BIOLOGY 1A03 Lecture 2: Theme 5: Module 2 - Meiosis

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BIOLOGY 1A03 Full Course Notes

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The inheritance of chromosomes meiosis: gametes passing genes to the next generation, genetic information is transmitted from generation to generation within a family. As you can imagine, if human gametes were produced by mitosis, they would be diploid and would give rise to offspring with double the chromosome number: this would lead to fatal effects in the offspring. It is the unique haploid gametes that are produced by each parent, which passes genetic information on to the next generation: prophase i condensation and synapsis, each sexually reproducing organism has a characteristic diploid and haploid number. Prior to the start of meiosis, as in mitosis, interphase allows for the duplication of chromosomes to occur. Sister chromatids are duplications of the same chromosome and therefore, are genetically identical to each other: in contrast, non-sister chromatids are replicas of different chromosomes, which are genetically similar, but not identical.

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1. _____ Females that reproduce sexually versus asexually are said to incur a âtwo-fold cost of sexual reproduction.â Which of the following costs are associated with sexual reproduction for females?

A) MALES! That do-nothing lot really does not directly contribute offspring to the next generation. Females would be better off making all daughters.

B) FEMALES! In most natural settings, those daughters compete with their mothers for potential mates and resources needed to successfully reproduce.

C) MITOSIS! Since sexual reproduction can produce many more offspring than asexual reproduction, there are energetic costs associated with the mitotic divisions required to produce those offspring.

D) MEIOSIS! Every time a gamete is produced, the parent loses a little bit of itself. Half to be precise since only half of its genes are passed on to sexually produced offspring.

E) DIVERSITY! Mothers maximize fitness if all offspring are genetically just like them. Sexual reproduction causes genetic diversity, which ultimately lowers the fitness of both parents.

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).

Just to keep you in practice, a couple of the much beloved multiple multiple choiceâ¦

1. An increase in CO₂ levels in the blood directly or indirectly causes which of the following?

A) an increases in ventilation rate

B) a decrease in the pH

C) a decrease in hemogloginâs binding affinity for O₂

D) a decrease in the O₂ levels in the blood

E) an immediate increase in the number of red blood cells found in the blood stream

2. _____ Females that reproduce sexually versus asexually are said to incur a âtwo-fold cost of sexual reproduction.â Which of the following costs are associated with sexual reproduction for females?

A) MALES! That do-nothing lot really does not directly contribute offspring to the next generation. Females would be better off making all daughters.

B) FEMALES! In most natural settings, those daughters compete with their mothers for potential mates and resources needed to successfully reproduce.

C) MITOSIS! Since sexual reproduction can produce many more offspring than asexual reproduction, there are energetic costs associated with the mitotic divisions required to produce those offspring.

D) MEIOSIS! Every time a gamete is produced, the parent loses a little bit of itself. Half to be precise since only half of its genes are passed on to sexually produced offspring.

E) DIVERSITY! Mothers maximize fitness if all offspring are genetically just like them. Sexual reproduction causes genetic diversity, which ultimately lowers the fitness of both parents.

BIOLOGY 1A03 Lecture 2: Theme 5: Module 2 - Meiosis

BIOLOGY 1A03 Full Course Notes

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