Grade 12 Biology - Genes and Protein synthesis

One gene-one enzyme hypothesis

the hypothesis, proposed by Beadle and Tatum, that each gene is unique and codes for the synthesis of a single enzyme

One gene - one polypeptide hypothesis

the hypothesis that each gene is unique and codes for the synthesis of a single polypeptide; the restated version of the one gene—one enzyme hypothesis

Central dogma

The fundamental principle of molecular genetics, which states that genetic information flows from DNA to RNA to proteins

Transcription

mechanism by which the information coded in nucleic acids of DNA is copied into the nucleic acids of RNA; something rewritten in the same language

Translation

mechanism by which the information coded in the nucleic acids of RNA is copied into the amino acids of proteins

Deoxyribonucleic acid

-Double stranded

-Adenine pairs with thymine

-guanine pairs with cytosine

-Deoxyribose sugar

Ribonucleic acid

-single stranded

-adenine pairs with uracil

-guanine pairs with cytosine

-ribose sugar

messenger RNA (mRNA)

the end product of the transcription of a gene; mRNA is translated by ribosomes into a protein

Characteristics and functions:

-varies in length, depending on the gene that has been copied

-acts as the intermediary between DNA and the ribosomes

-is translated into protein by ribosomes

-is the RNA version of the gene encoded by DNA

transfer RNA (tRNA)

a carrier molecule that binds to a specific amino acid and adds the amino acid to the growing polypeptide chain

Characteristics and functions:

-functions as the delivery system of amino acids to ribosomes as they synthesize proteins

-is very short, only 70 to 90 base pairs long

Ribosomal RNA (rRNA)

an RNA molecule within the ribosome that bonds the correct amino acid to the polypeptide chain

Characteristics and functions:

-binds with proteins to form the ribosomes

-varies in length

RNA polymerase

An enzyme that reads a DNA strand and creates a complementary strand of RNA

Template strand

the DNA strand that is copied into an mRNA molecule during gene transcription

precursor mRNA (pre-mRNA)

the initial RNA transcription product

Genetic code

The specific coding relationship between bases and the amino acids they specify; the genetic code can be expressed in terms of either DNA or RNA bases

Codon

a group of three base pairs that code for an individual amino acid

Start codon (Initiator codon)

the codon that signals the start of polypeptide chain and initiates translation

Stop codon

a codon that signals the end of a polypeptide chain and causes the ribosome to terminate translation

promoter

A nucleotide sequence that lies just before a gene and allows for the binding of RNA polymerase

TATA box

a region of the promoter that enables the binding of RNA polymerase

Coding strand

the DNA strand that is not being copied but contains the same sequence as the new RNA molecule

poly(A) tail

a chain of adenine nucleotides that are added to the 3’ end of the pre-mRNA molecule to protect it from enzymes in the cytosol

5’ cap

a sequence of seven Gs that is added to the start of a pre-mRNA molecule; ribosomes recognize this site and use it as the site of initial attachment

Exon

A sequence of DNA or RNA that codes for part of a gene

intron

a non-coding sequence of DNA or RNA

spliceosome

an enzyme-protein complex that removes introns from the mRNA

small ribonucleoprotein (snRNP)

a protein that binds to introns and signals them for removal

alternative splicing

a process that produces different mRNA from pre-MRNA (exons and introns), allowing more than one possible polypeptide to be made from a single gene

1. Initiation (termination sequence)

An RNA polymerase molecule binds to the DNA upstream from the beginning of the gene to be transcribed and the DNA begins to unwind at the front of the RNA polymerase, which begins moving along the DNA

2. Elongation (termination sequence)

During transcription, RNA nucleotides are base paired one after another with the template DNA bases

3. Termination (termination sequence)

The RNA copy is released when the entire gene has been transcribed. The unwound region of the DNA rewinds into a double helix.

Transcription in Prokaryotes

-Transcription occurs throughout the cell

-A single type of RNA polymerase transcribes all types of genes

-Bases are added quickly (15 to 20 nucleotides per second)

-The promoters are less complex than those in eukaryotes

-A protein binds to the mRNA and cleaves it, or the mRNA binds with itself

-There are no introns

-Transcription results in mRNA ready to be translated into protein by ribosomes

Transcription in Eukaryotes

-Transcription takes place in the nucleus

-Different RNA polymerases are used to transcribe genes that encode protein (RNA polymerase II) and genes that do not encode protein (RNA polymerase I, III)

-Bases are added slowly (5 to 8 nucleotides per second)

-The promoters are immediately upstream of protein-coding genes, and they are more complex than those in prokaryotes

-Nuclear proteins bind to the polyuracil site and terminate transcription

-There are both introns and exons

-Transcriptions result in pre-mRNA, which must be modified to protect the final mRNA from degradation in the cytosol and to remove introns

Anticodon

the complementary sequence of base pairs on a tRNA that corresponds to a codon on an mRNA

Aminoacylation

The process by which a tRNA molecule is bound to its corresponding amino acid

Aminoacyl-tRNA

a molecule of transfer RNA bound to its associated amino acid

Reading frame

a particular system for separated a base pair sequence into readable codons

polysome

a complex that is formed when multiple ribosome attach to the same mRNA molecule in order to facilitate rapid translation

Translation in Prokaryotes

-mRNA is translated by ribosomes in the cytosol as it is being transcribed from DNA

-mRNA bases pair directly with a ribosomal binding site, just upstream of the start codon

-mRNA 5’ cap is involved

-15 to 20 elongation cycles per second

-stop codon appears and a release factor binds so that the polypeptide

-mRNA strand can be translated by multiple ribosomes simultaneously, even as it is being transcribed from DNA

Insulin

A hormone produced in the pancreas that lowers the blood glucose level by promoting the uptake of glucose by the body cells

Lac operon

A cluster of genes that contains the DNA sequences to regulate the metabolism of lactose

Operator

The region in the operon that regulatory factor bind to

Repressor protein

A protein that binds to the operator to repress gene transcription

Inducer

A signal molecule that triggers the expression of an operon’s genes

Corepressor

A signal molecule that binds to a regulatory protein to reduce the expression of an operons genes

Transcriptional

Regulates which genes are transcribed (DNA to mRNA) or controls the rate at which transcription occurs

Post-transcriptional

Controls the availability of mRNA molecules to ribosomes; pre-mRNA molecules undergo changes in the nucleus, resulting in final mRNA before translation occurs

Translational

Controls how often and how rapidly mRNA transcripts will be translated into proteins

Post-translational

Controls when proteins become fully functional, how long they are functional, and when they are degraded

Point mutation

A change in a single nucleotide within a gene

Substitution

The replacement of one base pair in a DNA sequence by another base pair

Insertion

The addition of a base pair (small-scale mutation) or larger coding region (large-scale mutation) to a DNA sequence

Deletion

The removal of a base pair (small-scale mutation) or larger coding region (large-scale mutation) from a DNA sequence

Inversion

Two adjacent bases trading places (small-scale mutation) or the reversal of a sequence of DNA (large-scale mutation)

Single nucleotide polymorphism (SNP)

A difference in the DNA between individuals caused by point mutations

No mutation

This is the normal condition

A missense mutation

Occurs when a change of a single base pair or group of base pairs results in the code for a different amino acid. The protein that is synthesized will have a different sequence and structure, and it may be non functional or function differently. A missense mutation can be beneficial if it creates a new, desirable effect

A nonsense mutation

Occurs when the change in a single base pair or group of base pairs results in a premature stop code in the gene. The polypeptide is cut short and, most likely, will be unable to function

A silent mutation

Occurs when the change in one or more base pairs does not affect the functioning of the gene. The mutated DNA sequence codes for the same amino acid as the non-mutated sequence, and the resulting protein is not altered.

A frameshift mutation

Occurs when one or more nucleotides are inserted into or deleted from a DNA sequence, causing the reading frame of codons to shift in one direction or the other. This results in multiple missense and/or nonsense effects. The frameshift mutation “shifts” the reading frame by one or more steps, and every amino acid coded for after this mutation is affected. Any deletion or insertion of base pairs in multiples of three does not cause frameshift because the reading frame is unaltered. Tay-Sachs disease is a result of insertion of four base pairs

Translocation

The movement of entire genes or sequences of DNA from one chromosome to another

Spontaneous Mutation

A mutation that is caused by an error in DNA replication

Induced mutation

A mutation that is caused by an environment agent

Mutagen

An environment agent that directly alters the DNA within a cell

Human genome project

The joint government and private sector research project that sequenced the human genome

Variable number tandem repeats (VNTRs)

Non-coding, repeating sequences of DNA that vary in length between homologous chromosomes and between individuals

LINEs (long interspersed nuclear elements)

Repetitive DNA sequences, approximately 6500 bp in length on average, interspersed throughout the genome

SINEs (short interspersed nuclear elements)

Repetitive DNA sequences, approximately 500 bp in length on average, interspersed throughout the genome