Cambridge Pre-U Biology - 1.6 Genes and Protein Synthesis PART 1 Sample

MAKING SENSE OF

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• This is it! This is how the whole thing works!

• You now have enough biology knowledge to understand howall the pieces of the puzzle go together to build livingorganisms.

• In this topic you will learn how the genetic code works tomake polypeptides and RNA which go onto give cells theirstructure and function and therefore build organisms.

• You will learn the details of exactly how this works and howthese instructions are inherited by the next generation.

• Finally you will learn about what can go wrong!2

MAKING SENSE OFMAKING SENSE OF

Gene expressionVariationInheritance

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Mutations

The genetic code and protein synthesis

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a) Define a gene as a unit of inheritance or as an ordered sequence ofnucleotides located at a particular locus on a particular chromosome,which codes for a particular protein or, in certain cases, a functional orstructural RNA molecule and discuss the limitations of these definitionswith reference to introns, exons and promoters.

b) Describe the genetic code and discuss the extent to which it is true thatthe code is universal to all organisms.

c) Explain protein synthesis in terms of transcription and translation,including the roles of DNA, mRNA, tRNA and ribosomes.

d) Describe, in outline, eukaryotic introns, exons and the splicing of mRNA.

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• By now you should know what DNA looks like andhow it divides but we still haven't covered how itactually works!

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• The sequence of bases in DNA is actually a code tomake proteins.

• We have already explained how important proteinsare to living organisms.

• Machinery in cells can read this code andmanufacture the proteins (protein synthesis).

• Every three bases on the DNA codes for an aminoacid.

• This is called the triplet code.

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• There are 64 combinations of bases (43) for only 20amino acids, so amino acids are usually coded for bymore than one triplet code.

• For example the codes TGA, TGG, TGT and TGC all codefor the amino acid threonine.

• This is known as a degenerate code, but its good becauseit means that often mutations in the DNA don’t actuallychange the eventual protein produced so it protects theorganism.

• Most of your DNA actually doesn't’t code for proteins butit regulates when and where to make them.

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• The triplet code works thesame in all living organisms(with a couple of exceptions).

• Therefore it’s (almost)universal.

• That means we can swap DNAbetween organisms!

• This is explained later in thecourse.

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• As mentioned there are a couple of exceptions to thegenetic code being universal.

• The majority of these exceptions are found in the DNAinside the mitochondria (mtDNA) which code for differentamino acids than the nuclear DNA.

9By Jfitz1974 - Using Adobe Photoshop Elements, CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=41045504

Recently some modifications havealso been discovered in the nuclearDNA of some species.

https://en.wikipedia.org/w/index.php?curid=41045504

Copyright © 2017 Henry Exham10

So what exactly is a

gene?


• You probably know this definition:

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A gene is a section of DNA that codes for a particular protein.

However, at this level we can go into this definition further and actually see why this is not a good definition at all.



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By OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148169

But what about proteinswith quaternary structurelike this one? Are theycoded for by one gene?

https://commons.wikimedia.org/w/index.php?curid=30148169



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By OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148169

No they are not, they are made ofmultiple polypeptides coded for byseparate genes and then modified bythe Golgi apparatus.

A gene is a section of DNA that codes for a particular polypeptide.

Therefore . . .



• But in actual fact genesdon’t always code forpolypeptides, they canalso code in certaincases for a functionalor structural RNAmolecule.

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A gene is a section of DNA that codes for a particular polypeptide.

By Yikrazuul - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10242870

Large ribosomal subunit (50S) of Haloarcula marismortui. The ribosomal proteins are shown in blue, the rRNA in ochre.



• Another problem with this definition is that it is notone discrete section of DNA that makes up the actualgene. Its more complicated than that!

• A better definition of a gene’s structure is to say anordered sequence of nucleotides located at aparticular locus (plural loci) on a particularchromosome.

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A gene is a section of DNA that codes for a particularpolypeptide or in some cases an RNA molecule.


• But genes are not all at one locus! They are actuallyinterrupted by non-coding sequences called introns. Andbegin with sections called promoters.

• The coding sequences in the gene are called exons.

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A gene is a section of DNA that codes for a particularpolypeptide or in some cases an RNA molecule.


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Cambridge Pre-U Biology - 1.6 Genes and Protein Synthesis PART 1 Sample

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Transcript of Cambridge Pre-U Biology - 1.6 Genes and Protein Synthesis PART 1 Sample