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By the end of this section, you will be able to:
  • Describe the different steps in protein synthesis
  • Discuss the role of ribosomes in protein synthesis

The synthesis of proteins consumes more of a cell’s energy than any other metabolic process. In turn, proteins account for more mass than any other component of living organisms (with the exception of water), and proteins perform virtually every function of a cell. The process of translation, or protein synthesis, involves the decoding of an mRNA message into a polypeptide product. Amino acids are covalently strung together by interlinking peptide bonds in lengths ranging from approximately 50 amino acid residues to more than 1,000. Each individual amino acid has an amino group (NH 2 ) and a carboxyl (COOH) group. Polypeptides are formed when the amino group of one amino acid forms an amide (i.e., peptide) bond with the carboxyl group of another amino acid ( [link] ). This reaction is catalyzed by ribosomes and generates one water molecule.

Illustration shows two amino acids side-by-side. Each amino acid has an amino group, a carboxyl group, and a side chain labeled R or R'. Upon formation of a peptide bond, the amino group is joined to the carboxyl group. A water molecule is released in the process.
A peptide bond links the carboxyl end of one amino acid with the amino end of another, expelling one water molecule. For simplicity in this image, only the functional groups involved in the peptide bond are shown. The R and R' designations refer to the rest of each amino acid structure.

The protein synthesis machinery

In addition to the mRNA template, many molecules and macromolecules contribute to the process of translation. The composition of each component may vary across species; for instance, ribosomes may consist of different numbers of rRNAs and polypeptides depending on the organism. However, the general structures and functions of the protein synthesis machinery are comparable from bacteria to human cells. Translation requires the input of an mRNA template, ribosomes, tRNAs, and various enzymatic factors.

Click through the steps of this PBS interactive to see protein synthesis in action.

Ribosomes

Even before an mRNA is translated, a cell must invest energy to build each of its ribosomes. In E. coli , there are between 10,000 and 70,000 ribosomes present in each cell at any given time. A ribosome is a complex macromolecule composed of structural and catalytic rRNAs, and many distinct polypeptides. In eukaryotes, the nucleolus is completely specialized for the synthesis and assembly of rRNAs.

Ribosomes exist in the cytoplasm in prokaryotes and in the cytoplasm and rough endoplasmic reticulum in eukaryotes. Mitochondria and chloroplasts also have their own ribosomes in the matrix and stroma, which look more similar to prokaryotic ribosomes (and have similar drug sensitivities) than the ribosomes just outside their outer membranes in the cytoplasm. Ribosomes dissociate into large and small subunits when they are not synthesizing proteins and reassociate during the initiation of translation. In E. coli , the small subunit is described as 30S, and the large subunit is 50S, for a total of 70S (recall that Svedberg units are not additive). Mammalian ribosomes have a small 40S subunit and a large 60S subunit, for a total of 80S. The small subunit is responsible for binding the mRNA template, whereas the large subunit sequentially binds tRNAs. Each mRNA molecule is simultaneously translated by many ribosomes, all synthesizing protein in the same direction: reading the mRNA from 5' to 3' and synthesizing the polypeptide from the N terminus to the C terminus. The complete mRNA/poly-ribosome structure is called a polysome    .

Questions & Answers

what is Alimentary canal?
Princess Reply
what is commenialism
jamex Reply
Do you mean commensalism?,it is a feeding relationship that has to do with two different species feeding, that is one is benefiting and the other is unaffected.
hamidat
commensalism is a feeding relationship that has to do with different species feeding, one is gaining and the other is unaffected.
hamidat
what is the work of phloem tissue
Rose Reply
What is osmoregulation?
Jacklin
osmoregulation is the maintenance of internal body through the aid of water
favour
mention four characteristics of enzymes
Leah
mention four characteristics of enzymes
Agorong
The phloem tissue is responsible for the distribution of manufactured food in the shoot.
hamidat
Enzymes speed up the rate of chemical reactions,they are catalytic protein.
hamidat
to transport the manufactured food
Ronald
A skeleton is any film structure that gives mechanical support to the body and provides protection to the softer parts of the body.
Patrick Reply
nice
Kabir
A skeleton is a frame or bony part of a body that aid in the movement of different parts of the body.
hamidat
What is a skeleton?
Chipo Reply
is a bone without meat
Stanley
is a frame work of d body that provide support nd rigidity 4 d body
tunz
is a structure of the body without the organs and the skin
Rose
what is an electron microscope?
Chr Reply
is a microscope that is used to study small organisms in the cell.
Rose
is a microscope which uses electricity to magnify
jamex
okay
Rose
what is commenialism
jamex
what is an antigen?
Luyando
an antigen is any thing that reduces the action of a gene
hamidat
what is biochemist
Lenard Reply
biochemistry:-is the study of chemical reaction with living organisms
Kabir
HOW MANY DNA STRANDS DOES CORONA VIRUS HAVE?
Baramox
GO AND ASK GOOGLE.
hamidat
what are the scientific method
Precious Reply
describe the functioning of the Golgi body in animal cells
Naiga Reply
used to package minerals in the cell
Rose
what features does red blood have that allows it to effectively move through the blood and transport oxygen? list at least four features and explain how they help RBC's carry out their functions.
Alice Reply
has nucleus, haemoglobin
favour
what is biology
kenneth Reply
it's the study of living organisms and their interactions with one another and the environment
Precious
Biology is the study of life.
hamidat
is the study of living organisms and there structure
Rose
describe the structure of DNA
Mafashion Reply
it a double helical structure negatively charged as a results of a phosphate backbone and the two strands are joined together by hydrogen bonds
Ebenezer
Bring out clearly the process of clothing
Irene Reply
How is the region of unwinding called
Irene
what is hoemositasis
Violet
homeostasis is the regulation of a constant internal environment
hamidat
what is DNA replication
jamex
is de maintance of constant internal environment
jamex
is an unfavourable conditions cost by capavic bacteria
hashim Reply
no idea
hamidat
no idea
Precious
Describe the following terms used in nutrition, parasitism, saprophytic, symbiosys
Meymo Reply
parasitism involves gaining and losing
hamidat
a saprophyte does not cause harm to it Host.
hamidat
in symbiosis the two organisms are benefiting
hamidat

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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