The Evolution Myth (Ukázka, strana 99)

a given chromosome receives a part of the genetic information from the partner’s (homologous) chromosome.

Decalogue – defined here after Ambrose Bierce, The Devil’s Dictionary (1911): ‘a series [...] ten in number – just enough to permit an intelligent selection for observance, but not enough to embarrass the choice’. dominant – a characteristic of a trait (or an allele). The trait is dominant when it is phenotypically expressed in an individual genotype, possessing two different alleles at a given locus on a pair of homologous chromosomes. (Compare recessive.) epigenetic – the term used to indicate any factor that can affect the phenotype without changing the genotype. gene – a unit of heredity. At the molecular level, a sequence of bases which occurs in chromosomal DNA and carries the information for the production of a functional product (mainly a protein molecule). gene pool – all the genes (alleles) present at a given locus within a defined population. gene targeting – a method introducing a mutation into a target gene within an intact cell, resulting in gene inactivation or an altered expression of the gene.

It usually occurs in recombination with a homologous chromosome from which a chromosome fragment, with the gene segment closely related to the target gene, is transferred into the acceptor gene. genome – all genes carrying the entire genetic information of a gamete or of a somatic cell. genotype – usually considered with respect to a single character, when it describes the alleles at both loci on the pertinent homologous chromosomes. homologous chromosomes (homologs) – twin chromosomes of one type, one inherited from each parent. The term is also used for two chromosomes of one type, each of which belongs to different, closely related species. imprinting – the different expression of alleles within a given locus, depending on whether the expressed allele has been inherited from the male parent or the female, that is, whether the locus is on the chromosome inherited from the male or the female. karyotype (karyotyping) – the nuclear chromosome set of an individual. Karyotyping is the process of preparing the chromosomes of an individual and their arrangement in the standard order; in human genetics, used for diagnostics. locus – the position of a gene on a chromosome. meiosis – the special process of the diploid germline cell division by which gametes are produced, containing the haploid number of chromosomes. mutation – a term broadly used at three different levels of DNA organization: genome mutation, chromosome m., and gene m. In all three categories of muta98

Ukázka elektronické knihy, UID: KOS200911

tion, it means a heritable change in the sequences of genomic DNA. A heritable change of DNA outside the genomic sequence is usually called variation. mutation rate – this rate has no physical dimension of velocity, but means a quantitative estimation of how many times we encounter a new change within a gene searched for in a given generation. It is expressed as a ratio of mutated genes to all genes of a given locus within the observed generation. For example, if we find ten children affected as a result of a mutated dominant allele within a new generation of 100,000 born children (200,000 alleles), then the mutation rate is 5 x 10-5. phenotype – the observable characteristics of an organism, in the strict sense, resulting from the expression of its particular gene(s). recessive – a characteristic of a trait (or an allele). The trait is recessive when both alleles (one from each parent), phenotypically expressed in an individual progeny genotype, are identical. In the genome of at least one of the parents the allele should be excluded from the expression to phenotype by the alternative (dominant) allele of the same locus on the homologous chromosome. (Compare dominant.) recombination – the formation of a DNA molecule that is within a chromosome of a given parent and contains a linked part not originating from this parent. The formation can be either natural by crossing-over or artificial by different techniques. telomere – a structure at both tips of a chromosome that protects the ends of the DNA molecule. trait – understood in genetics as a particular form of a character; the colour of a flower is a character; white and purple flowers are traits. transcription – the synthesis of an RNA molecule from a template DNA molecule. translation – the synthesis of a protein (polypeptide) from a template RNA molecule. transposon – a segment of DNA, which, once incorporated by a specific technique into a gene sequence (either into another locus or into a foreign host DNA), affects the function of the attacked gene. trisomy – a genome that has three representatives of a given chromosome type, instead of the usual pair of homologous chromosomes. variation – a phenotypic change of a trait evoked either by the environment (in Darwin’s thinking) or, at the molecular level, a change in a sequence in the DNA molecule. The change within a gene sequence is identical with mutation and the term variation is reserved for a heritable change outside the genomic sequence.

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APPENDIX 1

MITOCHONDRIAL DNA (mtDNA) As we saw in chapters ii and iv, the mtDNA molecule consists of 16,569 base-pairs and contains 37 genes. When considered in greater detail, some particular characteristics of the mt genetic code differ from the arrangement of the nuclear genome. For example, the sequence shows extreme economy of space, in that the genes have none or few non-coding bases between them, no intervening sequences (introns), and, strangely, no termination codons in most of the reading frames (which leads to some dissimilarity in mt gene expression).

The mtDNA comprises three types of unit (a five-carbon sugar deoxyribose, nitrogen-containing bases, and a phosphate group), and their pattern, based on complementary base pairing, is arranged in the same manner as the nuclear DNA, the scheme of which is presented in appendix 3. What is different here is that the mtDNA is packed into a circular chromosome, which is schematically illustrated in the following picture: The circular double-stranded DNA consists of a ‘heavy’ (H) strand and a ‘light’ (L) strand, which behave slightly differently during replication. Together, they contain those 37 genes, 28 of which are encoded with the H-strand and nine with the L-strand. The genes’ products are 22 mt-tRNA molecules, one molecule of 23S rRNA (a component of the large ribosomal subunit), and one molecule of 16S rRNA (a component of the small ribosomal subunit). The remaining 13 genes encode polypeptides. Seven of them, encoding subunits of slightly different forms of the important NADH dehydrogenase enzyme, are indicated in grey in the picture.

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