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Naturally Occurring Polymers—Animals 361

Why these differences? A higher rate of recombination increases the likelihood of at least one
crossover during meiosis of each chromosome arm. Such crossovers are necessary for normal mei-
otic disjunction of homologous chromosome pairs in eukaryote cells. Recombination occurs with
the greatest frequency during meiosis, the process where diploid cells with two sets of chromosomes
divide producing haploid gametes—sperm cells or ova—with each gamete having only one member
of each chromosome pair.
The “crossing over” is not entirely random. Even so, in general, the frequency of homologous
recombination in any region separating two points on a chromosome is proportional to the distance
between the points. A homologous genetic recombination is simply the recombination between two
DNAs of similar (not necessarily the same) sequence. Homologous recombination serves several
functions. First, it contributes to the repair of certain types of DNA damage. Second, it provides
a transient physical link between chromatids that encourages orderly segregation of chromosomes

during the first meiotic cell division. Third, it enhances genetic diversity.
Since such crossover sequences are important, it is possible that they are present to an extent in
each arm to insure that crossover occurs but the full answer is not currently known and since shorter
arms are shorter, the density or frequency of them is greater.
We need to remember that the present knowledge of the human genome is a rough map with-
out complete knowledge of the stop lights, detours, alternative routes, pot holes, and so on to use
the metamorphic language relating a paper map to the actual physical terrain. Scaffolds are being

built to fill these knowledge gaps with time allowing the scaffolding to become part of a solid
building.

10.11 CHROMOSOMES

The preliminary investigation of chromosomes has resulted in several reoccurring themes being

evident. One theme is that nature magnifies small differences, often the difference in only a single
base pair can lead to marked differences in our overall predicted health, and so on. The second
theme involves the interrelativeness of the genes with one another and with various proteins that are
created by them. A third theme will not be dealt with to much extent. That theme concerns the fact
that even though we talk about a common human genome, there are within this human genome suf-

ficient differences to make each of us individuals with our own aspirations and dreams, tendencies
toward particular foods and diseases, and so on. Even so, most of the human genome is the same
with the small variances, including our outward environment, resulting in a divergence population
of human beings.
We often think of the chromosomes as being flat with little or no geographical topology

because the sheet of paper or screen we view them on is flat. They are not flat and it is that

three- dimensional structure that assists the various genes to perform their function in designing
needed proteins. The secondary structure of these features is more or less helical with the vary-
ing clefts shown in Figure 10.16 causing the DNA to have these varying structures. The transfer
of information from the DNA template to protein, and less so RNA, synthesis is described in
Section 10.4.
Our bodies have about 100 trillion cells. Inside each cell is the nucleus and, with the exception
of egg and sperm cells, inside the nucleus are two copies of the human genome made from DNA
with protein building genes contained within chromosomes that compose the human genome.
There are a few exceptions to this, including the following. Not all genes are DNA, but some con-
tain RNA. Not all genes code for proteins. Some are transcribed into RNA that becomes part of
a ribosome or transfer RNA. While most gene-associated reactions are catalyzed by proteins, a
few are catalyzed by RNA. Again, while many proteins are designed by a single gene, some are
designed by several genes.
The human genome comes in 23 packages with each package being a chromosome. This num-
ber 23 is important. If we have more or less than 23 then we may well be in great trouble. Only

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