Sex, meiosis and biological variability

Assignment

Audesirk, Audesirk & Byers part of Chapter 11


Today's musical selection
Let me tell you about the birds and the bees

Eukaryotes, diploidy, and sexual reproduction

1-1.5 billion years ago, eukaryotic cells originated. Perhaps diploidy soon evolved because lethal and detrimental mutations would be over-ridden by dominant wild-type genes. Sexual reproduction and meiosis arose and gave rise to a powerful mechanism to insure biological variability, shuffling the genetic deck. Variability is very fundamental in evolution since new varieties may be more adapted and hence survive (to reproduction) better as selection pressures change.

Figure 11-20
Humans are diploid
Our gametes (sperm and eggs) are haploid
When sperm fertilizes egg, a diploid zygote is formed

With two homologous chromosomes...

Figure 11-16
...one is from the father and one is from the mother.
Both copies (alleles) of a gene might be the same (homozygous)...
...or they may be different

human and other life cycles

Figure 11-27
haploid (n), diploid (2n), gametes, ova, sperm, fertilization, zygote.

Figure 11-25
Note that while meiosis creates haploid gametes in humans, there are many organisms where meiosis creates a gamete-forming organism (alternation of generations, haploid life cycles).

(Previously, I described how mitosis creates two identical diploid "daughter" cells; here, you can see that mitosis can also create haploid cells from haploid cells in protists, fungi and plants.)

(An anthropocentric view is that meiosis only makes haploid gametes, but here you can see that it can make haploid organisms, again in protists, fungi and plants.)

Meiosis

Figure 11-21
(We will spend a long time with this figure.)
Meiosis. Note that there are two homologous chromosomes, 23 pairs for the human diploid number, 46. Theoretically, one could go to each gamete with one meiotic division. Instead, they align, duplicate, and divide twice.
Homologous chromosomes separate in first division
Sister chromatids separate in second division

Awesome variability

for 2 pairs of chromosomes, there are 4 possible combinations of chromosomes (centromeres) in the zygote. For 46 chromosomes, there are 2 to 23 power = 8.4 million.

The story goes that the king offered to pay the inventor of chess who wanted one grain of wheat for one square on the chess board, 2 for the second and so on. The king readily agreed but then found out that that was more wheat than the country would produce for decades, so he had the inventor beheaded instead. Behold the power of 2 to the power of (whatever).

Figure 11-22
Tetrad (bivalent), homologues have duplicated and are aligned next to eachother
Crossing-over (recombination) increases this beyond measure(shown for one pair of chromosomes)

Coming attractions

Unequal crossing over and gene duplication as major means of protein family evolution

Figure 12.33
Errors in meiosis (nondisjunction) can lead to offspring with aneuploidy (the wrong number of chromosomes) such as trisomy 21

Questions used in 2007 relating to this outline

In the bivalent there is/are [A] sister chromatids and [B] homologues
(a) [A] 1; [B] 1
(b) [A] 1; [B] 2
(c) [A] 2; [B] 1
*(d) [A] 2; [B] 2
(e) none of the above

What is the end result of spermatogenesis in the human male?
(a) Four diploid daughter cells
*(b) Four haploid gametes
(c) Four identical chromosomes lined up next to each other
(d) Two haploid spores
(e) Two haploid zygotes

"Meiosis creates haploid gametes." Why is this not the whole truth?
*(a) Meiosis creates haploid organisms when there is alternation of generations.
(b) Heterozygous gametes are not haploid.
(c) Many gametes are diploid.
(d) In plants, meiosis creates zygotes.
(e) In Protista, it is alleles that are made, not gametes.

Two different but closely related proteins may have arisen, over evolutionary time, by a process of
(a) trisomy.
(b) mitosis.
(c) fertilization.
(d) nondisjunction.
*(e) unequal crossing over.

A pair of sister chromatids might be X-shaped in the microscope
*(a) when they are joined at the centromere.
(b) during the G2 of interphase.
(c) because of the centriole pair.
(d) in anaphase.
(e) only for the X chromosome.

The tetrad would form
(a) during the preparation of the karyotype.
(b) in the G1 portion of the cell cycle.
*(c) during meiosis.
(d) only during interphase.
(e) after gametes are haploid.

Two to the twenty-third power = 8.4 million. Why are there even more possible gametes than that in the human?
(a) Diploidy adds to variability.
*(b) Crossing over adds to variability.
(c) Homozygosity adds to variability.
(d) That's how it would be if there were just one meiotic division, but there are two.
(e) That's how it would be in interphase, but gametes are in prophase.

If a human does not possess 46 chromosomes, what is the term for the abnormal chromosome number?
(a) loci
*(b) aneuploidy
(c) hybrids
(d) recessives
(e) bivalents

What processes take place during Anaphase II?
*(a) Centromeres divide and sister chromatids move to opposite poles.
(b) Centromeres do not divide and sister homologues move to opposite poles.
(c) Centromeres divide and bivalents move to opposite poles.
(d) Sister homologues attach to spindle fibers from opposite poles.
(e) Tetrads attach to spindle fibers from opposite poles.

What constitutes a tetrad (bivalent)?
(a) Sister chromatids aligned next to each other.
*(b) Duplicated homologous chromosomes aligned next to each other.
(c) Homologous chromosomes aligning at the center of the cell during anaphase I.
(d) The karyotype when mitosis is blocked with colchicine.
(e) Homologous chromosomes that are connected during telophase I.

"Mitosis creates genetically identical diploid daughter cells." Why is this statement not the whole truth?
(a) In the retrovirus, it makes identical RNA strands.
(b) It's not true at all; mitosis makes haploid gametes.
*(c) When there is alternation of generations, there can be mitosis of haploid cells.
(d) In the central dogma of cell biology, it makes proteins.
(e) It's not true at all; it is meiosis that makes identical diploid cells.

Children with Down syndrome
(a) are heterozygous.
*(b) have 47 chromosomes.
(c) accumulate glycolipids from an enzymatic deficiency.
(d) have a mutation of a gene on chromosome 21.
(e) are born when older men decide to have children.

Name a cell that could have 22 autosomes and one Y chromosome.
(a) a human egg
*(b) a human sperm
(c) a human diploid cell
(d) a body cell from a Klinefelter's syndrome individual
(e) a cell from a Turner's syndrome individual

Questions used in 2002 relating to this outline (and other outlines)

Disregarding crossing over, in meiosis, [A] separate in the first division, while [B] separate in the second division.
(a) A-single DNA strands, B-centrioles.
(b) A-alleles, B-genes.
(c) A-homozygotes, B-heterozygotes.
(d) A-chromatin, B-tetrads.
*(e) A-homologous chromosomes, B-sister chromatids.

Most people have two copies of chromosome 21. These two copies would be called
(a) alleles.
(b) tetrads.
*(c) homologues.
(d) spindles.
(e) centromeres.

Name a cell that could have 22 autosomes and one Y chromosome.
*(a) a human sperm cell
(b) a human ovum
(c) a human diploid cell
(d) a body cell from a Klinefelter's syndrome individual
(e) a cell from a Turner's syndrome individual

When sperm and egg form the zygote, this is called
(a) nondisjunction.
(b) meiosis.
(c) mitosis.
(d) crossing over.
*(e) fertilization.

The tetrad would form
(a) during the preparation of the karyotype.
(b) in the G1 portion of the cell cycle.
*(c) during meiosis.
(d) only during interphase.
(e) after gametes are haploid.

Haploid is a term that would apply to
(a) interphase.
(b) chromosomes viewed for the karyotype.
(c) daughter cells of a mitotic division
*(d) gametes.
(e) zygotes.

In flowering plants, meiosis
(a) makes gametes just as it does in animals.
(b) makes sperm and eggs.
(c) makes zygotes, in contrast with the situation for animals.
*(d) gives rise to a separate gamete-forming organism.
(e) makes all the cells of the plants.

When does crossing over occur?
(a) during G2
(b) during fertilization
(c) during mitosis
*(d) when the tetrad is present
(e) during interphase

Crossing over occurs
(a) during the preparation of the karyotype.
(b) in the G1 portion of the cell cycle.
*(c) during meiosis.
(d) only during interphase.
(e) after gametes are haploid.

Making haploid cells from diploid cells is called
(a) nondisjunction.
*(b) meiosis.
(c) mitosis.
(d) mapping.
(e) fertilization.

Nondisjunction of chromosome #21 in humans leads to
(a) Tay-Sachs disease.
(b) Crossing over.
(c) Klinefelter's syndrome.
(d) Turner's syndrome.
*(e) Down's syndrome.

A person has 44 autosomes plus one X chromosome plus one Y chromosome. Which statement is true for this person?
*(a) The Y chromosome came from the father.
(b) The X chromosome becomes a Barr body.
(c) The X chromosome could have come from either the father or the mother.
(d) The Y chromosome's inactivation is described by the Mary Lyon hypothesis.
(e) This person has Klinefelter's syndrome.

This page was last updated 7/31/08

Return to Bio 110 Syllabus

return to Stark home page