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DNA Processes: Transcription and Translation

By: Sidney London and Melissa Hampton

• DNA and RNA transfers genetic information from one generation to the next

• DNA is widely accepted to be the primary source of heritable information

• Transcription and Translation covert genetic information into polypeptides that can be used by the organism

Background & Purpose

Structure: DNA• -Sugar-phosphate backbone connected to nitrogenous base

(Thymine, Adenine, Cytosine and Guanine)• The two strands are held together by hydrogen bonds

between the nitrogenous bases to form a double helix

• RNA is synthesized from the 5’ end to the 3’ end

The RNA produced has nucleotides organized in triplets called codons

Each codon represents 1 amino acid

The RNA synthesis is catalyzed by the RNA polymerase

The process follows the same base pairing rules as DNA but in RNA Uracil substitutes for Thymine

• In DNA the present sugar is deoxyribose and in RNA it is ribose

• RNA exists in a single strand

Structure: RNA

Overview of Transcription

The synthesis of RNA uses information in DNA Occurs in the nucleus of a cell The information is imply transcribed from DNA to RNA A way to remember transcription is when a gene is transcribed it

is rewritten from DNA to RNA and a scribe is someone who writes Transcription occurs in all organisms The purpose of transcription is to get RNA from DNA so that the

RNA can produce proteins needed by the organism The DNA strand serves as a template for creating a new

complementary RNA strand The resulting RNA is a transcript of the protein building

instructions from the gene

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3 Stages of Transcription

Initiation, Elongation, & Termination

• A promoter establishes where RNA synthesis will begin

(TATA box is used in eukaryotic organism)• Transcription factors a collection of proteins that

help the binding of RNA polymerase and the initiation of transcription

• The promoter and the transcription factors combined are known as the transcription initiation complex

Step 1: Initiation

Step 2: Elongation

• RNA continues to untwist the double helix and adds nucleotides to the 3’ end of the growing RNA molecule

The new RNA molecule begins to peel away from the DNA template

The DNA double helix reforms

Step 3: Termination

The process varies in prokaryotes and bacteria

In bacteria, a transcribed terminator functions as a termination signal and causes the polymerase to detach from the DNA and release the new RNA strand

In eukaryotes, RNA polymerase II transcribes a DNA sequence into a polyadenyaltion signal sequence which then code for the polyadenyaltion signal in the RNA strand

Then proteins associated with the RNA cut it free from the polymerase

o After Termination, the new RNA strand is processed before being dispatched into the cytoplasm

o During this stage, both ends of the primary strand will be altered

o The 5’ end will have a Guanine added to it o The 3’ end will have about 50-250 Adenine molecules

added to it forming a Poly-A Tail o These new features help the strand with its exit from

the nucleus, help protect it from damage and make it easier for ribosomes to attach

RNA Processing

o These primary RNA strands have sections that are non-coding that are interspersed throughout the strand .

o These are called Introns (Introns-Interspersed)

o Also present are coding strands called exons

o Exons are the nucleotides that are eventually expressed as amino acids (Exons-Expressed)

o Splicing is a process similar to cutting and pasting where spliceosomes move along the strand and cut out introns and join together the remaining exons to form the final strand that will be released from the nucleus

Splicing

Video: Transcription

https://www.youtube.com/watch?v=ztPkv7wc3yU

o Translation is the synthesis of a polypeptide whose amino acid sequence comes from the nucleotide sequence of the RNA strand

o A change of language takes place, from nucleotide codons to amino acids (change of language-translate-translation)

o When written, codons are three letters representing three bases (UAG, GUA…)

o Each codon codes for a specific amino acid

o There are also three stages of Translation…

Overview of Translation

{Initiation, Elongation, & Termination

3 Stages of Translation

• Translation takes place in the ribosomes of a cell

• First, the formation of the translation initiation complex occurs

• The complex is made up of mRNA, an initiator, tRNA, and a small ribosomal unit which is then later joined by a larger ribosomal unit• Initiation factors are

proteins that hold the complex together

• Then, the anticodon on the tRNA binds to each codon of the mRNA & converts the nucleotides to amino acids

Initiation

• Formation of peptide bonds that connects the amino acids into a polypeptide chain

Elongation

• There are certain nucleotide sequences that code as a stop signal and not as a amino acid (UAG, UAA, UGA)

• When the tRNA reaches these sequences, a release factor binds to that sequence and inserts water into the chain, hydrolyzing (breaking) the reaction

• The polypeptide chain is released through a tunnel in the ribosomal unit into the cytoplasm

• The complex then breaks apart

Termination

Video: Translation

https://www.youtube.com/watch?v=-zb6r1MMTkc