If we compare living beings of certain species with their descendants, we can notice that the descendants in all essential features are similar to their parents. However, species is not invariable in the course of longer periods. Sometimes the changes within species appear. If the individuals marked by some new features with the individuals of the old kind cannot reproduce any more, a new kind originates. Consequently there is constantly a contradiction between the steady inheritance of features and the phyletic evolution.

Descendants do not receive the features from the parents, not even protein for certain structures or the enzymes for the specific course of physiological processes, but rceive only the possibility to form the materials independently in the cells. These possibilities for the signs formation are called the inherited abilities. Transmitting the inherited abilities of the parents to the descendants is called inheritance. An inherited ability is genetic information which is inherited from one generation to another.

The genetics is the biological science which examines the storage of the genetic informations in the genetic material, in the chromosomes, transmitting the genetic information with cell divisions as well as their realisation in the cells and in the developing organism.

The inherited abilities, called also genes, lie in the chromosomes which are in the nucleus. They are visible during the cell division as filariform, colorable parts of the karyoplasm.

The human genes are divided in a cell into more than 100000 different n-chromosomes and lie in them at a certain place. The n-chromosomes are distinguished externally by their size and form.

The gender cells contain the chromosome set of n-chromosomes, haploid chromosome set, spermatozoa and ova of the person 23.

The homologous chromosome consists of the n-chromosomes in every body cell because n-chromosomes of the father and n-chromosomes of the mother gather in zygote (primary embryonic cell) by the fertilization.

Every body cell of higher plants and animals contains thereby 2n-chromosomes which are in pairs. The gene for a certain sign lies at the same certain place on two homologous chromosomes. This genetic pair often shows small differences (e.g., in hair colour: light or dark). The total number of the 2n-chromosomes is called a diploid chromosome set. Different species are characterized by different amounts, e.g., a human has 46, a cow – 60, potato – 48. A chromosome consists of two absolutely same neighboured lying halves. Similar chromosome halves are the feature typical for both genes.

8.1 Origin of a diploid chromosome set (example n=3)

Chromosome halves consist chemically essentially of a molecule DNA. These are very big molecules which elements are the nucleotides. Every nucleotide of a DNA consists of a sugar with five C atoms, a residual phosphate and an organic base. One of four bases is occured with the nucleotides of the DNA which is shortly called A, G, C and T.

8.2 Replication of the DNA

The nucleotides are connected into longer chains and the residual phosphate and sugar are changed, the bases are connected on each side with the residual sugar. Two parallel chains are combined to a double chain in the DNS molecule. The bases join in pairs and are connected with each other by bridges. Only the complementary bases are able of this base mating: A with T by a certain form of the bridge, G with C by another.

Both chains are turned helical around each other. The storage of the genetic information is based on the order of the bases in the DNA chains.

An identical duplication of the DNA which occurs between two cell divisions and is called replication. Both chains of a DNA molecule are separated from each other in the bridges between the complementary bases, and new complementary nucleotides are attached to every single chain by twos. They then form a nucleotide chain, so that two new, absolutely identical double chains appear.

All genes with their full programme are transmitted for metabolism and development to both daughter cells by mitosis with the cell division of the body cells, so with the growth of a living being.

The inherited abilities are also transmitted with the formation of the gender cells, but with the last division in the form of meiosis.

Mitosis always leads to daughter nucleus whose chromosome set is exactly the same to that of the original nucleus. The both halves of the chromosomes are separated from each other during replication and are distributed to both daughter nucleuses.

Mitosis runs in four successive phases:

The chromosomes become visible, short and thick. The nucleus membrane disappears. The cytocentrum divides and both parts go to the cellular pole. Between them the plasma forms the fibres which are attached to the middle of a chromosome.

The chromosomes are arranged in the middle level of the cell.

The halves of every chromosome are separated. A half reaches one cytocentrum, the other reaches the opposite one. Now they are called daughter chromosomes.

Around both groups of daughter chromosomes a new nucleus membrane is formed. The chromosomes are extended again and then are not to be recognised individually under the microscope any more.

The division of the cytoplasm and its components and the formation of the distinguishing membrane ordinarily occur by the end of mitosis.

A chromosome сonsists essentially of halves which double before the next mitosis by replication, so that the chromosome includes two opposite lying halves at the beginning of the next mitosis.

8.3 Phases of the mitosis. Example 2n=6

Meiosis leads to daughter nucleuses which chromosome set, as opposed to the original nucleuse, is decreased by half. As a result, the embryonic cells have only the haploid chromosome set, so that after the fertilization the zygote, i.e. the product from two embryonic cells, receives the diploid set.

8.4 Phases of meiosis. Example 2n = 6

The human, like most multicellular living beings, in the process of meiosis the old embryonic cells increase by mitosis, as the last division which provides pure embryonic cells.

Meiosis consists of two successive nucleuse divisions; but a duplication of the chromosomes occurs only with one, so that four haploid nucleuses arise from a diploid nucleuse with meiosis.

The decrease of the chromosome number occurs in the first division of the meiosis, while not chromosome halves are distributed to both poles, but the whole chromosomes. Two homologous chromosomes are distributed to opposite poles. Some chromosomes of the father with others of the mother can gather in a pole. The second division is similar to mitosis.

The signs of the cell and the organism, first of all their typical metabolism processes, are determined by the species-specific proteins which influence the metabolism processes as enzymes or are occured as building materials. The qualities of a protein molecule are caused by the order of its amino acids. This order is fixed as genetic information in the DNA.

Formation of the genetic information consists in the fact that the information from the nucleus will be transmitted on substances. They then bring the information to the ribosomes. There the rows of the bases is "transformed" into the rows of the amino acids and the suitable amino acids are connected like this.

The connection of amino acids into a polipeptide chain according to the information being in the genes is called a protein biosynthesis.

The carriers of the genetic information about the ribosomes are the RNA. The RNA consists of a chain of nucleotides, however, is does not double in contrast to the DNA. Every nucleotide of the RNA contains a similar sugar like with the DNA, a residual phosphate and one of four bases A, C, G and U (instead of T).

The information of the DNA to the RNA is transfered in the nucleus, where both DNA chains are separated sometimes from each other, so that a complementary RNA chain can be formed to a DNA chain. The  single free nucleotides are bound for the RNA to the bases of the DNA that in each case C is connected with G and vice versa, T with A and A with U. These nucleotides form a coherent RNA chain which walks by the pores of the nucleus membrane to a ribosome.

Here a certain amino acid from the cytoplasm is brought to three nucleotides of the RNA in the ribosome where the amino acids are connected with each other, until the signal for the end of the amino acid chain is given by a certain group of three nucleotides. Now the formed polypeptide becomes free of the ribosome.

8.5 Protein biosynthesis

The genetic code is a key for the transtition of the nucleotide consequence to the amino acid consequence. As there are  20 different amino acids for the formation the different protein, the code must show at least 20 different signs which consits from three nucleotides UUC caused, e.g., that the amino acid phenylalanine is attached to the growing polypeptide chain. The genetic code is valid uniformly for all living beings.

Mutations are based on changes of the DNA. If, for example, a nucleotide is substituted during replication with a wrong nucleotide, it leads to the change of a gene and, as a result, to the change of the amino acid order of a protein.

If a mutation affects an embryonic cell, the change is bequeath, if it does not make the reproduction generally impossible. A mutation occures relatively rare. The direct causes of the mutation are not known yet but the frequency of the appearance can be increased by X-rays or by some chemical materials considerably.

Most mutations are injurious because they disturb the development of the affected organisms. Mutations sometimes prove, however, also individuals who are better adapted in a certain environment than others become more frequent by the natural selection. This cooperation of mutations and selection is the cause of the phyletic evolution. It can be also used by people for the development of new domestic animals and useful plants, first of all since it is used rays or chemical materials consciously to the increase of the mutation rate for these purposes.

People must be protected against the negative changes which the mutations have for them. For this purpose we must exactly know the factors which cause the mutations, and which illnesses and damages are caused by mutations.

Then we must decrease the effect of the corresponding injurious environmental factors, e.g., by avoiding unnecessary X-rays, by following the measures for the ray protection in labs, by avoiding of taking a lot of needless drugs and luxuries. The ban of all nuclear weapons is exceptionally important for this reason.

As we are not able yet to avoid the appearance of injurious mutations and their inheritance, it comes to so-called hereditary diseases. In most cases, the parents may be healthy, although one of two homologous chromosomes contains the features of disease, and if the changed genes of a mother and a father meet, the baby may have a mutation.

The medicine knows more than 2000 illnesses of the most different kinds caused by mutations.

Phenylketonuria

One of the innate illnesses of the metabolism is phenylketonuria (PKU).

PKU is based on the loss of a metabolism enzyme by which the baby is not able to diminish the amino acid phenylalanine. Thereby it comes to a too high concentration of this material in the body liquids and the tissues. This disturbs among other things the metabolism of the nerve cells what leads to heavy spiritual and physical damages.

It is possible to prevent the effect of this illness, if baby’s food contains little phenylalanine at leat until he/she is one year old. To recognise PKU with a newborn child, the phenylalanine level is examined child’s blood.

Others genetically conditioned illnesses can be diagnosed during the pregnancy.

1. Answer the questions using the information from the text:

INHERITED ABILITIES AND THEIR MATERIAL CARRIER

1. What do the inherited abilities mean?

2. Define the concept Inheritance.

3. What is a gene?

4. Where do the genes have their certain place?

5. Where do both chromosomes of homologous pair come from?

6. What does the diploid chromosome set mean?

7. Where does the haploid chromosome set occur?

8. What parts does a nucleotide of a DNA consist of?

9. How is the duplication of the chromosomes halves before a cell division called?

10. Which division kind is occured with the growth of the organisms?

FORMATION OF THE INHERITED CHARACTERISTICS AND MUTATIONS

1. Why is the consequence of the bases in the DNA important for a forming protein molecule?

2. What function does the RNA have?

3. What does the protein biosynthesis mean and where does it take place?

4. What does the genetic code mean?

5. What is a mutation in a cell caused by?

6. What can mutations be caused by?

THE CHROMOSOMES AND THEIR FUNCTION

2. Use the following special terms in the gaps

Structure, chromosome, enzyme, gene, meiosis, ribosome, inheritance, diploid, genetically, haploid, homologous

Not the signs, but ………………. are bequeath. ………………… means the passing on of structure to the descendants. The segment of the DNA, …………………. Information for the formation of a sign is called …………………….. More than 100000 genes of the person lie separately in the nucleus in 46 …………………..

From the nucleus the genetic information is copied and brought to ………………. where accordingly to a gene a certain protein molecule is formed. This protein is mostly …………………… which regulate a metabolism process and formation of a sign.

The zygote and all body cells arising from the zygote are …………………, i.e., they contained 2n chromosomes. Two chromosomes which contain the genes for the same signs are called …………………… chromosome pair. During …………………. are separated homologous chromosomes and are distributed to two cells, so that ……………… gender cells originate.

THE HEREDITARY FACTORS AND THEIR MATERIAL BEARERS

3. Model answer: the hereditary factors mean the inherited possibility for the formation of a sign.

1. Explain the statement that the qualities are not bequeathed.

2. Describe the origin of a new kind.

3. Describe the structure of a DNA molecule.

4. Describe the replication of the DNA molecule before mitosis.

5. Describe mitosis.

6. Why is meiosis necessary? Describe this process breafly.

7. Compare mitosis and meiosis.