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In subjects such as biology,
details of a phenomenon or process...
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can be better understood if they are exposed
in a logical way, transiting in a timely manner...
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from the abstract to the concrete; or
vice versa. All in a coherent context
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I will make an effort to achieve the above
exposing in a basic way the process...
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of DNA transcription and
protein translation or synthesis
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Let's start explaining this topic by
what do we know...
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we know that DNA contains
instructions to build a living being, right?
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But... how does DNA become
a living being?
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The details in this regard overwhelm even the scientists, however...
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the fundamental principle is the following:
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A fragment of the DNA is copied
or transcribes into a molecule called ribonucleic acid or RNA for short
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It is from this copy that proteins are synthesized or produced
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Proteins are like the basic building blocks of a living being...
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like Wally the cat, or like you or me
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The copy or transcription of DNA in RNA is done in the cell nucleus...
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in the membrane core of the eukaryotic cell and in the nucleus without membrane of the prokaryotic cell...
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but the translation or synthesis of proteins happens in a cellular structure called ribosome
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Here we see a pair of ribosomes in the cytoplasm
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Resembling perhaps an acorn, the ribosome is composed of two parts: one smaller than the other
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Here there are a lot of ribosomes in the
prokaryotic cell
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The DNA is lodged in the nucleus and organizes in chromosomes like that of this eukaryotic cell
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For reasons I'm not going to list here...
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the DNA of the nucleus can’t go directly to the cytoplasm...
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but the DNA fragments transcribed in the form of RNA can come out of the nucleus...
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...and take the information or message to the cytoplasm
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It is very clear why this ribonucleic acid is called messenger RNA or mRNA
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Unlike DNA that is composed of two strands, RNA is a molecule of a single strand or chain of nucleotides
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Remember that each DNA nucleotide consists of a deoxyribose sugar...
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a nitrogenous base and a phosphate group
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Deoxyribose is a sugar of five carbon atoms
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I'm pointing them with the laser indicator
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In chemistry, we number these
carbons from the one that joins the nitrogenous base
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You see? We have five!
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The fifth and third carbon are two important extremes...
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We call these extremes:
5 prime phosphate and 3 prime hydroxyl
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Here is the scheme of a nucleotide. It has its ends five prime phosphate and three prime hydroxyl
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Other nucleotides are chained at these ends to form a strand of DNA
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In this way we have a strand in
5’-end and 3’-end direction
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By its molecular structure the other strands of DNA will necessarily have an...
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inverse direction. DNA strands are antiparallel
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To copy a fragment of DNA in form of
ARN it is enough to use a strand…
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of the first as a template
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ARN form their nucleotides with a ribose sugar
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and by this molecular reason one of its four nitrogenous bases differs from those of DNA
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RNA uses uracil as a nitrogenous base instead of DNA thymine
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But both bases are –let's say
equivalents
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In addition to the number of strands, now you now other molecular differences between DNA and RNA
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I have given you some preliminary concepts
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But how is messenger RNA synthesized from a DNA template strand?
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The process of transcription is more complex in eukaryotic cells than in prokaryotic cells...
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but to simplify it, I will explain it in a general way...
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that is, with the essential points valid in both cases
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In transcription there is always an enzyme RNA polymerase...
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that moves in 3'-end - 5'-end direction
on the template thread
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I hope it makes more sense now...
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why I first explained to you what the hell was that "3 prime end-5 prime end"
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In its path through the template strand, RNA polymerase builds the strand of messenger RNA
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How is it done? Don't worry! Here I will show you
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I promise you that the explanation is coming. I was engrossed with the music...
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and the dancing enzyme. But first, I warn you that this enzyme can form...
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different types of RNA
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All these types of RNA are important to build living beings from proteins
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But in the transcript, the protagonist is the messenger RNA
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The transcription or formation of messenger RNA has three stages: initiation, elongation and termination
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The initiation consists in indicating the RNA polymerase at which point of the ADN sequence...
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it must initiate transcription or
RNA synthesis
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There are sequences of nitrogenous bases - in DNA - called promoter centers that...
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that indicate to this enzyme where to start the transcription or copy in the form of RNA
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Coming up next, let's see it in more detail!
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RNA polymerase binds to these promoter centers and forms a...
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transcription bubble, where -in cooperation with other enzymes- begins...
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to partially unwind the DNA molecule
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Here we see the unwound strands in the transcription bubble and I have added the letters of each nitrogenous bases...
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to represent them better
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Remember that in DNA each of the nitrogenous bases of a strand matches a specific base of the other
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In DNA, bases couples are always
adenine and thymine; guanine and cytosine
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The bases are united through bridges of
hydrogen, but in the transcription...
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these are broken by enzymatic action
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The unique RNA chain is constructed in the transcription bubble...
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in the opposite direction to that of the template strand, that is...
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in a direction from 5 prime to 3 prime, given that it is a kind of negative of the DNA strand template
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In the initiation stage, the RNA polymerase assembles the first RNA nucleotides or ribonucleotides...
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because it uses ribose -remember- and
these nucleotides are found there in the nucleus
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The enzyme selects those that correspond...
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to each of the nitrogenous template bases
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I explained earlier that in RNA, uracil replaces thymine
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this means that when the RNA polymerase finds an adenine in the template strand...
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places a ribonucleotide with uracil in the RNA strand
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When there are enough ribonucleotides and the RNA chain does not break...
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the RNA polymerase is released from the promoter...
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and the elongation happens
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In this phase, RNA polymerase continues its path through the template strand while assembling...
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in the same way the rest of ribonucleotides
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In the back of the transcription bubble, the DNA template strand...
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meets again with its original partner to re-form...
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the double helix, as if it were the end of a «closure»
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Transcription continues until the RNA polymerase encounters a DNA termination signal...
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and then the transcription bubble will...
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disassembles and releases the RNA polymerase and newly formed messenger RNA
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And now what happens with the messenger RNA?
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Messenger RNA carries this information to ribosomes
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Ribosomes use messenger RNA
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to synthesize proteins in
cooperation with transfer RNA
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The RNA message is encrypted...
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in the famous genetic code,
which is like a biological language...
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The -between quotes- words in the genetic code are encrypted in a...
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sequence of nitrogenous bases
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To form a word of this code a minimum of three nitrogenous bases is required
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This unit of three nitrogenous bases or triplet is called «codon»
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In protein synthesis, a codon can be translated into an amino acid or a start or...
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termination signal for the translation process
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Several triplets or codons can encode the same amino acid and that is why it is said that there are synonymous codons...
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The translation starts with the binding of the messenger RNA to the ribosome...
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starting from a triplet or initiation codon that is usually adenine, uracil and guanine...
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and which also encodes the amino acid «methionine»
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Note that the ribosome covers two
codons of the messenger RNA that are going to...
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attach to two transfer RNA
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The transfer RNA is...say, like a piece of a puzzle
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At one end it coincides with a codon of mRNA...
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and in the other end carries a specific amino acid
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It's like an amino acid transporter
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Each amino acid transported by the transfer RNA binds or chains with the next...
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by a peptide bond
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When that happens the transfer RNA that transported it
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is free and can be used to
transport the same type of amino acid...
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in another protein synthesis
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We observe how the process continues chaining or linking...
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amino acids transported by transfer RNA
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The ribosome goes through the
codons of messenger RNA in...
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5 prime end - 3 prime end direction
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The translation process ends until you come across a...
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codon of completion to which no amino acid corresponds, meaning that...
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does not translate into any amino acid
it's just an end signal
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At this point the ribosome separates from the messenger RNA and also the amino acids...
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chain is free. And so it ends
the process of translation or synthesis...
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because a protein is a long chain
of amino acids that is folded... or bent
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All right! You have reached the end of this video.
If you have doubts with something of the exposed ...
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you can go back to the video, repeat it or pause it when you need it...
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my presentation has been general and basic. To deepen and encompass more details...
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your participation is required. For
example, you can investigate the...
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differences of these processes in
eukaryotic and prokaryotic cells or...
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other factors or enzymes involved in transcription or translation but I didn't...
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include in the video, as well as the 20
main amino acids that make up ...
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the proteins, and codons that
encode them. I hope I have been helpful...
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See you another time!
