Ploidy refers to the number of complete sets of chromosomes (genes) present in a
ID: 140547 • Letter: P
Question
Ploidy refers to the number of complete sets of chromosomes (genes) present in a cell. The number of chromosomes in a complete set is a fixed characteristic of a species. Humans are diploid with two sets of 23 chromosomes. Fruit flies are diploid with two sets of 4 chromosomes. Sexually reproducing organisms are diploid, each somatic cell of an organism has two complete sets ("di" is a prefix indicating 2.). To complete a life cycle and reproduce new members of a species, mating is required. Mating brings a donation from a male and a donation from a female together. However, if the male and female cells remain diploid, the fertilization will result in a tetraploid zygote. In general, these are not viable. A mechanism is needed to form special haploid cells, gametes. The diploid set is split precisely into two complete haploid sets. That mechanism is meiosis.
The complete set of chromosomes of a species is also the complete set of genes of a species, the genome. (Yes, there is a minor contribution from mitochondria and chloroplasts. Inheritance of this information is "cytoplasmic".)
Please answer the following questions:
1. How many chromosomes in a brain cell of a fruit fly? How many chromosomes are in a liver cell of a human? Explain both answers.
2. How many chromosomes are in a fruit fly unfertilized egg? How many chromosomes are in a human egg? Explain both answers.
3. Why is meiosis necessary for sexually reproducing organisms? How many chromosomes are in a human zygote? Relate the two answers.
4. What is a gene allele? Suppose there is a gene called "bingo" and it is present once in a complete haploid set. How many alleles of "bingo" does a diploid individual have? How many copies or versions of the gene "bingo" are present in a diploid individual?
5. Modern DNA sequencing clearly identifies alleles by showing the different DNA sequence of each allele. These alleles may be termed structural alleles (I coined the term for this problem.). Classical genetics only dealt with phenotypes or visible traits. As a result, alleles of genes were identified by functionality, usually normal or non-functional. Using your knowledge of gene structure and gene expression, postulate a relationship between structural alleles and functional alleles. (include relative amounts; more, less, or equal).
Explanation / Answer
Ans. 1 With the exception of sex cells (eggs and sperm), there will be the same number of chromosomes in all cells of the body. If fruit fly skin cells have eight chromosomes then brain cells will also have eight chromosomes. These body (somatic) cells undergo mitosis. Fruit fly somatic cells have eight chromosomes.
In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair, the sex chromosomes, differ between males and females. So the human liver cell (somatic cell)will have 46 chromosomes.
Ans.4 Drosophila unfertilized egg cell will have 4 chromosomes as egg cell is haploid and haploid number of chromosomes in drosophila is 4.
Human egg cell will have 23 chromosomes as haploid number of chromosomes in humans are 23.
Ans.3 Meiosis is important because it produces gametes, female or male germ cells. During meiosis a germ cell divides to make four new sex cells. As a result of fertilization, two gametes (an egg and a sperm cell in humans) join together to form a fertilized egg or zygote. A zygote is a eukaryotic cell (includes a nucleus) composed of all the genetic material needed to produce a new human being.
A human zygote is diploid, and it has 46 chromosomes. The secondary oocyte contained 23 chromosomes and the sperm cell contained 23 chromosomes. Oocytes and sperm cells are both haploid cells, so they only have half a set of chromosomes. When the sperm cell enters the oocyte they combine and fuse together to form the zygote. The zygote now has both haploid sets of chromosomes, which means it has a full set of 46 chromosomes. Different organisms have different numbers of chromosomes, but in humans the zygote has 46.