Are recessive genes transcribed



 

A gene is a section on deoxyribonucleic acid (DNA) that contains the basic information for the production of a biologically active ribonucleic acid (RNA). During this manufacturing process (called transcription) a negative copy is made in the form of the RNA. There are different RNAs, the best known is the mRNA from which a protein is translated during translation. This protein has a very specific function in the body, which can also be referred to as a characteristic. In general, genes are therefore called Hereditary disposition or Hereditary factor because they are the carriers of genetic information that is passed on to the offspring through reproduction. The expression, that is the level of activity or the level of activity of a gene, is precisely regulated in every cell.

Research into the structure and function and inheritance of genes is the subject of genetics. Research into the entirety of all genes in an organism (the genome) is a matter of genomics. genomics).

history

In 1854 Johann Gregor Mendel began to investigate the inheritance of traits in peas. He was the first to suggest the existence of factors that are passed on from parents to offspring. In his crossbreeding attempts, he described that traits can be inherited independently of one another, as well as dominant and recessive traits. He developed the hypothesis that there can be homozygous and heterozygous states and thus laid the basis for differentiating between genotype and phenotype.

1900 is considered the year of the "rediscovery" of the Mendelian rules, as the botanists Hugo de Vries, Erich Tschermak and Carl Correns took up the fact that there are quantifiable rules according to which the factors that were responsible for the expression of characteristics are passed on to the offspring be passed on. Correns coined the term investment or. Hereditary disposition. William Bateson recalled in 1902 Mendel's Principles of Heredity because there are two variants of the hereditary factors in every cell. He named the second element Allelomorph after the Greek word for "other" and thus coined the term allele. Archibald Garrod, a British doctor, had studied metabolic diseases and found that they were inherited through families. Garrod realized that the laws were also valid for humans and assumed that the genetic makeup was the basis for them Chemical individuality of people.

August Weismann put in his Lectures on the theory of descent 1904 the discovery that there is a difference between body cells and germ cells, and that only the latter are able to produce new organisms. Germ cells should contain a "hereditary substance" made up of individual elements that he Determinants called. These determinants should be responsible for the visible expression of the limbs, for example.

The name "Gen" was first coined in 1909 by the Dane Wilhelm Johannsen. He named the objects that genetics are concerned with after the Greek word genos (Gender). For him, however, they were just a unit of account. Already three years earlier, William Bateson had known the science of heredity as genetics named after the Greek word genetikos (Creation). At this point the chemical nature of the genes was still completely unclear. In the first years of the 20th century, geneticists also looked at insects and later birds after various plants in order to test the laws of inheritance. In combination with the chromosomes discovered in 1842 and named in 1888, this gave rise to the chromosome theory of inheritance. It had been observed through improved staining techniques that chromosomes first double and then divide with the cells. Therefore, they had come into question as carriers of the genetic make-up. During this time there was controversy between the proponents of Johannsen and Mendel's hypothesis that genes are material and their critics, who dismissed a connection between genes and chromosms as “physicalism” and “Mendelism” and continued to regard genes as abstract entities.

Thomas Hunt Morgan was also convinced that it could not be physical units that were responsible for the various characteristics and tried to refute Mendelism. He began in 1910 with cross-breeding attempts on black-bellied fruit flies. However, his work produced the opposite: the final proof that genes are on chromosomes and are therefore of material origin. Together with his colleagues, including Calvin Bridges, Alfred Sturtevant and Hermann Muller, he found many natural mutations and examined in innumerable crosses the probability that two traits are inherited together. They were able to show that genes are located at certain points on the chromosomes and are lined up one behind the other. Together, the group created the first gene map over many years. Since the crossing over could also be observed under the microscope, it was known that chromosomes can exchange sections. The closer two genes are to each other on the chromosome, the greater the likelihood that they will be inherited together and not separated by a crossing over event. This made it possible to provide information about the removal of two genes which, according to Morgan, in centiMorgan can be specified.

Some time later, Hermann Muller began experimenting with X-rays and was able to show that irradiating flies greatly increases their mutation rate. This discovery from 1927 was a sensation because it was actually shown for the first time that genes are physical objects that can be influenced from outside.

In 1928, Frederick Griffith demonstrated for the first time in the experiment known as "Griffiths Experiment" that genes can be transferred from organisms to others. The process he demonstrated was transformation. In 1941, George Wells Beadle and Edward Lawrie Tatum showed that mutations in genes are responsible for defects in metabolic pathways, showing that specific genes encode specific proteins. These findings led to the “one-gene-one-enzyme hypothesis”. Oswald Avery, Colin MacLeod and Maclyn McCarty showed in 1944 that DNA contains genetic information. In 1953 the structure of DNA was deciphered by James D. Watson and Francis Crick, based on the work of Rosalind Franklin. In 1969, Jonathan Beckwith was the first to isolate a single gene.

The definition of what a gene is exactly has changed constantly and has been adapted to new findings. To attempt a current definition, it took 25 scientists from the Sequence Ontology Consortium at the University of Berkeley two days in early 2006 to achieve a version that everyone could live with. A gene is accordingly a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and / or other functional sequence regions (a localizable region of genomic DNA sequence that corresponds to a hereditary unit and is associated with regulatory, transcribed and / or functional sequence regions). [1]

And this definition is not final either. The ENCODE (ENCyclopedia Of DNA Elements) project, in which the transcriptional activity of the genome was mapped, found new, complex regulatory patterns and established that the transcription of non-coding RNA is much more widespread than previously known. So a gene is one Unit of genomic DNA sequence that encodes a contiguous set of potentially overlapping functional products (A gene is a union of genomic sequences encoding a coherent set of potentially overlapping functional products). [2]

construction

At the molecular level, a gene consists of two different areas:

  1. a segment of DNA from which a single-stranded RNA copy is produced by transcription
  2. all additional DNA segments that are involved in the regulation of this copying process.

There are different peculiarities in the structure of genes in different living beings. The drawing shows the structure of a typical eukaryotic gene that encodes a protein. In this case, the transcribed gene part (pre-mRNA) contains six areas, the introns, which are removed from the RNA during the maturation process (processing) and seven exons which are linked to form the mature mRNA.
Regulatory elements such as enhancers or promoters are located in front of the transcription unit or within the exons and introns. Depending on the sequence, various proteins such as transcription factors and RNA polymerase bind to this. The pre-mRNA (non-mature mRNA), which initially arises in the cell nucleus during transcription, is modified to mature mRNA in the maturation process. In addition to the directly protein-coding open reading frame, the mRNA also contains untranslated, i.e. non-coding, areas, the 5 'untranslated area (5' UTR) and the 3 'untranslated area (3' UTR). These areas serve to regulate the initiation of translation and to regulate the activity of the RNAses that break down the RNA again.

The structure of the prokaryotic genes differs from eukaryotic genes in that they have no introns. In addition, several different RNA-forming gene segments can be connected very closely in series (one then speaks of polycistronic genes) and their activity can be regulated by a common regulatory element. These gene clusters are transcribed together, but translated into different proteins. This unit of regulatory element and polycistronic genes is called an operon. Operons are typical of prokaryotes.

Genes encode not only the mRNA from which the proteins are translated, but also the rRNA and the tRNA as well as other ribonucleic acids that have other tasks in the cell, for example in protein biosynthesis or gene regulation. A gene that codes for a protein contains a description of the amino acid sequence of that protein. This description is in a chemical language, namely in the genetic code in the form of the nucleotide sequence of DNA. The individual 'chain links' (nucleotides) of the DNA - summarized in groups of three (triplets) - represent the 'letters' of the genetic code. The coding area, i.e. all nucleotides that are directly involved in the description of the amino acid sequence, is used as an open reading frame designated. A nucleotide consists of one part phosphate, one part deoxyribose (sugar) and a base. A base is either adenine, thymine, guanine or cytosine.


Genes can mutate, i.e. change spontaneously or through external influences (for example through radioactivity). These changes can take place at different points in the gene. As a result, after a series of mutations, a gene can exist in different states called alleles. A DNA sequence can also contain several overlapping genes. Genes duplicated by gene duplication can have identical sequences, but nevertheless be regulated differently and thus lead to different amino acid sequences, i.e. they would not be alleles.

Gene Activity and Regulation

Main articles: Gene expression and gene regulation

Genes are "active" when their information is rewritten into RNA, i.e. when transcription takes place. Depending on the function of the gene, mRNA, tRNA or rRNA are produced. As a result, a protein can, but does not have to be, translated from this activity in the case of mRNA. The articles Gene Expression and Protein Biosynthesis offer an overview of the processes.

The activity of individual genes is regulated and controlled by a large number of mechanisms. One way is to control the rate of their transcription into RNA. Another way is to break down the mRNA before it is translated via siRNA, for example. In the short term, gene regulation occurs through the binding and detachment of proteins, so-called transcription factors, to specific areas of the DNA, the so-called "regulatory elements". In the long term, this is achieved through methylation or the “packaging” of DNA segments in histone complexes. The regulatory elements of DNA are also subject to variation. The influence of changes in gene regulation including the control of alternative splicing should be comparable to the influence of mutations in protein-coding sequences. With traditional genetic methods - by analyzing inheritance patterns and phenotypes - these effects cannot normally be separated from one another in inheritance. Only molecular biology can provide information here. An overview of the regulatory processes of genes is given in the article gene regulation.

Organization of genes

In all living things, only part of the DNA codes for defined RNAs. The rest of the DNA is called non-coding DNA. It has functions in gene regulation, for example for the regulation of alternative splicing, and has an influence on the architecture of the chromosomes.

The location on a chromosome where the gene is located is called the gene location. In addition, genes are not evenly distributed on the chromosomes, but sometimes occur in so-called clusters. Gene clusters can consist of genes that happen to be in close proximity to one another, or they can be groups of genes that code for proteins that are functionally related. However, genes whose proteins have a similar function can also be located on different chromosomes.

Just as there are non-coding DNA, there are sections of DNA that code for several different proteins. This is because of the overlapping open reading frames.

Genetic Variation and Genetic Variability

As genetic variation denotes the occurrence of genetic variants (alleles, genes or genotypes) in individual living beings. It arises through mutations, but also through processes in meiosis ("crossing over"), through which the grandparents' genes are distributed differently to the sex cells.

Genetic variability on the other hand is the ability of an entire population to produce individuals with different genes. Not only genetic processes play a role here, but also mechanisms of partner choice. Genetic variability plays a crucial role in the ability of a population to survive under changed environmental conditions and is an important factor in evolution.

Special genes

RNA genes in viruses

Although genes are present as DNA fragments in all cell-based life forms, there are some viruses whose genetic information is in the form of RNA. RNA viruses infect a cell, which then immediately begins to produce proteins as directed by the RNA; there is no need to transcribe DNA to RNA. Retroviruses, on the other hand, translate their RNA into DNA when infected, with the help of the enzyme reverse transcriptase.

Pseudogenes

A gene in the narrower sense is usually a nucleotide sequence that contains the information for a protein that is immediately functional. In contrast, pseudogenes are copies of genes that do not encode a full-length functional protein. These are often the result of gene duplications and / or mutations, which subsequently accumulate in the pseudogene without selection and have lost their original function. However, some seem to play a role in regulating gene activity. The human genome contains around 20,000 pseudogenes.

Jumping genes

They are also known as transposons and are mobile sections of genetic material that can move freely within the DNA of a cell. They cut themselves out of their ancestral location in the genome and can be reinserted at any other point. Biologists led by Fred Gage from the Salk Institute for Biological Studies in La Jolla (USA) have shown that these jumping genes not only occur in the cells of the germ line, as previously assumed, but are also active in nerve progenitor cells. Research by Eric Lander et al. (2007) show that transposons have an important function in that they act as creative factor be able to quickly spread important genetic innovations in the genome in the genome.[3]

Typical genome sizes and number of genes

Organisms or other biological systems Number of genes Base pairs
plant >25.000 108-1011
human ~24.800 3·109
Fly 12.000 1.6·108
mushroom 6.000 1.3·107
bacterium 500-7.000 106-107
Escherichia coli ~5.000 4,65·106
Mycoplasma genitalium 500 106
Dna virus 10-300 5.000-200.000
Rna virus 1-25 1.000-23.000
Viroid 0 150-400

Individual evidence

  1. Helen Pearson: What is a gene? Nature 441, 398-401 (May 25, 2006) PMID 16724031
  2. Gerstein et al .: What is a gene, post-ENCODE? History and updated definition Genome Res. 2007 Jun; 17 (6): 669-81. PMID 17567988
  3. http://www.nature.com/nature/journal/v447/n7141/abs/nature05805.html Eric Lander et al: Genome of the marsupial Monodelphis domestica reveals innovation in non-coding sequences, Nature 447, 167-177 (10 May 2007)

literature

  • Ernst P. Fischer: History of the gene. Fischer, Frankfurt 2003, ISBN 3-596-15363-8
  • Benjamin Lewin: Molecular Biology of Genes. Spectrum Academic Publishing House, Heidelberg 2002. ISBN 3-8274-1349-4 (German)
  • Benjamin Lewin: Genes 8. Pearson Prentice Hall, London 2004. ISBN 0-13-143981-2 (English)
  • Inge Kronberg: Which genes make humans human? In: Biology in our time. 34.2004,4, pp.206-207. ISSN 0045-205X
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