14.4 The Cell Cycle

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This facilitates their recognition by a huge (upper M Subscript r Baseline tilde 10 Superscript 6Mr ∼10⁶) multiprotein complex called

the proteasome, which carries out proteolysis into peptides, which may be presented

to the immune system, and ultimately to amino acids.

14.4.1

The Chromosome

In eukaryotes, the nucleic acid is present as long linear segments (each containing

thousands of genes) called chromosomes, because they can be coloured (stained)

and hence rendered visible in the optical microscope during cell division.

Chromosomes are terminated by telomeres. The telomere is a stretch of highly

repetitive DNA. Since during chromosome replication (see below) the DNA poly-

merase complex typically stops several hundred bases before the end, telomeres

prevent the loss of possibly useful genetic information.

Germline cells are haploid; that is, they contain one set of genes (like bacte-

ria). When male and female gametes (eukaryotic germline cells) fuse together, the

zygote, the single-celled progenitor of the adult organism, therefore contains two sets

of genes (i.e., two double helices), one from the male parent and one from the female

parent. This state is called diploid. The normal descendants of the zygote, produced

by mitosis, remain diploid. Many plants, and a few animals, have more than two sets

of genes (four = tetraploid, many = polyploid), widening the possibilities for the

regulation of gene expression. Polyploidy is a macromutation that greatly alters the

biochemical, physiological, and developmental characteristics of organisms. It may

confer advantageous tolerance to environmental exigency (especially important to

plants because of their immobility) and open new developmental pathways. Cancers

(Sect. 14.5) are characterized by aneuploidy, which, typically, leads to unpredictable

further development. The unpredictability is autocatalytic, since once entire chro-

mosomes are missing or duplicated, there is a chance that some of the machinery for

copying the DNA is affected (cf. Sect. 14.7.3). Cancer is characterized by aneuploidy.

The two (or more) forms of the same gene are called alleles. The inheritance

of unlinked genes (i.e., genes on different chromosomes; genetic linkage refers to

the association of genes by virtue of their being located on the same chromosome)

follows Mendel’s laws. ¹⁶ If, for a given gene, two alleles are known, denoted as A and

a, occurring with probabilitiespp and1 minus p equals q1 −p = q, respectively, there are three possible

genotypes in the population (AA, Aa, and aa), with probabilities of occurrence of

p squaredp²,2 p q2pq, andq squaredq², respectively (this is the Hardy–Weinberg rule). The Aa genotype is

called heterozygous (the two parental alleles of a gene are different).

16 1. Phenotypical characters depend on genes. Each gene can vary, the ensemble of variants being

known as alleles. In species reproducing sexually, each new individual receives one allele from the

father and one from the mother. 2. When an individual reproduces, it transmits to each offspring

the paternal allele with probability 1/2 and the maternal allele with probability 1/2. 3. The actual

transmission events are independent for each independently conceived offspring.