Gene expression
Post transcription modifications
- In prokaryotes, the mRNA is now ready to be translated into protein. - In eukaryotes, the mRNA can contain non-coding sections (introns stay inside the nucleus) that are not needed for protein synthesis. These introns are removed by enzymes in the nucleus. The remaining coding exons (exit the nucleus) are rejoined (splicing) to form mature mRNA. Known as coding sequence
Factors regulating phenotypic expression of genes
1. Proteins bind to specific DNA sequences (during transcription and translation) 2. Through the products of other genes (activators and repressors) 3. Via environmental exposure
Transcription (continued)
1. RNA polymerase reads the unwound DNA strand and builds mRNA molecule, using complementary base pairs. E.g. A in the dna binds to U in the RNA. This continues until RNA polymerase crosses a 'stop' sequence in the gene. Transcription ends and the mRNA strand is complete, detaching from DNA
Transcription factors
1. Transcription is the key on/off control point 2. If a gene is not transcribed in a cell, it can't be used to make a protein in that cell. if a gene does not get transcribed, it is likely to be used to make a protein (expressed). in general, the more a gene is transcribed, the more protein that will be made.
Key ideas for gene expression
Genes are switched on and off by regulatory proteins called transcription factors Transcription factors bind to the sequence in the DNA that come before the gene sequence that is to be transcribed. they either: 1. Activate the gene, and allow the RNA polymerase to bind or, 2. Repress the gene, blocking RNA polymerase from binding.
Homologous genes
Genes in different organisms that share similar sequence and function
Hox transcription factor
Genetic sequences maintained over evolutionary time are thought to be especially important to the basic development of even distantly related ancestors
Hox genes in vertebrates
Hox genes play more roles in vertebrate development. They help specify the difference between an arm and a leg. In the nervous system, their expression in segmented embryonic structures, directs the development of different brain regionsq
Example: coat colour of siamase cats
In coat colour in cats the C gene codes for the production of the enzyme trysinase, the first step in the production of this pigement. A mutant allele of the gene C allows normal pigment production of only at temperature below body temperature. The mutant allele has been selected for in the selective breeding of a Siamese cats. At higher temperatures, the protein product is inactive or less active, resulting in less pigment.
Transcription factor gene and cell differentiation
In mammals, gender is determined by the sex chromosome. typically, an individual with two X chromosomes is female, whereras an individual with one X and one Y chromosome is male. The Y chromosome has genes that the set the organism on the path to developing into a male. the single most influential gene is the process is the sex - determining region Y (SRY) gene, located on the p (small) arm of the Y chromosome.
gene expression continued
In prokaryotes, certain proteins can be controlled by nutrient availability. this allows organisms such as bacteria to rapidly adjust their transcription patterns in response to environmental conditions. The control of gene expression in eukaryotes is more complex than that in prokaryotes. in general, a greater number of proteins are involved, and can act together to control gene expression. Cells can control which genes get transcribed and which transcripts get translated. Regulation of transcription and translation occurs in both prokaryotes and eukaryotes, but is far more complex in eukaryotes.
Transcription
Once the mRNA molecule has detached, the DNA 'zips up' again and twists itself back into a double helix structure. The mRNA now leaves the nucleus through a gap and enters the cytoplasm where enzymes, ribosomes and amino acids are found
SRY gene
Provides instructions for making a regulatory protein that primarily launches the genetic program for testes development
Hox genes are arranged in clusters
The genes on the left control the patterning in the head, and the other genes on the right control patterning in the tail
The more non-coding DNA there is
The more complex the organism is
tRNA
acts as a "bridge" between mRNA and the amino acid.
Number of genes does not imply complexity
more regulation leads to complexity
Regulatory proteins
products of genes that regulate the expression of genes other than their own
Gene expression
refers to a gene being transcribed into mRNA and translated into a polypeptide, or transcribed into functional RNA. In gene expression, a gene is "turned on" to produce a polypeptide. Some genes codes for proteins that serve to maintain basic cellular processes, and so are constantly expressed. Not all genes in the body are turned on at the same time, or in the same cells or parts of the body
gene regulation
refers to the processes within cells that enable a gene to be expressed, just in particular cells and at certain specific times and rates
Promotor
region of DNA that initiates transcription of a particular Gene. Promotor sequence is typically located directly upstream or at the 5' end of the transcription intitate site
Regulatory proteins
that bind to the DNA are called transcription factors (TFS). They are proteins that help turn specific genes on or off by binding to to nearby DNA. Most of these activate gene expression (activators) but some repress it (repressors)
protein synthesis
the formation of proteins by using information contained in DNA and carried by mRNA
Genome
A gene consists of enough DNA to code for one protein. A genome is the sum of an organism's DNA.
Translation
The ribosome continues to read the sequences on the mRNA in groups of 3. The 2nd tRNA molecules bonds with the next three exposed bases. This one brings a different amino acid (TYR). The two amino acids are held closely together (with a bond formed between them). A maximum of two tRNAS can be bound to mRNA at the same time.
Translation
The ribosome moves along the mRNA 'reading' the next three bases on the ribosome. A 3rd tRNA molecule brings a third amino acid (LYS), which joins to the 2nd one. At this point, the first tRNA leaves. the polypeptide chain continues to grow until the "stop" codon is exposed on the ribosome. At this point, translation ceases and the polypeptide chain is released.
Pre-transcriptional control
Transcription only occurs during interphase when the chromosomes exist as chromatin threads. Transcription begins when RNA polymerase unzips the DNA, exposing the nucleotide bases.
Epigentics
involves genetic control by factors other than an individual's DNA sequence. Epigenetics changes can switch genes on or off and determine which proteins are transcribed (without affecting the genetic code itself)
Non-coding DNA
1. centromeres - hold duplicated chromosomes (sister chromatids) together, and are the site of attachment for spindle fibres to move chromosomes in cell division 2. Telemeres - Regions of repetitive DNA at the end of chromosomes. They protect against chromosomal deterioration during replication 3. Functional RNA - includes all ribosomal RNA and transfer RNA, which are necessary for protein synthesis, but are not translated into proteins 4. Introns - DNA bases that are found in between exons (coding DNA).
Polypeptides
A protein consists of one or more polypeptide chains arranged into a 3D shape. Polypeptide molecules consist of amino acids joined together by polypeptide bonds. A polypeptide chain can serve as a protein on its own. However, lots of proteins are made up of several polypeptide chains. 20 different amino acids
mRNA
Acts as the messenger between the DNA carrying genetic material and the ribosomes (protein production workshops)
Genome QCAA defintion
All the genetic material in the chromosomes of an organism. including it's genes and DNA sequences
.DNA Methylation
An epigenetic mechanism that can occur is when a methyl group is added to DNA. this often modifies the function of the genes and affects gene expression.
Repressors
Are regulatory genes that bind to the DNA in order to switch off gene expression i.e. they are transcription factors that repress transcription They may bind to the promotor of a specific gene to block RNA polymerase from binding to prevent transcription
Other species have environmental factors that alter gene expression
Changes in environment (both intracellular or extra cellular) can trigger changes in the amount of regulatory proteins or transcription factors produced, leading to a change in gene expression. The presence of drugs and chemicals can alter the gene expression
Changes to gene expression
Changes to hox gene expression changes a segment's identity For example: First segment of the thorax normally grows legs Second grows legs and wings Third segment growths legs and halteres.
Differential gene expression
Differences between cells are not due to genes present within the nucleus but through the expression of those genes. Differential gene expression occurs when different genes are expressed in different sets of cells with the same genome. This leads to differentiation and specialization. The genes that are not expressed in a cell are not lost or destroyed and can "switched on" if given the correct environment and stimulus
Hox genes regulate other genes
Different hox proteins regulate different sets of genes. As regulatory proteins are very powerful. A single Hox protein can regulate the activity of many genes (turning genes on and off). These genes work together to carry out programs during embryonic development e.g. programs for building a leg
cell differentiation 1
Different transcription factors which bind to DNA, will activate or repress the transcription of different genes. Through the action of these transcription factors, cells specialise into one of hundreds of different cell types in the human body. Transcription factors regulate cell differentiation for tissue formation and morphology
Differentiaion
Differential gene expression controls the structural changes that occurs during growth. In many animals (e.g. insects) there is extensive differentiation during metamorphosis In insects metamorphosis is controlled by hormones made by the endocrine glands near the front of the body. Such hormones can meditate a change in gene expression
Transcription factors that regulate morphology
E.g. fruit flys a single mutation could dramatically alter the structural development of an organism. It was discovered that embryonic development is regulated by a small subset of genes. Each gene is expressed in a precise location within the animal and development
The genome consists of coding and non-coding DNA
Eukaryotic genomes are comparatively large. the genome is thought to consist of 3.2 billion. In the genome 2% is made up of protein - coding DNA (exons) the other 99% is non-coding segments of DNA (introns)
Protein Synthesis - central dogma of biology
Everything a cell does is determined by the proteins it produces, and the cell's genome directs the production of these proteins. DNA is too large to pass through the nuclear pores and by remaining in the nucleus, it is protected from the rest of the cell. Consequently, it requires another nucleic acid, known as RNA 2 processes involved in protein synthesis 1. Transcription 2. Translation
Hox transcription factors
Homeotic transformations because one part seemed to have replaced by another. Hox genes demonstrate that antennal cells carry all of the information necessary to become leg cells. Every cell in an organism carries within its DNA, all the information necessary to build the entire organism
Environmental exposure regulates the phenotypic expression of genes. identical vs fraternal twins
Identical twins share all their genes and their home environment. E.g. For twins with schizophrenia 50% identical twins share this disease, while only 10-15% of fraternal twins do. the difference is evidence for a strong genetic component in susceptibility to schizophrenia.
Functional RNA
RNA molecule that is not translated into proteins. It is still transcribed from DNA. In general, functional RNA is involved in regulating gene expression. the number of functional RNAs within the human genome is unknown. E.g. Transfer RNA and Ribosomal RNA
The genetic code is written in triplets
The smallest unit of DNA that passes information from gene to polypeptide is the triplet code The triplet code is a sequence of there nucleotides (bases). For example, the triplet AUG is the code of the amino acid methionine. The position of the code along the DNA strand leads to methionine being placed as the corresponding position in the polypeptide that is being produced. Any set of three bases can determine the identity of one amino acid.
Key points for transcription
Transcription is when the DNA sequence of a gene is copied to make an RNA molecule RNA polymerase is the enzyme that unzips the DNA molecule, reads the unwound strand and makes the mRNA. The mRNA molecule is made using complementary base pairing In RNA, A-U (Unlike dna replication, A-T)
Key points for translation
Translation is when the MRNA is decoded to build a protein. Translating sequence of nucleotide bases (languages of mRNA) to sequence of amino acids (language of proteins). Ribosomes are the protein factories, they work with the tRNA to decode the mRNA and build the protein. The ribosome reads the codon, and the tRNA brings the amino acid with the corresponding anticodon to the ribosome.
Genetic and environmental contributions
Twins share the same genes but their environments become more different as they age. this unique aspect of twins makes them an excellent contribute to certain traits, especially behaviour and diseases. Even though they are identical twins have the same genes. they show different characteristics, such as different weights and heights. This is due to environmental conditions affecting gene expression
Transcription
When the DNA sequence of a gene is copied to make an RNA molecule. Transcription starts with RNA polymerase 'unzipping' the DNA The weak hydrogen bonds between the bases are broken which causes the two strands of DNA to seperate. Only one strand is used to direct the synthesis of mRNA.
Translation
mRNA is decoded to build a protein Translation is the 2nd part of protein synthesis. It is the process in which the genetic code in mRNA is translated into a sequence of amino acids. After mRNA leaves the nucleus, it moves to a ribosome, which consists of rRNA and proteins. The ribosomes reads the sequence of codons in mRNA, and molecules of tRNA (transfer RNA) bring amino acids to the ribosome in the correct sequence.
Gene QCAA defintion
region's of DNA that are made up of nucleotides; the molecular unit of heredity
Activators
regulatory proteins that bind to sections of non-coding DNA to activate gene expression i.e. they switch a gene on Activators enable the DNA to unwind from histone proteins and expose the gene for transcription. They assist the binding of RNA polymerase, which begins transcription of the gene