gene expression: video · gene regulation in eukaryotes gene regulation in eukaryotes remains very...
TRANSCRIPT
Transcription and Translation: Overview
Transcription and Translation: Overview
Transcription and Translation: Overview
Transcription
Transcription Terms
template strand: strand of the DNA double helix used to make RNA
coding strand: strand of DNA that is complementary to the template strand
RNA polymerase: the enzyme that synthesizes RNA from the DNA template
Transcription
Transcription: Gene Structure of Eukaryotes
Eukaryotic Gene Structure
Transcription: mRNA Processing in Eukaryotes
Spliceosome
Translation
Translation: Triplet Codons and Redundancy
Translation: Polypeptide Synthesis
Translation: Polypeptide Synthesis
Alternative Splicing
The human genome has about 30 000 genes but our proteome (the total number of different proteins) is much larger.
How can this occur?
Many genes can produce more than one protein because the mRNA transcript contains different combinations of exons. This process is called alternative splicing.
Protein Variation
Alternative Splicing
Protein Variation
Spliceosome
Prokaryotic Transcription & Translation
Transcription and translation follows a similar pattern in prokaryotic organisms with one major difference:
Prokaryote genomes do not contain introns.
Processing of pre-mRNA transcripts still occurs in prokaryotes to produce mature mRNA but obviously spliceosomes are not
necessary for the removal of introns.
You should also remember that prokaryotic DNA exists as a single circular chromosome that is not bound by nuclear membrane.
Prokaryotic Transcription & Translation
Gene Regulation in Prokaryotes
Bacteria have groups of genes that are controlled together and are turned on/off as required.
One example, the lac operon is a set of genes in bacteria used for lactose metabolism
Some bacteria use the disaccharide lactose as an energy source
Bacteria produce the enzymes to break down lactose to glucose and galactose only when lactose is present.
Gene Regulation
The lac operon
The regulatory gene is responsible for the expression of the repressor protein. The lac operon itself is responsible for 3 proteins with specific
functional roles in the metabolism of lactose.
Significantly most regulatory genes in prokaryotes are found next to the genes they regulate.
Gene Regulation
Gene Regulation in Eukaryotes
Gene regulation in eukaryotes remains very poorly understood. Indeed, until very recently, “introns” were called “junk DNA” and thought to serve no significant purpose in gene expression. Advances in the past 10 years
have shown this is not the case.
An important difference between gene regulation in eukaryotes vs prokarytoes is in the loci of the regulating genes. In eukaryotes regulatory
genes can be located far from the genes they affect- they can even be located on different chromosomes.
Gene Regulation
A mutation is a change in a DNA sequence. This may occur at the DNA or chromosome level. Mutations may be categorised as;
HarmfulBeneficialNeutral
A change in DNA sequence means a change in the protein product that a gene might code for.
Changes mean diversity so mutations are an important contributor to evolution and polymorphism.
Mutations
Mutations
Mutations occur spontaneously and randomly throughout the lifetime of all organisms.
The effects of mutation varies depending on the location both within the chromosome (or gene) and the body of the organism.
Mutations in the DNA can be caused by mistakes in DNA replication or repair. This is a natural process.
Environmental factors that increase the rate of mutations are called mutagens. These factors include radiation, various chemical and high temperatures.
Germline mutations- Originate in meiosis- Affects gametes
- When inherited will affect all cells of the offspring
Somatic mutations- Originate in mitosis- Affect only cells that descend from changed cell
Mutations
Point Mutations involve the substitution of a single nucleotide for another.
The results of this may vary* a different amino acid might be coded for (missense)* a stop codon might result (nonsense)* the amino acid may not change at all (silent)
Gene Mutations
Sickle Cell Anaemia
Sickle cells contain abnormal hemoglobin that causes the cells to have a sickle shape. Sickle-shaped cells don’t move easily through your blood vessels and tend to get stuck in the blood vessels.
The clumps of sickle cells block blood flow in the blood vessels that lead to the limbs and organs. Blocked blood vessels can cause pain, serious infections, and organ damage
Deletions and Insertions will remove or add nucleotides to the sequence
If the deletion or insertion does not occur as a multiple of three it can cause a frameshift mutation
Frameshift mutations tend to have more severe effects on phenotype.
Gene Mutations
Cystic Fibrosis
The most common mutation, ΔF508, is a deletion (Δ) of three nucleotides that results in a loss of the amino acid phenylalanine (F)
Deletion and Insertion are essentially similar but the regions affected may involve large segments areas and even whole genes.
Chromosomal/Block Mutations
Duplication involves the
repetition of a particular sequence.
Inversion occurs where a
segment is rotated or reversed.
Translocation occurs where one segment is transferred to another chromosome.
Chromosomal/Block Mutations
The extra or missing chromosomes may be autosomal or sex chromosomes
Such changes are due to non-disjunction. These evens are errors in chromosome segregation during aeiosis. Pairs of homologous chromoms may fail to separate in Meiosis I or the centromere may fail to separate sister chromatids in Meisosis II.
AneuploidyChanges to a number of specific chromosomes
Turner Syndrome44 + X
Klinefelter Syndrome44 + XXY
Occurs in 1/25000 births. Common symptoms
* Short stature* Swelling of hands and feet* Broad chest* Low hairline* Webbed neck
AneuploidyChanges to a number of specific chromosomes
Occurs in 1 in 500-1000 births. Varied characteristics
* Sterility* Laguage impairment* Youthful build* Rounded body type* Hypogonadism
PolyploidyChanges to the number of whole sets of chromosomes
Polyploid organisms have more than two sets of chromosomes.
Very common in plants because they can reproduce asexually. Rare in animals.
Polyploid plants are usually more robust than diploid plants.
Throughout history we have selected out polyploid plants because they have a high yield and are more disease-resistant. Many of our crops today are polyploid.
Wheat was the first crop to be domesticated originating in Asia about 10000 years ago. Today, bread wheat is hexaploid.
Different strawberry species worldwide can be diploid, tetraploid, hexaploid, ocotoploid or even decaploid.