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Protein Synthesis

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Biology

Proteins are essential for the functioning of cells and of all life. Proteins are polypeptides made of monomeric amino acids. In nature, there are hundreds of different amino acids, but just 20 of them make up the proteins in the human body and other animals. Don't worry, you don't need to know the structures of each amino acid, that's for university-level biology.

What are proteins?

Protein : a large and complex molecule that plays several critical roles in the body.

Proteins include enzymes like DNA polymerase used in DNA replication, hormones like oxytocin secreted during labour, and also antibodies synthesized during an immune response.

All cells contain proteins, making them highly important macromolecules that are essential in every organism. Proteins are even found in viruses, which are not considered living cells!

Protein synthesis is an intelligent process consisting of two main steps: transcription and translation .

Transcription is the transfer of a DNA base sequence into RNA .

Translation is the 'reading' of this genetic RNA material.

Different organelles, molecules, and enzymes are involved in each step, but don't worry: we'll break it down for you so you can see which components are important.

The process of protein synthesis begins with DNA found in the nucleus. DNA holds the genetic code in the form of a base sequence, which stores all the information needed to make proteins.

Genes encode proteins or polypeptide products.

What are the transcription steps in protein synthesis?

Transcription is the first step of protein synthesis and it happens inside the nucleus, where our DNA is stored. It describes the stage in which we make pre-messenger RNA (pre-mRNA), which is a short single-strand of RNA complementary to a gene found on our DNA. The term 'complementary' describes the strand as having a sequence that is opposite to the DNA sequence (ie, if the DNA sequence is ATTGAC, the complementary RNA sequence would be UAACUG).

Complementary base pairing occurs between a pyrimidine and purine nitrogenous base. This means in DNA, adenine pairs with thymine while cytosine pairs with guanine. In RNA , adenine pairs with uracil while cytosine pairs with guanine.

pre-mRNA applies to eukaryotic cells as these contain both introns (non-coding regions of DNA) and exons (coding regions). Prokaryotic cells make mRNA directly as they do not contain introns.

As far as scientists know, only around 1% of our genome codes for proteins and the rest does not. Exons are DNA sequences that code for these proteins, while the rest are considered introns as they do not code for proteins. Some textbooks refer to introns as 'junk' DNA, but this is not entirely true. Some introns play very important roles in the regulation of gene expression.

But why do we need to make another polynucleotide when we already have DNA? Simply put, DNA is far too large a molecule! Nuclear pores mediate what comes in and out of the nucleus, and DNA is too large to pass through and reach the ribosomes, which is the next location for protein synthesis. That is why mRNA is made instead as it is small enough to exit into the cytoplasm.

Read and understand these important points first before reading the steps of transcription. It'll be easier to understand.

  • The sense strand, also known as the coding strand, is the DNA strand containing the code for the protein. This runs from 5 'to 3'.
  • The antisense strand, also known as the template strand, is the DNA strand that does not contain the code for the protein and is simply complementary to the sense strand. This runs 3 'to 5'.

You might find some of these steps very similar to DNA replication, but don't get them confused.

  • The DNA containing your gene unwinds, meaning the hydrogen bonds between the DNA strands are broken. This is catalyzed by DNA helicase.
  • Free RNA nucleotides in the nucleus pair with their complementary nucleotides on the template strand, catalysed by RNA polymerase. This enzyme forms phosphodiester bonds between adjacent nucleotides (this bond forms between the phosphate group of one nucleotide and the OH group at the 3 'carbon of another nucleotide). This means the pre-mRNA strand being synthesized contains the same sequence as the sense strand.
  • The pre-mRNA detaches once the RNA polymerase reaches a stop codon.

Protein Synthesis [+] rna transcription [+] StudySmarterFigure 1. A detailed look into RNA transcription. Wikimedia Commons CC-BY-SA-3.0, Hawk-Eye

Enzymes involved in transcription

DNA helicase is the enzyme responsible for the early step of unwinding and unzipping. This enzyme catalyses the breaking of the hydrogen bonds found between complementary base pairs and allows the template strand to be exposed for the next enzyme, RNA polymerase.

RNA polymerase travels along the strand and catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides. Adenine pairs with uracil while cytosine pairs with guanine.

Remember: in RNA, adenine pairs with uracil. In DNA, adenine pairs with thymine.

What is mRNA splicing?

Eukaryotic cells contain introns and exons. But we only need the exons as these are the coding regions. mRNA splicing describes the process of removing introns so we have an mRNA strand containing just exons. Specialized enzymes called spliceosomes catalyze this process.

Protein Synthesis [+] mRNA splicing [+] StudySmarterFigure 2. mRNA Splicing, Source: Wikimedia Commons, Seru

Once splicing is complete, the mRNA can diffuse out from the nuclear pore and towards the ribosome for translation.

What are the translation steps in protein synthesis?

Ribosomes are organelles responsible for the translation of mRNA, a term that describes the 'reading' of the genetic code. These organelles, which are made of ribosomal RNA and proteins, hold the mRNA in place throughout this step. The 'reading' of the mRNA begins when the start codon, AUG, is detected.

First, we'll need to know about transfer RNA (tRNA). These clover-shaped polynucleotides contain two important features:

  • An anticodon, which will bind to its complementary codon on the mRNA.
  • An attachment site for an amino acid.

Ribosomes can harbour a maximum of two tRNA molecules at a time. Think of tRNAs as the vehicles delivering the correct amino acids to the ribosomes.

Below are the steps for translation:

  • The mRNA binds to the small subunit of a ribosome at the start codon, AUG.
  • A tRNA with a complementary anticodon, UAC, binds to the mRNA codon, carrying with it the corresponding amino acid, methionine.
  • Another tRNA with a complementary anticodon for the next mRNA codon binds. This allows the two amino acids to come close.
  • The enzyme, peptidyl transferase, catalyses the formation of a peptide bond between these two amino acids. This uses ATP.
  • The ribosome travels along the mRNA and releases the first bound tRNA.
  • This process repeats until a stop codon is reached. At this point, the polypeptide will be complete.

Protein Synthesis [+] mrna translation in the ribosome [+] StudySmarterFigure 3. Ribosome mRNA translation Source: Wikimedia Commons Public Domain

Translation is a very quick process because up to 50 ribosomes can bind behind the first so that the same polypeptide can be made simultaneously.

Enzymes involved in translation

Translation features one main enzyme, peptidyl transferase, which is a component of the ribosome itself. This important enzyme uses ATP to form a peptide bond between adjacent amino acids. This helps form the polypeptide chain.

What happens after translation?

Now you have a completed polypeptide chain. But we are not done yet. Although these chains can be functional by themselves, the majority undergo further steps to become functional proteins. This includes polypeptides folding into secondary and tertiary structures and Golgi body modifications.

Protein Synthesis - Key takeaways

  • Transcription describes the synthesis of pre-mRNA from the template strand of DNA. This undergoes mRNA splicing (in eukaryotes) to produce an mRNA molecule made of exons.
  • The enzymes DNA helicase and RNA polymerase are the main drivers of transcription.
  • Translation is the process by which the ribosomes 'read' the mRNA, using tRNA. This is where the polypeptide chain is made.
  • The main enzymatic driver of translation is peptidyl transferase.
  • The polypeptide chain can undergo further modifications such as folding and Golgi body additions.

Protein Synthesis

Protein synthesis describes the process of transcription and translation in order to make a functional protein.

The first step of protein synthesis, transcription, takes place inside the nucleus: this is where (pre-) mRNA is made. Translation takes place at the ribosomes: this is where the polypeptide chain is made.

The ribosomes are responsible for the translation of the mRNA and this is where the polypeptide chain is made.

DNA holds the code for a gene in its sense strand, which runs 5 'to 3'. This base sequence is transferred onto an mRNA strand during transcription, using the antisense strand. At the ribosomes, tRNA, which contains a complementary anticodon, delivers the respective amino acid to the site. This means the building of the polypeptide chain is 

purely informed by the gene. 

Transcription starts off with DNA helicase which unzips and unwinds the DNA to expose the template strand. Free RNA nucleotides bind to their complementary base pair and RNA polymerase catalyses the formation of phosphodiester bonds between adjacent nucleotides to form pre-mRNA. This pre-mRNA undergoes splicing so that the strand contains all coding regions. 


mRNA attaches to a ribosome once it exits the nucleus. A tRNA molecule with the correct anticodon delivers an amino acid. Peptidyl transferase will catalyse the formation of peptide bonds between amino acids. This forms the polypeptide chain which can undergo further folding to become fully functional.

Final Protein Synthesis Quiz

Question

How many amino acids are there in the human body?

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Answer

20 amino acids.

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Question

What is transcription and where does it take place?

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Answer

Transcription describes the process of making a pre-mRNA molecule. This occurs inside the nucleus.

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Question

What is the difference between introns and exons?


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Answer

Introns are non-coding regions of DNA whereas exons are coding regions of DNA.

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Question

What kind of mRNA do eukaryotic and prokaryotic cells make?


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Answer

During transcription, eukaryotic cells make pre-mRNA while prokaryotic cells make mRNA.

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Question

The sense strand runs ___ to ___.


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Answer

5 'to 3'.

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What is the difference between the sense strand and the antisense strand?

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Answer

The sense strand is the DNA strand containing the code for the protein. The antisense strand, also termed the template strand, is complementary to the sense strand.

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Question

 What are the two enzymes involved in transcription? What are their functions?


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Answer

DNA helicase and RNA polymerase. 


DNA helicase unwinds DNA by breaking hydrogen bonds between complementary base pairs. 


RNA polymerase catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides.

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Question

In RNA, adenine pairs with what nucleotide?

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Answer

Uracil.

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In DNA, adenine pairs with what nucleotide?


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Answer

Thymine.

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What is mRNA splicing?


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Answer

The removal of introns from pre-mRNA in eukaryotic cells so the resulting mRNA strand contains just exons.

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What is translation and where does it take place?


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Answer

Translation describes the process of building the polypeptide chain by 'reading' the mRNA. This occurs at the ribosomes.

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Question

What are the two most important features of tRNA?


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Answer

tRNA contains an anticodon that can bind to its complementary mRNA codon. 


tRNA contains an attachment site for an amino acid. 

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Question

What is the main enzyme involved in translation? What is its function?

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Answer

Peptidyl transferase. This catalyses the formation of a peptide bond between amino acids using ATP.

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Question

What is the start codon in mRNA? What amino acid does it code for?


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Answer

AUG. These codes for methionine.

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How many tRNA's can the ribosomes harbor at any given time?


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Answer

Two tRNAs.

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 What is the end product of transcription?

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pre-mRNA for eukaryotic cells. mRNA for prokaryotic cells.

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What does the mRNA contain?

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Answer

mRNA contains codons that are derived from the template strand of DNA. These codes for the protein/polypeptide to be synthesized.

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What is the main function of mRNA?


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Answer

mRNA transfers the codons for a gene from the nucleus to the ribosomes found in the cytoplasm.

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Question

What process does pre-mRNA undergo before translation?


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Answer

pre-mRNA in eukaryotic cells undergo mRNA splicing. This is where unwanted introns are removed, catalysed by spliceosomes.

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Question

Where does translation take place?


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Answer

Ribosomes.

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Question

What is the main function of tRNA?


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Answer

tRNA delivers amino acids to the ribosomes to add to the polypeptide chain.

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What are the components that are common to both mRNA and tRNA?


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Answer

A phosphate group, a ribose pentose sugar and a nitrogenous base.

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What are the RNA nitrogenous bases?


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Answer

Adenine, uracil, cytosine and guanine.

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What is the structure of mRNA?


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Answer

mRNA is a short single-stranded structure that contains codons corresponding to a protein / polypeptide.

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Question

What are the functionally important regions in tRNA? What role do these regions play?


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Answer

The anticodon loop. These bind to the complementary codons found on mRNA.


The amino acid attachment site. This is where the amino acid temporarily binds for transport. 

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Question

What enzyme allows the binding of the correct amino acid to its corresponding tRNA?


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Answer

Aminoacyl-tRNA synthetase.

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Question

What pentose sugar do DNA and RNA contain?


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Answer

DNA contains deoxyribose. RNA contains ribose.

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Question

Which nucleic acid contains uracil? What is its complementary base pair?


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Answer

RNA. Uracil pairs with adenine.

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Question

Compare the structure of mRNA and tRNA.


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Answer

mRNA is a single-stranded helical structure. tRNA is also a single-stranded structure and it is folded into a cloverleaf shape.

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Question

mRNA contains codons while tRNA contains ______ anticodons. 


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Answer

Complementary.

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Question

How many amno acids are found in the human body?

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Answer

20 amino acids.

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