PDF 12-3 The Structure of RNA The Central Dogma

12-3 The Structure of RNA

The Central Dogma

12-3 RNA and Protein Synthesis

Phenotype

A gene is a SEQUENCE of DNA that codes for a protein (or functional RNA). Phenotype is the individual's observable trait resulting from its genotype (genes). Gene expression is the overall process that begins with the transcription of a DNA SEQUENCE (gene) into an RNA SEQUENCE which is translated into the amino acid SEQUENCE of a functional polypeptide/protein, resulting in a particular phenotype.

This is considered the central dogma of biology because it is fundamental to how every living thing has come to be. The role of RNA in this process is as a middleman or worker that physically creates proteins according to the instructions (blueprint) in DNA. Like DNA, RNA is a nucleic acid and is similar to DNA in its variable SEQUENCE of nucleotides.

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RNA & Protein Synthesis The Structure of RNA

Codon

Unlike DNA, RNA: 1) is a single polynucleotide 2) has ribose instead of deoxyribose 3) has uracil instead of thymine The three main types of RNA are: 1) Messenger RNA (mRNA) 2) Transfer RNA (tRNA) 3) Ribosomal RNA (rRNA) Each group of three nucleotides in mRNA is known as a codon. Messenger RNA carries the message (SEQUENCE) of a gene that is to be translated into a polypeptide/protein.

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RNA & Protein Synthesis

Ribosome

Types of RNA

Amino acid

Anticodon

Ribosomal RNA

Ribosomes are the smallest, non membrane-bound organelles where translation (protein synthesis) occurs. Ribosomes are made of ribosomal RNA and about 80 different proteins. The function of rRNA is to recognize and bind mRNA & tRNA, like an enzyme active site, to catalyze translation (protein synthesis).

Transfer RNA

tRNA is "T" or "hairpin" shaped.

One end of tRNA is bound to a

specific amino acid.

The other end of tRNA has a triplet

of nucleotides called the anticodon.

tRNA interacts with both rRNA and

mRNA when it is time for the amino

acid is carries to be added to a

growing polypeptide chain.

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RNA & Protein Synthesis RNA Transcription

RNA RNA polymerase

DNA

Template Strand

Coding Strand

The process that produces all three types of RNA is transcription, which takes place in the nucleus. There are mRNA genes, tRNA genes and rRNA genes. RNA polymerase, along with numerous transcription factors, binds to DNA at a promoter sequence. Once transcription is initiated, RNA polymerase unwinds and unzips the DNA. Using one strand as a template, RNA nucleotides from the nucleoplasm are matched up with DNA nucleotides according to the base pairing rule, resulting in an RNA plynucleotide complementary to the DNA template polynucleotide. In this way, the RNA nucleotide SEQUENCE is determined by the DNA nucleotide SEQUENCE.

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RNA & Protein Synthesis RNA Processing

Once transcribed, RNA processing takes place in the nucleoplasm.

Exon

Intron

Pre-mRNA is mRNA after it has been

DNA

transcribed, but before being processed. Introns are non-coding sequences of DNA and pre-mRNA to be cut out during RNA processing. Exons are coding sequences of DNA and

Transcription

Pre-mRNA

pre-mRNA that will be spliced together

during processing.

Following transcription, introns are cut out of pre-mRNA and exons are spliced together, then cap and tail sequences are added at either end.

Processing

mRNA

This is all performed by the activity of

ENZYMES!

The cap and tail sequences help the

Cap

mRNA leave the nucleus through nuclear

Tail

pores and protect it from degradation by

hydrolytic enzymes.

RNA splicing allows for a variety of final mRNA polynucleotides to be produced

from one gene and pre-mRNA by joining exons in a variety of ways. This may

explain how in humans, 100,000 proteins are produced by about 25,000

genes. It also allows for the evolution of new genes through "exon shuffling".25

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