Chapter 14 - Bio

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The Central Dogma

One gene - one polypeptide // Beadle and Tatum

The Central Dogma - cells are governed by a cellular chain of command

The Flow of Genetic Information

Gene expression - the process by which DNA directs protein synthesis

DNA - TRANSCRIPTION - RNA - TRANSLATION - PROTEIN

----------------------> -------------------->

Basic Principles of Transcription

synthesis of RNA under the direction of DNA

produces messenger RNA (mRNA)

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Basic Principles of Translation

synthesis of a polypeptide, which occurs under the direction of mRNA

Ribosomes are the sites of translation

RNA

The bridge between DNA and protein synthesis

RNA has ribose sugar and the base uracil (U) rather than thymine (T)

usually single stranded

TEMPLATE DNA 5' - ACCTAGTTGC - 3'

Trascription RNA 3' - UGGAUGAACG - 5'

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Molecular Components of Transcription

RNA polymerase

Promoter

Terminator in bacteria

Transcription

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Transcription in Eukaryotes

Occurs in the nucleus

RNA processing produces the mature mRNA

mRNA goes to the cytoplasm for translation on ribosomes

RNA Processing

WHEN - modify pre mRNA before genetic messages are dispatched to the cytoplasm

WHERE - nucleus

Two types of modifications:

"Capping and Tailing" - both ends of the primary transcript are altered

"Splicing" - interior parts of the molecule are cut out and other parts are spliced together

Functions of Altercations of mRNA ends

facilitates the export of mRNA to the cytoplasm

protects mRNA from hydrolytic enzymes

helps ribosomes attach to the 5' end

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often translation isn't sufficient to make a functional protein

polypeptide chains are modified after translation or targeted to specific sites in the cell

Protein Folding

During synthesis, a polypeptide chain spontaneously coils and folds into its 3D shape

Post - Translational Modifications

Proteins may also require this before doing their jobs

Attach sugar, lipids, phosphate groups to certain amino acids

one or more amino acids are removed from the leading end of the polypeptide chain

a polypeptide chain is cleaved into multiple pieces

Multiple polypeptide assemple a protein that has quaternary structure

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Targeting Polypeptides to Specific Locations

Two populations of ribosomes are evident in cells: free ribosomes (in the cytosol) and bound ribosomes (attached to the ER)

Free ribosomes mostly synthesize proteins that function in the cytosol

Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell

Polypeptide synthesis always begins in the cytosol

synthesis finishes in the cytosol unless the polypeptide signals the ribosome to attach to the ER

Polypeptides destined for the ER or for secretion are marked by a signal peptide

A signal-recognition particle (SRP) binds to the signal peptide

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Making Multiple Polypeptides

In bacteria and eukaryotes multiple ribosomes translate an mRNA at the same time

Once a ribosome is far enough past the start codon, another ribosome can attach to the mRNA

Strings of ribosomes called polyribosomes (or polysomes) can be seen with an electron microscope

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Transcription in Prokaryotes

Absence of a nuclear membrane

mRNA transcription and translation are coupled

translation of mRNA can begin before transcription has finished

mRNA is translated as soon as it is made

no processing

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Mutations

mutations - changes in the genetic material of a cell or virus

small scale mutations - chemical changes in just one or a few nucleotide pairs of a gene

the change of a single nucleotide (point mutation) in a DNA template strand can lead to the production of an abnormal protein

Types of Small-Scale Mutations

Small scale mutations within a gene can be divided into two general categories

Nucleotide-pair substitutions. A nucleotide-pair substitution replaces one nucleotide and its partner with another pair of nucleotides

One or more nucleotide-pair insertations or deletions (frameshift mutations)

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Substitutaions

Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code

Missense Mutations still code for an amino acid, but not the correct amino acid. Substitution mutations are usually missense mutations

Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein

Insertations and Deletions

inserations and deletions are additions or losses of nucleotide pairs in a gene

These mutations have a disastrous effect on the resulting protein more often than substitutions do

Insertion and deletion of nucleotides may alter the reading frame of the genetic message, producing a frameshift mutation