Figure 4.1 shows a picture of human replicated chromosomes (adding up to a total of 12 billion base pairs of DNA). At this stage (after DNA replication but before cell division), each chromosome consists of two identical "chromatids" (the results of DNA replication) connected at a special structure called the "centromere". At the ends of the chromosomal arms are "telomeres", which are special DNA-protein structures essential for chromosome stability. Along the chromosomal arms, regions called "euchromatin" contain most of the genes, while regions called "heterochromatin" consist largely of highly repetitive nucleotide sequences. The relative amounts and locations of euchromatin and heterochromatin vary among species and among different chromosomes within the same species. Figure 7.19 b on page 279 shows a common situation of regions near the telomeres and centromere being heterochromatic.

1. When somatic (non-sex) cells divide, how do they manage to get exactly two copies of each gene into each of the two daughter cells?

Mitosis is shown in Figure 4.3 and described on pages 137-140. All of the chromosomes get localized to the metaphase plate, and are attached to spindle fibers (micotubules) at the centromere (the specific attachment structure is called the kinetochore). With a "go" from the spindle checkpoint, the centromeres split, and the fibers (which are shortening) pull the chromatids apart. So, each of the two daughter cells gets a one-chromatid version of each chromosome (two #1's, two #2's, two #3's, etc.).

2. What happens during the TWO ROUNDS of chromosome separation and cell division which constitute meiosis ( i.e., meiosis I and meiosis II )?

Figure 4.4 shows what happens to a single pair of homologous chromosomes (such as your two chromosomes #5) during the two stages of meiosis.

Figure 4.7 shows meiosis for an organism that has a diploid chromosome number of four (two pairs). The resulting four gamete cells contain haploid genomes consisting of two single-chromatid chromosomes.

3. What is the chromosomal explanation for the Mendelian Principle of Independent Assortment ?

Figure 4.11 shows the crucial point that the alignment of the chromosome pairs in meiotic metaphase I is random. All possible alignment combinations are equally likely. "Independent assortment" of genes on different (i.e., nonhomologous) chromosomes results directly from this fact.


It is important to understand thoroughly the four figures 4.3, 4.4, 4.7, and 4.11.



Problem S-2. "Drawing Chromosomes in Anaphase".

Consider a hypothetical organism whose genome consists of two pairs of chromosomes, #1 and #2, with #1 being somewhat larger than #2. Assume that the organism is heterozygotic for gene alpha on chromosome #1 and heterozygotic for gene beta on chromosome #2.

Using Figure 4.11 as your guide to size, shape, and color, make good drawings of the chromosomes in cells that are in (a) anaphase of mitosis, (b) anaphase I of meiosis, and (c) anaphase II of meiosis. Chromosomes should be drawn so they look like those in Figure 4.11; chromosomes should not be drawn to look like "narrow lines", "the letter X", "little fat blobs", or "a four-leaf clover".

Show (label) all of the alleles of alpha and beta at the correct locations on your diagrams.

It is important to be able to make clear drawings depicting chromosomes (with appropriate designations of the alleles they carry) in the various stages of mitosis and meiosis.

Problem S-3. "Siblings Genotypes".

Identical twins arise from a single fertilized egg, and thus they have identical genotypes for all of their genes. What about non-identical twins or regular siblings, that arise from separate fertilized eggs. Is it possible that such twins or regular siblings could have the same genotype for all of their genes? To answer this, calculate the following for two human siblings, a girl and a boy.

a) What is the probability that the girl and boy have inherited the same chromosome #1 from their mother?

b) What is the probability that the girl and boy have inherited the same pair of chromosomes #1 (one from the mother and one from the father)?

c) What is the probability that the girl and boy have inherited the same pairs of both chromosomes #1 and #2?

d) What is the probability that the girl and boy have inherited the same pairs of all 22 autosomes?

e) Over the past million or so years, perhaps a hundred billion human beings have lived, and most of them have had at least one sibling. Based on the answer to (d) above, is it likely or unlikely that there has ever been a case of two human siblings (other than identical twins) with the same autosomal genotype?