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PENN STATE HARRISBURG / Biology / BIOL 110 / What is a sequence of three nucleotides that form a genetic code unit

What is a sequence of three nucleotides that form a genetic code unit

What is a sequence of three nucleotides that form a genetic code unit

Description

DNA -> RNA -> PROTEIN


What is a sequence of three nucleotides that together form a unit of genetic code in a dna or rna molecule?



∙ CODONS

o Codon: a sequence of three nucleotides that together form a unit of genetic code in a  DNA or RNA molecule.

o 64 codons – 61 code for amino acids, 3 triplets are to stop translation

o Codons must be read in the correct READING FRAME (correct groupings) in order for the  specified polypeptide to be produced

∙ The genetic code is universal – shared by simplest bacteria to the most complex animals. ∙ We have considered a gene to be:  

o A discrete unit of inheritance

o A region of specific nucleotide sequence in a chromosome

o A DNA sequence that codes for a specific polypeptide chain


What is the 1st stage of gene expression?



o In summary: A gene can be defined as a region of DNA that can be expressed to produce  a final functional product, either a polypeptide or a RNA molecule.

∙ Genes can be transcribed and translated after being transplanted from one species to another. ∙ TRANSCRIPTION

o The 1st stage of gene expression

o 3 stages of transcription: Initiation, Elongation, Termination

o INITIATION: Promoters signal the start point 

▪ Transcription Factors mediate the binding of RNA Polymerase and the initiation  of transcription Don't forget about the age old question of What is the meaning of persuasion in social psychology?
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▪ Transcription Initiation Complex – The whole completed assembly of  


What are the 3 stages of transcription?



transcription factors and RNA Polymerase II bound to a promoter.

▪ TATA Box – a promoter that is crucial in forming the initiation complex in  

eukaryotes

o ELONGATION: of the RNA strand

▪ RNA polymerase moves along the DNA and untwists the double helix, 10 to 20  bases at a time

▪ Transcription progresses at a rate of 40 nucleotides per second in eukaryotes ▪ A gene can be simultaneously transcribed by several RNA Polymerases

▪ Nucleotides are added to the 3' end of the growing RNA molecule

o TERMINATION: different in bacteria and eukaryotes

▪ Bacteria: the Polymerase stops transcription at the end of the terminator and  the mRNA can be translated without further modification

▪ Eukaryotes: RNA Polymerase II transcribes the polyadenylation signal sequence ,  the RNA transcript is released 10-35 nucleotides past this polyadenylation  

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∙ Eukaryotic cells modify RNA after transcription.  

o RNA Processing: Enzymes in the eukaryotic nucleus modify pre-mRNA before the  genetic messages are dispatched to the cytoplasm.  

o During RNA Processing, both ends of the primary transcript are usually altered.

o Usually some of the interior parts of the molecule are cut out, and the other parts  spliced together.  

o Alteration of mRNA ends: The 5' end receives a modified nucleotide 5' cap. The 3' end  gets a poly-A tail.

∙ Split Genes and RNA Splicing

o Most eukaryotic genes and their RNA transcripts have long noncoding stretches of  nucleotides that lie between coding regions called Introns  

o Exons- the other regions that are eventually expressed and usually translated into amino  acid sequences.  

o RNA Splicing – removes introns and joins exons, creating an mRNA molecule with a  continuous coding sequence – carried out by Spliceosomes (consist of a variety of  proteins and several small nuclear ribonucleoproteins (snRNPs) that recognize the splice  sites.  If you want to learn more check out Is cal state fullerton a community college?

∙ Ribozymes: are catalytic RNA molecules that function as enzymes and can splice RNA  o The discovery of Ribozymes showed that not all biological catalysts are proteins.

∙ Alternative RNA Splicing: Some introns may contain sequences that regulate gene expression – some genes can encode more than one kind of polypeptide, depending on which segments are  treated as exons during splicing.  Don't forget about the age old question of What is the kelvin scale named after?
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o Consequently – the number of different proteins an organism can produce is much  greater than its number of genes

o Proteins often have domains. These are conserved parts of a given protein sequence  and (tertiary) structure that can evolve, function, and exist independently of the rest of  the protein chain.

o Different exons code for different domains of the protein.

o Exon shuffling may result in the evolution of new proteins.  

∙ Translation is the RNA directed synthesis of a polypeptide. Genetic information flows from  mRNA to protein through the process of translation.  

∙ A cell translates an mRNA message into protein with the help of transfer RNA (tRNA). Transfer  RNA transfer amino acids to the growing polypeptide in a ribosome. Translation is a complex  process in terms of its biochemistry and mechanics.  

o Molecules of tRNA are not identical.  

▪ Each carries a specific amino acid on one end

▪ Each has an anticodon on the other end, the anticodon base pairs with a  complementary codon or mRNA.  

▪ Consists of a single RNA strand that is only about 80 nucleotides long.  

▪ Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a  cloverleaf.

∙ Ribosomes

o Couple tRNA anticodons with mRNA codons in protein synthesis

o 2 ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA) o A ribosome has 3 binding sites for tRNA:  

▪ 1. The P Site holds the tRNA that carries the growing polypeptide chain.

▪ 2. The A Site holds the tRNA that carries the next amino acid to be added to the  chain.  

▪ 3. The E Site is the exit site, where discharged tRNAs leave the ribosome

Types of RNA:

Messenger RNA (mRNA)

Carries information specifying amino acid  sequences of proteins from DNA to ribosomes.

Transfer RNA (tRNA)

Serves as translator molecule in protein  synthesis, translates mRNA codons into amino  acids.

Ribosomal RNA (rRNA)

Plays catalytic (ribozyme) roles and structural  roles in ribosomes.

Primary Transcript

Is a precursor to mRNA, tRNA, or rRNA , before  being processed, some intron RNA acts as a  ribozyme, catalyzing its own splicing

Small Nuclear RNA (snRNA)

Plays structural and catalytic roles in  

spliceosomes, the complexes of protein and RNA  that splice pre-mRNA

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