Gene Expression

Nucleic Acid

Nucleic Acid provides instructions to make proteins. There are two types of nucleic acids: DNA and RNA which carry genetic information necessary for cell functioning.

Each Nucleotide consists of:

5 carbon sugars, a phosphate group, 1 to 5 nitrogenous (or nucleotide) bases.

DNA

Consists of many nucleotides joined together. These two chains are bonded together with weak hydrogen bonds. DNA acts as a transmission of genetic information.

DNA Location and Base Pairing

Adenine - Thymine

Guanine - Cytosine

DNA is found in the nucleus.

Nucleotides of DNA consists of:

Sugar (deoxyribose), a phosphate group, four nitrogenous bases adenine, thymine, cytosine, and guanine.

RNA

RNA is used to transfer genetic code needed for the creation of proteins from the nucleus to the ribosome. It has the sugar ribose, has the base uracil instead of thymine, and is typically single-stranded, found mostly in cytoplasm.

Two main forms of RNA are called:

Messenger RNA (mRNA) and Transfer RNA (tRNA)

Messenger RNA (mRNA)

Carries the “message” that codes for a specific protein in the sequences of its bases from the nucleus to the cytoplasm where proteins are synthesised.

Single stranded and is long enough to contain one gene only also has a short lifetime and is degraded soon after it is used.

Transfer RNA (tRNA)

It is responsible for bringing amino acids to the ribosome so that they can be assembled.

It is short length of RNA

Amino Acids

Building blocks of protein. Amino acids combine to form proteins.

Proteins

Protein plays a role in every function of the human body. The protein structure is a long chain of amino acids.

Protein Function determined by:

1) The sequence of amino acids since there are millions of combinations

2) The final folding of the protein - achieved through the bonding and interactions of amino acids within and between polypeptide chains.

Polypeptide

A protein is made up of one or more chains of amino acids

Factors of Amino Acids

Human body can make non-essential amino acids but it cannot make essential amino acids which is what you have to get from your diet.

Proteins in Human Body

Protein in the immune system, Signalling Proteins, Proteins in the muscle, Structural Proteins, Enzymes, Cell Membrane, Proteins in the Blood.

Protein Synthesis (Genes to Protein)

DNA is transmitted from parents to offsprings which can determine some of the offsprings characteristics.

Genes

A DNA molecule is divided up into units called genes. Each gene provides instructions for a functional protein needed to perform a job in the cell. Many genes provide instructions to build polypeptides.

Central DOgma

During expression of a Protein-Coding Gene, the information flows from DNA → RNA → Protein. The flow of information is known as central dogma. This involves two major steps: Transcription and Translation.

Transcription Definition

Is the process where mRNA is made from the template strand of DNA in the nucleus, it all begins when a cell needs a particular protein to be made.

Process of Transcription

1) An enzyme in the body (RNA Polymerase) unzips the DNA by breaking the hydrogen bonds, exposing the nucleotides. Once open and accessible, one of the strand can then be read and used as a template strand.

2) An enzyme adds nucleotides to the template strand, to ensure that the correct nucleotides are added, the enzyme follows the complementary base pairing rule. The enzyme is making mRNA C-G and A-U

3) Transcription forms a single mRNA strand. It is complete when (RNA polymerase reaches the terminator sequence) mRNA detaches and moves out of the nucleus into the cytoplasm and attacks to a ribosomes in preparation for translation.

Coding Strand

The new mRNA strand ends up with exactly the same sequence of bases (expect with T replaced by U) as the other separated strands of DNA. The coding strand function is to increase the durability of the DNA strand. It reduces the risk that the DNA can be damaged, in the event that the template strand is damaged it acts as a template for repairing enzymes.

Triplet

Three bases of DNA that will code for an amino acid.

mRNA Codon

3 bases of mRNA that will code for an amino acid.

Template Strand

Single strand of DNA that provides a sequence of bases for mRNA to be built from. Complimenting bases (RNA) pairing allows this to be copied.

Translation

Translation is the process where the mRNA is read by the ribosome in the cytoplasm, and an amino is created based on the codons on the mRNA.

Steps of Translation

1) A triplet of 3 bases on the mRNA codes for 1 amino acid

2) When mRNA leaves the nucleus through the nuclear pore it travels to a ribosome and binds to it. The ribosome moves along the mRNA from the start codon until the stop codons reached.

3) Each sequence of 3 bases (codon) on the mRNA is read by the ribosome and matched to the complementary unpaired three base sequences (anticodon on the tRNA)

4) tRNA carries the amino acid to the ribosomes and drops it off. The specific amino acid attached to the tRNA is then added (peptide bond forms) to the polypeptide chain being made.

5) This process continues until the ribosome encounters a stop codon. At this point the mRNA will leave the ribosome, the polypeptide chain will also detach.

Ribosomes

Are the structures where polypeptides (proteins) are built. They are made up of protein and RNA and form the bonds between the amino acids.

Anticodon

One end of tRNA has a sequence of three nucleotides which can bind to specific mRNA codon.

Genetic Code

During translation, the nucleotides sequence of an mRNA is translated into the amino acid sequence of a polypeptide. Cells decode mRNA by reading their nucleotides in groups of threes codons. Three “stop” codons mark the end of a protein. One “start: codon, AUG, marks the beginning of a protein.

Redundancy

Most amino acids have more than one codon for each amino acid. It ensures that even if a mistake is made, there is still a chance the amino acid coded will be the same.

Degeneracy

Most amino acids where there are multiple codons for it, only differ by the third base. Degeneracy leads to redundancy.

Protein Final Stage

The polypeptide chain then has to fold up into different structures with specific functions. Protein folding up depends on the sequence of its amino acid. The structure of the protein when it has finished folding determines the function of the protein.

Mutation

A permeant, random change to the DNA sequence of a cell or organisms DNA. The altered DNA sequence in turn may lead to the production of a different protein. Mutation may arise due to errors during DNA replication or by mutagens.

Mutagens causes Mutations

Radiation - UV Light, Sun, X-rays

Chemical - Acids, Alcohols

Infectious Agents - Viruses, Bacteria

Point Mutations

Are changes to one base in the DNA code. Changes to DNA may have an effect on the final polypeptide by changing the sequence of amino acids that the DNA coder for.

Substitution

One nucleotide is substituted for another. May or may not change the amino acid sequence.

Insertion

One nucleotide is added to the DNA strand.

Deletion

One nucleotide is removed from the DNA strand.

Silent Mutation

Even though there is a change in the DNA sequence, there is no change in the amino aid coded for.

Missense Mutation

The change in the DNA sequence, there is a different amino acid coded for.

Nonsense Mutation

Change in the DNA sequence results in a STOP codon being coded for - short protein.

Frameshift mutation

Occurs when the addition or removal of a base alters the reading frame of the gene. When a nucleotide is added or removed, an overall change in the codons that are read by the ribosome. When a reading frame is shifted, it will cause a completely different string of amino acids.

Metabolic Pathway

Is a series of stepwise biochemical reactions with each step controlled by an enzyme. The product of one step becomes the substrate for the next step. Each enzyme is coded for by one specific gene. A specific enzyme catalyses each step in the metabolic pathway binding to the substrate or substrates.

What can disrupt the metabolic pathway?

Any change in the genes, enzyme or substrate, anywhere in the metabolic pathway.

An error in any step of the process will mean end product is not made - disruption in function.

How can the gene be disrupted?

Change in DNA sequence - Mutations

Change in RNA sequence - Transcription error - mutation

Translation error - mistake on tRNA - incorrect amino acids added

Frameshift Mutations

How can the enzyme be disrupted?

Denaturation - High or very low temp, too high or too low pH. If enzyme disrupted - will not perform on metabolic pathway

Genotype

Unique sequence of DNA.

Phenotype

The expression of the genotype of an organism

Environemental Effects on Phenotype

The phenotype of an organisim is determined by the interaction between its genes and its environment. PHENOTYPE = GENOTYPE + ENVIRONMENT