DNA, RNA and protein synthesis

chapter 20 DNA, RNA and protein synthesis



KEY POINTS


image The flow of information from DNA through mRNA to protein is known as the central dogma of molecular biology.

image DNA is a nucleic acid which has the blueprint for the synthesis of all the proteins of the body.

image A gene is a portion of DNA, identified by a sequence of nucleotides, which provides information for the synthesis of not only protein but also of tRNA and rRNA.

image Transcription is the process by which RNA is synthesised.

image Three types of RNAs act together to apply the information as dictated in DNA to protein synthesis.

image Messenger RNA (mRNA) carries information for protein synthesis from the nucleus to cytoplasm.

image Transfer RNA (tRNA) carries the correct amino acid to mRNA.

image Ribosomal RNA (rRNA) is associated with protein making up the ribosome, the site for protein synthesis.

image Template refers to the DNA strand that is active in transcription.

image Translation is the interpretation of the codes in mRNA into an amino acid sequence of the protein to be synthesised.

image Codon is the sequence of three nucleotides that specifies one amino acid.

image Proenzymes or zymogens are enzymes synthesised in an inactive form for export out of the cell.

Earlier chapters have shown how the cell can use a number of different proteins to create all of its components. Some of these proteins are enzymes and catalyse the formation of other parts of the cell such as lipids. Others are structural proteins that provide a scaffold within the cell. The properties of all of these proteins are determined by the sequence of amino acids within the protein. How then does a cell ‘know’ what that sequence of amino acids should be?


The information that instructs the cell on how to make each and every one of its proteins is encoded in the DNA contained in the cell. As outlined in Chapter 9, DNA is made up of many molecules of the four different nucleotides A, G, C and T joined together as a large polymeric molecule. The information contained in DNA is determined by the sequence of the different nucleotides that arises when moving along the DNA strand.


This chapter explains the way in which this information is organised in the DNA and the way in which the cell uses the information to synthesise proteins.



Genes


The information stored in the DNA of chromosomes is divided up into discrete, heritable units or genes. Each gene has a particular role in the cell. The majority of genes provide the instructions or code for making proteins, as shown in Figure 20-1.


image

FIGURE 20-1 The relationship of genes to proteins.


Genes are the biological unit of heredity that determines the protein to be synthesised and thus the characteristics of the individual such as eye or hair colour. The genome is a collective term that refers to all the DNA in an organism.


Some genes do not encode for protein, rather provide the instructions of specialised RNA molecules such as tRNA and rRNA (Fig 20-2), which play an active role in protein synthesis.


image

FIGURE 20-2 Genes provide the information for the synthesis of not only protein, but also tRNA and rRNA.



Chromosomes


The DNA in a mammalian cell is contained within the nucleus. The DNA molecules are associated with a number of specialised DNA-binding proteins called histones. When cells are ready to divide the histones and DNA coil up into very dense structures that can be seen under the microscope. These complexes of DNA and histones are known as chromosomes.


In humans there are 23 pairs of chromosomes in a ‘normal’ cell, giving 46 chromosomes in total. For 22 pairs, each chromosome of the pair contains the information for the same genes and are said to be homologous. The exceptions to this are the sex chromosomes. These chromosomes are designated as X and Y. In human females each cell has two copies of X and therefore both copies contain the same information. However, in human males each cell has one X and one Y chromosome and each of these chromosomes contains different information.


When humans reproduce specialised sex cells, or gametes, are made; sperm in males and ova in females. When these cells are made the pairs of chromosomes are split up such that each member of each pair ends up in different cells. When fertilisation occurs the chromosomes from one sperm and one ovum are mixed together to give the full complement of 46 chromosomes in the developing fetus.



The processes in protein synthesis


Using DNA to store the information required for the building of a cell results in two problems. First, DNA does not generally leave the nucleus, yet protein synthesis occurs outside the nucleus in the cytoplasm and the rough endoplasmic reticulum. Second, DNA consists of only four different nucleotides yet proteins can be made from up to 20 different amino acids. Two key processes are used to make sure that the message can leave the nucleus and that the sequence of nucleotides can specify where each of the 20 amino acids sits in the polypeptide chain of a protein.


The first of these processes is known as transcription (see below). In this process the information contained in a gene is copied or transcribed into an RNA molecule, known as messenger RNA (mRNA), which can leave the nucleus.


The second of these processes is known as translation (see below). In this process the copied information in the mRNA is used to define the sequence of amino acids in a protein. To do this the information contained in a gene is grouped into codons. A codon represents a sequence of three nucleotides that specify one amino acid, for example GCG codes for alanine. Each codon can be thought of as a three-letter word and the gene as a sentence made up of a sequence of these three-letter words to constitute the specific protein. Since there are four different bases making up the nucleotides, then there are 4 × 4 × 4 or 64 different code words possible. But there are only about 20 amino acids in nature. So what is the meaning of the remaining code words? The genetic code, or the specific sequence of nucleotides coding for a specific protein, is degenerate. This means that several different codons may specify the same amino acid; for example, GCU, GCC, GCA and GCG all code for alanine. In other words, it does not have a unique set of code words, just as in the English language different words are used to describe the same thing, for example, list, catalogue, inventory, schedule and register. In addition, some codons may indicate start and stop signals, just as a sentence is not complete without punctuation marks.


The flow of information from DNA to protein via mRNA is known as the central dogma of molecular biology.



Characteristics of a mammalian gene


Mammalian genes differ greatly in their size and in the size of protein they code for. However, all genes have some common characteristics.


Within the DNA of a chromosome are small stretches of DNA or motifs that identify a gene. These include a promoter region, which contains motifs that indicate the beginning of a gene. This defines the place at which the cell should start copying the DNA into mRNA. Yet other motifs instruct the cell where to stop copying the DNA. The message in the gene is not continuous, as shown in Figure 20-3.


image

FIGURE 20-3 The DNA of the human β-globin gene used as an example to illustrate the existence of exons or amino-acid-specifying sequences, interrupted by introns or nonsense-specifying sequences in the DNA of the eukaryotic genes.


In the eukaryotic gene there are exons and introns

Related posts:

  1. Unifying principles of biology
  2. Amino acids and proteins
  3. Catabolism of macromolecules and energy generation
  4. Anabolic metabolism

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Jun 11, 2016 | Posted by in BIOCHEMISTRY | Comments Off on DNA, RNA and protein synthesis

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