Good Afternoon, My name is Whitney Barfield and Today we will be discussing Macrmolecular Synthesis: Replication, Transcription and translation. This lecture is normally performed by Dr. Kanaan. However, I will be presenting this lecture today and all of the other lectures related to this topic will be performed by her.
Now, We will begin by discussing genetics. Genetics deals with the molecular structure of genes, how they replicate, and how they are inherited from parents to offspring. Genes are segments of deoxyribonucleic acid that encode for functional products called proteins. And proteins facilitate biological reactions.
Now genes are coiled very tightly with other proteins called histones into structures called chromosomes, therefore, these chromosomes are said to carry our hereditary information. There the specific collection of an individuals genes is said to be its genotype. Each genotype codes for all of the particular characteristics of an organism. And the proteins that are expressed is referred to as the individuals phenotype. The phenotype describes all of the observable properties of the organism.
Now lets talk about DNA. DNA is a type of nucleic acid. The other type being ribonucleic acid or RNA. DNA is a polymer containing several monomers called nuceotides.
Nucleotides are composed of three different components—a five carbon sugar called a deoxyribose that has an hydrogen group at its two prime carbon, a phosphate group, and one of four different bases adenine, thymine cytosine and guanine. Now Adenine and guanine are classified as purines because they contain a double ring While pyrimidines cytosine, thymine and Uracil (which replaces thymine in RNA) contain a single ring structure
Now the nucleotides join and the sugar and the phosphate group bond making up the DNA backbone—this is called a phosphodiester bond Two separate dna strand align parallel to one another joining at the bases via hydrogen bonding If you looked at it—it would be as if one strand was pointing upward in the 5’ to 3’direction (5’ having a terminal phosphate group and 3’ having a terminal hydroxyl group)
Adenine always binds with thymine forming a double hydrogen bon and guanine always binds with cytosine forming a triple hydrogen bond
A sugar and a base without a phosphate group is referred to as a nucleoside. The bond between the base and the deoxyribose is referred to as a glycosidic bond.
DNA and RNA are similar in that they are carriers of genetic information. However DNA is a much more stable molecule due to the presence of a hydrogen group at the 2’ carbon, while RNA has a much more unstable hydroxyl group at the 2’ carbon. And as I said earlier Uracil replaces thymine in RNA
Now in order for this genetic information to be passed during cell division it has to be replicated effectively.
Replication has several key steps including pre-priming, priming, elongation and termination.
In order for DNA to be replicated, it has to first be separated. To facilitate this separation, proteins referred to as helicases (because they break the helical structure of the DNA) facilitate the separation of the DNA strands. And to ensure that these strands don’t rehybridize, other small proteins called single stranded binding proteins bind to the DNA. This stabilizes the seperation of the two strands.
Now as you can see here the helicases have unwound the DNA and the Single stranded binding proteins have bound and stabilized the strands apart from one another forming a replication fork. This makes the bases available for templating reactions.
Now, once the replication fork has been formed, an enzyme called a primase is linked directly to the DNA helicase to form a unit called a primosome. The primase can then move along the DNA adding short segments of RNA called primers. Because synthesis always occurs in the 5’ to 3’ direction only one primer is needed for the leading strand (or the DNA strand orientated in the 5’ to 3’ direction) while several primers are needed for the 3’ to 5’ strand.
AN enzyme called DNA polymerase three then facilitates the addition of new complementary DNA sequences to the 3’ termini of the primers. The leading strand can be synthesized continuously, while the lagging strand forms short discontinuous DNA fragments called Okazaki fragments.
. DNA polymerase 1 then digests the RNA primers and replaces it with DNA, and another enzyme called DNA ligase then fills in the gaps forming a continuous template Now instead of one DNA molecule you have two and that is how replication occurs
Now in order for this DNA to be express its encoded protein, it must be transcribed into a single stranded RNA molecule specifically messenger RNA. Messenger RNA is a complementary single stranded copy of the DNA template. Transcription also occurs in the 5’ to 3’ direction.
In bacteria transcription begins with the binding of an RNA polymerase consisting of five subunits called a holoenzyme to a binding site called a promoter. The sigma subunit recognizes the Adenine Thymine rich region (or promoter) approximately ten bps upstream the start site of transcription
RNA polymerase travels down the template strand and uses base pairing complementary with the DNA template to create an RNA copy. Only one strand of the DNA template will be used for transcription and the RNA complementary copy will have Uracil in the place of thymine.
Elongation continues until it reaches an inverted repeat sequence in the DNA template—forming a stem loop structure. Or termination occurs with the help of rho a termination factor which actively unwinds the DNA-RNA hybrid formed during transcription
Now after the mRNA transcript is made, it has to undergo translation. Translation is the process of converting an mRNA transcript into an amino acid (polypeptide) sequence that will later fold into its functional protein product. Now in translation there are three types of RNA that are used mRNA which carries the information for the amino acid sequence. rRNA which combine to form the ribosome on which protein synthesis occurs tRNA whose primary role is to capture the appropriate amino acid and bring it to the ribosome and make sure that it is inserted into the right point on the growing amino acid chain
As I said earlier polypeptide synthesis occurs on ribosomes. Ribosomes are made up of a large subunit and small subunit. In bacteria the large subunit is 70S and the small subunit is 30S. While in Eukaryotes the large subunit is 60S, while the small suunit is 40S
To begin protein synthesis the small subunit of the ribosome binds to the mRNA upstream of the start codon (a codon is three adjacent bases that code for a specific amino acid) (AUG). This is called the Shine Dalgarno sequence.
Next, an enzyme called amino-acyl tRNA synthase adds the correct amino acid to its tRNA. When this occurs the tRNA is said to be charged. The tRNA The start codon in translation is AUG and a tRNA
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