.
Sexual Reproduction- Has two processes: meiosis, involving halving the number of chromosomes and crossing over to get more gene combinations and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. 3. Distinguish between the following pairs of terms:
Chromosome- structures that makeup DNA
Chromatid- DNA wrapped around protein called histone
Homologous chromosomes- same but separated
Sister chromatids- same but connected by centromere
Synapsis- the entire process of pairing two homologous chromosomes
Chiasma- sites of crossing over on chromatids
Anaphase I-Centrosome does not split
Anaphase II-Centrosome splits and seperates sister chromosomes
Haploid- contains one set of chromosomes
Diploid- contains two sets of chromosomes
4. Explain how haploid and diploid cells differ from each other. State which cells in the human body are diploid and which are haploid.
Haploid=n (humans=23 chromosomes) germ cells
Diploid=2n (humans=46 chromosomes) somatic cells
5. Explain why fertilization and meiosis must alternate in all sexual life cycles.
Meiosis maintains the normal diploid number by halving the number to make the gametes and then allowing the diploid number to be restored through fertilization.
6. Distinguish among the three life-cycle patterns characteristic of eukaryotes, and name one organism that displays each pattern.
Haploid life cycle- the organism spends the majority if its life with only half the chromosomes it can have. In fact, the only haploid life cycle to be diploid is the zygote stage.
Diploid life cycle- Humans have 46 chromosomes, while at the same time our gametes are haploid with only 23 chromosomes.
Haplo-diloid life cycle- Typically organisms with this life cycle grow are diploid organisms, but when they reach maturity, they release haploid spores. These haploid spores fall to the ground and form haploid structures that will eventually develop into diploid organisms.
7. List the phases of meiosis I and meiosis II and describe the events characteristic of each phase.
In meiosis I, the cell goes through prophase I, Metaphase I, Anaphase I, and Telophase I. In prophase I Synapsis occurs and the chromosomes form tetrads. Chaismata occurs here. Genetic information is swapped. In Metaphase they align in the middle of the cell, and in Anaphase I they are pulled apart as full chromosomes still. In telophase I the cell splits into two daughter cell with chromosomes with different genetic info. Then In Meiosis part two, the basic mitosis happens. Half the number of chromosomes.
8. Recognize the phases of meiosis from diagrams or micrographs.
9. Describe the process of synapsis during prophase I and explain how genetic recombination occurs.
In prophase I replicated homologs pair up and become physically connected in a synaptonemal complex. At this stage genetic recombination occurs when alleles of the chromosomes are exchanged, they pull apart but still are connected where synapsis occurred called chiasmata.
10. Describe three events that occur during meiosis I but not during mitosis.
1. Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
2.At the metaphase plate, there are paired homologous chromosomes, instead of individually replicated chromosomes
3.At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate 11. Explain how independent assortment, crossing over, and random fertilization contribute to genetic variation in sexually reproducing organisms.
Crossing over occurs during prophase I of meiosis I. It involves the switching of genes between the non-sister chromatids of homologues which allows the novel mixture of maternal and paternal genetic material with new, recombinant chromosomes. Another layer of variation occurs during independent assortment which is the random lining up of the homologues during metaphase I of meiosis I. Between different gametes, there are 2n different possibilities of how the homologues could line up. Maternal and paternal chromosomes line up during. Finally, it is complete random which sperm fertlizes which egg which creates even more potential variation in the offspring since they have two separate sets of genes.
12. Explain why heritable variation is crucial to Darwin‘s theory of evolution by natural selection
Heritable variation adds more combinations of genes, this allows for more possibilities of traits for your offspring which then makes a greater chance of finding the correct genes for environmental success.