Usc bridge Nurs 500 bridge module 3.4

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3.4 gene function and protein synthesis
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DNA directs its own synthesis and serves as a master blueprint for protein synthesis. Cells also make lipids and carbohydrates but DNA does not dictate their structure. Historically DNA is said to specify only the structure of protein molecules, including the enzymes that catalyze the synthesis of all classes of biological molecules. Proteins are composed of polypeptide chains that make up amino acids. Genes are as segment of DNA molecule that carries instructions for creating one polypeptide chain. the 4 nucleotide bases are the letters of the genetic alphabet AGTC. Each sequence of these bases is a triplet or a word that specifies a particular amino acid. AAA is the amino acid phenylalanine, CCT is glycine.DNA bases in the gene and amino acids in the polypeptide are 3:1.

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A gene is a neucleotide segment of DNA molecule that carries the instructions to synthesize one polypeptide chain. The information sequence that determines the nature of a protein.

A, G, C, T are the letters of the genetic alphabet and the information of DNA is found in the sequence of these bases.

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Each sequence of three bases called a triplet specifies a particular amino acid. Ie: AAA or amino acid phenylalanine and CCT calls for glycine. The sequence of triplets in each gene forms a sentence that tells exactly how a polypeptide is to be made.


RNA synthesized on of the DNA strands is:

mRNA, tRNA, and rRNA.

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1st step- Transcription: DNAs information is encoded in mRNA.

2nd step is Translation: the information carried by mRNA is decoded and used to assemble polypeptides.

These steps take a DNA code and process it into a protein sequence.

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RNA is the go between the DNA molecule and the polypeptide or the protein that it encodes. RNA is made in the nucleus and is carried into the cytoplasm, where the protein making machinery is.

RNA differs from DNA in that it is single stranded. has the sugar ribose instead of deoxyribose and the base uracil for thymine.

3 types of RNA act to carry out DNAs instructions:

  1. Messenger RNA (mRNA)- carries the coded information to the cytoplasm where protein synthesis occurs.
  2. Ribosomal RNA (rRNA)- along with proteins, forms the ribosomes, which consist of 2 subunits one large and one small that combine to form functional ribosomes which are the site of protein synthesis.
  3. Transfer RNA (tRNA)-ferry amino acids to the ribosomes to decode mRNAs message for amino acid sequence in the polypeptide to be built.
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relatively long nucleotide strands resembling half DNA molecules. mRNA carries the coded information to the cytoplasm where protein synthesis occurs.

Transcription- The uncoiled DNA strand not used as a template is called the coding strand for the mRNA and the information is encoded to mRNA.

Once the mRNA molecule is made it detaches and leaves the nucleus via a nuclear pore and head for the protein synthesis machinery-the ribosome.

Steps in transcription processing are: pre-mRNA -> RNA processing -> mRNA.

Genes are composed of exons and introns. Exons are informational sequences and seperated by introns which are noncoding often repetitive segments.

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Transcription transfers information from DNA base sequence to th complementary base sequence of a mRNA molecule. Once mRNA molecule is made it detaches and leabes the nucleus and heads for protein synthesis machinery, the ribosome. Transcription cannot begin until the gene-activating chemicals called transcription factors stimulate histones. Transcription factors then bind to the promoter a special DNA sequence that contains the start point of the gene to be transcribed and specifies which DNA strand is going to serve as the template. RNA polymerase- enzyme that oversees synthesis initiates transcription.


Transcription happens in three phases: Initiation, Elongation, Termination.

Initiation- RNA polymerase pulls apart the strands of DNA double helix so transcription can begin at the start point in the promoter.

Elongation-Using incoming RNA nucleotides as substrates, RNA polumerase aligns them with complementary DNA bases on the template strand and links them together, continues to unwind the helix, link new template strands then rewinds the hydrogen bonded double helix strands back together.

Termination- when the polymerase reach a special base sequence called a termination signal, transcription ends and the newly formed mRNA seperates from the DNA template.


if DNA has the sequence of AAA then the segment of mRNA synthesized on it will have a sequence of?


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RNA processing

Processing of mRNA- Before translation can begin editing and further processing are needed to clean up the mRNA transcript. The DNA is transcribed sequentially. mRNA initally made is called pre-mRNA is littered with introns that must be removed. Large RNA protein complexes called spliceosomes snip out the introns and splice together the remaining exon coded sections in the order in which they occured in the DNA producing functional mRNA.

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Variation on transcription: Retrovirus

In a retrovirus the genome is RNA. A virus infects a cell and uses reverse transcriptase to catalyze the viral RNA into a double stranded DNA that enters the nucleus making protein in our cells.


The molecule along which ribosome slide to dictate protein production is:



The step in which pre-mRNA is made into mRNA is called?

RNA processing.