here is a cell the basic unit of all living tissue in most human cells there is a structure called the nucleus the nucleus contains the genome in humans the genome is split between 23 pairs of chromosomes each chromosome contains a long strand of DNA tightly packaged around proteins called histones within the DNA are sections called genes these genes contain the instructions for making proteins when a gene is switched on an enzyme called RNA polymerase attaches to the start of the gene it moves along the DNA making a strand of messenger RNA out of free bases
in the nucleus the DNA code determines the order in which the free bases are added to the messenger RNA this process is called transcription before the messenger RNA can be used as a template for the production of proteins it needs to be processed this involves removing and adding sections of RNA the messenger RNA then moves out of the nucleus into the cytoplasm protein factories in the cytoplasm called ribosomes bind to the messenger RNA the ribosome reads the code in the messenger RNA to produce a chain made up of amino acids there are 20 different types
of amino acid transfer RNA molecules carry the amino acids to the ribosome the messenger RNA is read 3 bases at a time as each triplet is read a transfer RNA delivers the corresponding amino acid this is added to a growing chain of amino acids once the last amino acid has been added the chain folds into a complex 3d shape to form the protein you protein synthesis translation inside the body the process of translation occurs within every single cell each cell has a nucleus after transcription mRNAs move out of the nucleus and enter the cytoplasm this
mRNA strand acts as a template for protein synthesis present in the cytoplasm is an enzyme amino acyl tRNA synthetase the enzyme macro molecule has two binding sites one site recognizes the amino acid methionine this is followed by the binding of the ATP molecule and release of pyrophosphate resulting in activation of amino acid finally the tRNA and the activated amino acid bind together this amino acyl ated tRNA is known as met tRNA and is released from the enzyme this marks the commencement of first stage of protein synthesis the initiation stage during the eighth initiation stage a
small subunit of a ribosome binds to the mRNA strand the mRNA strand is made up of codons which are sequences of 3 bases then the ribosome subunit moves along the mRNA in five prime to three prime direction until it recognizes the Aug codon or the initiation codon at this point net tRNA possessing the anticodon UAC pairs up with the Aug codon of the M RNA then a large subunit of ribosome combines with a small ribosomal subunits the lab subunit shows three sides the acceptor site or the a site v dial site or the P site
the exit site or the e site this whole unit forms the initiation complex this is followed by the elongation stage during this stage another tRNA carrying molecule of an amino acid approaches the M RNA ribosome complex and fits in the a site then a bond is formed between methionine and the amino acid molecule on the tRNA as a result met tRNA becomes d isolated the ribosome then advances a distance of one codon and the D isolated tRNA shifts to the east side from where it dissociates meanwhile another tRNA carrying an amino acid molecule attaches to
the a site this is followed by the binding of the amino acid molecules [Music] repetition of this process leads to the formation of an amino acid chain this event is called elongation finally when the UAG codon or the stop codon reaches the a site elongation is terminated termination is the last stage of protein synthesis the chain of amino acid molecules is released from the ribosome this released amino acid chain is the protein and this part of protein synthesis is known as translation then the tRNA detaches from the mRNA ribosome detaches and dissociates into its small
and large subunits summary protein synthesis shows that the first stage involves the binding of met tRNA to M RNA and the small subunit of the ribosome the larger subunit of ribosome then combines with a small subunit second stage is the elongation stage in this stage the incoming amino acyl tRNA fits in the a site then a bond is formed between methionine and the amino acid molecule on the tRNA the process is repeated until a chain of amino acid molecules is formed the last stage of protein synthesis is the termination stage when the ribosome reaches the
stop codon you AG elongation stops and the newly formed a minor acid chain which is the protein macro molecule detaches from the ribosome subsequently ribosomal subunits along with the tRNA dissociate from the M RNA in order for our bodies to function we need to supply them with a variety of nutrients we get from our diet our bodies cannot use the food as it is when it enters our digestive system the process of chemical digestion uses different proteins and enzymes to break down the food particles into usable nutrients our cells can absorb and where are the
instructions to manufacture these and all the different types of proteins we need to stay alive the instructions to make proteins are contained in our DNA DNA contains genes a gene is a continuous string of nucleotides containing a region that codes for an RNA molecule this region begins with a promoter and ends in a terminator genes also contain regulatory sequences that can be found near the promoter or at a more distant location for some genes the encoded RNA is used to synthesize a protein in a process called gene expression for these genes expression can be divided
into two processes transcription and translation in eukaryotic cells transcription occurs in the nucleus where DNA is used as a template to make messenger RNA then in translation which occurs in the cytoplasm of the cell the information contained in the messenger RNA is used to make a polypeptide during transcription the DNA in the gene is used as a template to make a messenger RNA strand with the help of the enzyme RNA polymerase this process occurs in three stages initiation elongation and termination during initiation the promoter region of the gene functions as a recognition site for RNA
polymerase to bind this is where the majority of gene expression is controlled by either permitting or blocking access to this site by the RNA polymerase binding causes the DNA double helix to unwind and open then during elongation the RNA polymerase slides along the template DNA strand as the complementary bases pair up the RNA polymerase links nucleotides to the 3 prime end of the growing RNA molecule once the RNA polymerase reaches the terminator portion of the gene the messenger RNA transcript is complete and the RNA polymerase the DNA strand and the messenger RNA transcript dissociate from
each other the strand of messenger RNA that is made during transcription includes regions called exons that code for a protein and non-coding sections called introns in order for the messenger RNA to be used in translation the non-coding introns need to be removed and modifications such as a five prime cap and a 3 prime poly a tail are added this process is called introns splicing and is performed by a complex made up of proteins and RNA called a spliceosome this complex removes the intron segments and joins the adjacent exons to produce a mature messenger RNA strand
that can leave the nucleus through a nuclear pore and enter the cytoplasm to begin translation how is the information in the mature messenger RNA strand translated into a protein the nitrogenous bases are grouped into three letter codes called codons the genetic code includes 64 codons most codons code for specific amino acids there are four special codons one that codes for start and three that code for stop translation begins with the messenger RNA strand binding to the small ribosomal subunit upstream of the start codon each amino acid is brought to the ribosome by a specific transfer
RNA molecule the type of amino acid is determined by the anticodon sequence of the transfer RNA complementary base pairing occurs between the codon of the messenger RNA and the anticodon of the transfer RNA after the initiator transfer RNA molecule binds to the start codon the large ribosomal subunit binds to form the translation complex and initiation is complete in the large ribosomal subunit there are three distinct regions called the e P and a sites during elongation individual amino acids are brought to the messenger RNA strand by a transfer RNA molecule through complementary base pairing of the
codons and anticodons each Eddie codon of a transfer RNA molecule corresponds to a particular amino acid a charged transfer RNA molecule binds to the a site and a peptide bond forms between its amino acid and the one attached to the transfer RNA molecule at the P site the complex slides down one codon to the right where the now uncharged transfer RNA molecule exits from the e site and the a site is open to accept the next transfer RNA molecule elongation will continue until a stop codon is reached a release factor binds to the a site
at a stop codon and the polypeptide is released from the transfer RNA in the P site the entire complex dissociates and can reassemble to begin the process again at initiation the purpose of translation is to produce polypeptides quickly and accurately after dissociation the polypeptide may need to be modified before it is ready to function modifications take place in different organelles for different proteins in order for a digestive enzyme to be secreted into the stomach or intestines the polypeptide is translated into the endoplasmic reticulum modified as it passes through the Golgi then secreted using a vesicle
through the plasma membrane of the cell into the lumen of the digestive tract proteins are needed for most physiological functions of the body to occur properly such as breaking down food particles in digestion and the processes of transcription and translation make the production of proteins possible the job of this mRNA is to carry the genes message from the DNA out of the nucleus to a ribosome for production of the particular protein that this gene codes for there can be several million ribosomes in a typical eukaryotic cell these complex catalytic machines use the mRNA copy of
the genetic information to assemble amino acid building blocks into the three-dimensional proteins that are essential for life let's see how it works the ribosome is composed of one large and one small subunit that assemble around the messenger RNA which then passes through the ribosome like a computer tape the amino acid building blocks that's the small glowing red molecules are carried into the ribosome attached to specific transfer RNAs that's the larger green molecules also referred to as tRNA the small subunit of the ribosome positions the mRNA so that it can be read in groups of three
letters known as a codon each codon on the MRNA matches a corresponding anticodon on the base of a transfer RNA molecule the larger subunit of the ribosome removes each amino acid and joins it onto the growing protein chain as the mRNA is ratcheted through the ribosome the mRNA sequence is translated into an amino acid sequence there are three locations inside the ribosome designated the a site the P site and the e site the addition of each amino acid is a three step cycle first the tRNA enters the ribosome at the a site and is tested
for a codon anticodon match with the mRNA next provided there is a correct match the tRNA is shifted to the P site and the amino acid it carries is added to the end of the amino acid chain the mRNA is also ratcheted on three nucleotides or one codon thirdly the spent tRNA is moved to the east side and then ejected from the ribosome to be recycled as the protein synthesis proceeds the finished chain emerges from the ribosome it folds up into a precise shape determined by the exact order of amino acids thus the central dogma
explains how the four-letter DNA code is quite literally turned into flesh and blood you
