Protein synthesis is the mechanism of protein production determined by the DNA, which happens in two phases called transcription and translation.
The process takes place in the cytoplasm of cells and also involves RNA, ribosomes, specific enzymes and amino acids that will help in the sequence of the protein to be formed.
Stages of gene or genetic expression.
In short, DNA is "transcribed" by messenger RNA (mRNA) and then the information is "translated" by ribosomes (ribosomal RNA compounds and protein molecules) and by the transporter RNA (tRNA), which transports the amino acids, whose sequence will determine the protein to be formed.
Gene expression
The steps in the protein synthesis process are regulated by genes. Gene expression is the name of the process by which the information contained in genes (the DNA sequence) generates products genes, which are RNA molecules (in the gene transcription step) and proteins (in the translation step gene).
Gene Transcription
In this first phase, the DNA molecule opens, and the codes present in the gene are
transcribed for the RNA molecule. THE RNA polymerase enzyme it attaches to one end of the gene, separating the strands of DNA, and the free ribonucleotides pair with the strand of DNA that serves as a template.The sequence of nitrogen bases of RNA follow exactly the sequence of bases of DNA, according to the following rule:
- U with A (Uracil-RNA and Adenine-DNA),
- A with T (Adenine-RNA and Thymine-DNA),
- C with G (Cytosine-RNA and Guanine-DNA) and
- G with C (Guanine-RNA and Cytosine-DNA).
What determines the beginning and end of the gene that will be transcribed are specific nucleotide sequences, the beginning is the promoter region of the gene and the end is the terminal region. The RNA polymerase fits into the promoter region of the gene and travels to the terminal region.
genetic translation
THE polypeptide chain is formed by the joining of amino acids according to the sequence of nucleotides of the mRNA. This mRNA sequence, called codon, is determined by the base sequence of the DNA strand that served as the template. Thus, protein synthesis is the translation of information contained in the gene, which is why it is called gene translation.
Genetic Code: Codons and Amino Acids
There is a correspondence between the sequence of nitrogenous bases, which make up the codon of the mRNA, and the amino acids associated with him called genetic code. The combination of triples of bases form 64 different codons to which correspond 20 types of amino acids that will compose the proteins.
See the following figure for the circle of the genetic code, which must be read from the middle outwards, so example: the AAA codon is associated with the amino acid lysine (Lys), GGU is glycine (Gly) and UUC is phenylalanine (Phe).
Genetic Code Circle. The AUG codon, associated with the Methionine amino acid, is the initiation codon and the UAA, UAG and UGA codons without associated amino acids, are the stop codons.
The genetic code is said to be "degenerate" because many of the amino acids can be encoded by the same codon, such as the serine (Ser) associated with the UCU, UCC, UCA, and UCG codons. However, there is the amino acid Methionine associated with only one AUG codon, which signals the start of translation, and 3 stop codons (UAA, UAG and UGA) not associated with any amino acid, which signal the end of protein synthesis.
Learn more about Genetic Code.
Polypeptide Chain Formation
Schematic representation of the association between the ribosome, tRNA and mRNA for protein formation.
Protein synthesis begins with the association between a tRNA, a ribosome and an mRNA. Each tRNA carries an amino acid whose base sequence, called anticodon, corresponds to the mRNA codon.
The tRNA carrying a methionine, oriented by the ribosome, binds to the mRNA where the corresponding codon (AUG) is found, starting the process. Then it turns off and another tRNA turns on, bringing another amino acid.
This operation is repeated several times forming the polypeptide chain, whose amino acid sequence is determined by mRNA. When the ribosome finally reaches the region of the mRNA where there is a stop codon, the end of the process.
Who participates in the Synthesis?
Comparison between a DNA (double stranded) and an RNA (single stranded) molecule.
DNA
Genes are specific parts of the molecule of DNA, which have codes that will be transcribed into RNA. Each gene determines the production of a specific RNA molecule.
Not every DNA molecule contains genes, there are some that do not have the information for gene transcription, they are non-coding DNA, and their function is not well known.
RNA
the molecules of RNA are produced from a DNA template. DNA is a double strand, only one of which is used for RNA transcription.
The enzyme participates in the transcription process RNA polymerase. Three different types are produced, each with a specific function: mRNA - messenger RNA, tRNA - transport RNA and rRNA - ribosomal RNA.
Ribosomes
You ribosomes they are structures present in eukaryotic and prokaryotic cells, whose function is to synthesize proteins. They are not organelles because they do not have membranes, they are species of granules, whose structure is composed of the folded ribosomal RNA molecule, associated with proteins.
They are formed by 2 subunits and are located in the cytoplasm, free or associated with the rough endoplasmic reticulum.
Check the differences between DNA and RNA.
Exercises
1. (MACK) The UGC, UAU, GCC and AGC codons code, respectively, for the amino acids cysteine, tyrosine, alanine and serine; the UAG codon is terminal, that is, it indicates the interruption of the translation. A DNA fragment, which encodes the serine – cysteine – tyrosine – alanine sequence, has suffered the loss of 9The nitrogenous base. Check the alternative that describes what will happen to the amino acid sequence.
a) The tyrosine amino acid will be replaced by another amino acid.
b) The amino acid tyrosine will not be translated, resulting in a molecule with 3 amino acids.
c) The sequence will not be translated, as this altered DNA molecule is not capable of commanding this process.
d) Translation will be interrupted at the 2nd amino acid.
e) The sequence will not suffer damage, as any modification in the DNA strand is immediately corrected.
Correct alternative: d) The translation will be interrupted at the 2nd amino acid.
2. (UNIFOR) “The messenger RNA is produced in the ____I___ and, at the ____II___ level, it is associated with ____IIII___ participating in the synthesis of ____IV___ .” To correctly complete this sentence, I, II, III and IV must be replaced, respectively, per:
a) ribosome – cytoplasmic – mitochondria – energy.
b) ribosome – cytoplasmic – mitochondria – DNA.
c) nucleus – cytoplasmic – mitochondria – proteins.
d) cytoplasm – nuclear – ribosomes – DNA.
e) nucleus – cytoplasmic – ribosomes – proteins.
Correct alternative: e) nucleus – cytoplasmic – ribosomes – proteins.
3. (UFRN) An X protein encoded by the Xp gene is synthesized on ribosomes from an mRNA. For for the synthesis to take place, it is necessary that the steps take place, in the nucleus and in the cytoplasm, respectively, in:
a) Initiation and transcription.
b) Initiation and termination.
c) Translation and termination.
d) Transcription and translation.
Correct alternative: d) Transcription and translation.
4. (UEMA) The genetic code is a biochemical information system that allows the production of proteins, which determine the structure of cells and control all metabolic processes. Mark the correct alternative where the structure of the genetic code is found.
a) A random sequence of nitrogenous bases A, C, T, G.
b) A sequence of broken DNA bases indicates a sequence of nucleotides that must come together to form a protein.
c) A sequence of broken RNA bases indicates a sequence of amino acids that must come together to form a protein.
d) A random sequence of nitrogenous bases A, C, U, G.
e) A sequence of broken DNA bases indicates a sequence of amino acids that must come together to form a protein.
Correct alternative: e) A sequence of broken DNA bases indicates a sequence of amino acids that must come together to form a protein.
You may also be interested in:
- Cytoplasm
- protein structure
- Introduction to Genetics
