Ch 2: The Genetic Code Flashcards
What are proteins?
proteins expressed by a cell determine how a cell develops, synthesises and operates, they are essential to cell structure and functioning
What are enzymes?
group of enzymes that are able to increase the rate of chemical reactions (act as a catalyst) without undergoing any change themselves
each step in the pathway of metabolic reactions (sum of chemical reactions that occur constantly in each living organism) are regulated by an enzyme
What are the steps of transcription?
- In the nucleus, an area of DNA becomes unzipped with the use of the enzyme helicase
- One of the 2 strands is used as a template to create an mRNA molecule
- RNA polymerase promotes/assists in the complimentary base pairing of adenine with uracil (instead of thymine) and cytosine with guanine
- Introns are removed/sections of DNA are spliced out and the exons are ready to move out of the nucleus
- The mRNA, containing codons-triplets of nucleotide bases, detaches from the DNA and floats through one of the nuclear pores
What are the steps of translation?
- mRNA will locate a ribosome and a mRNA start codon (methionine) starts the process
- mRNA will align the first two codons within the boundary of the ribosome
- A specific tRNA float towards the codon containing the complementary anti-codon and the first amino acid attached to the tRNA
- The second anti-codon complementing the 2nd mRNA codon approaches with the specific amino acid attached. The chain of amino acids form in the order determined firstly by the DNA sequence, then by the mRNA codon sequence
- The amino acid bonds with the first amino acid, forming a peptide bond between the amino acids. Peptide bonds are covalent bonds
- The system of attaching complimentary anticodons continues until the polypeptide has been formed
- tRNA breaks away one at a time from its amino acid protein, ready to join another amino acid of the same type
- The final mRNA codon triplet acts as a STOP signal and the polypeptide breaks away from the final tRNA molecule and floats in the cytoplasm
What are the differences between DNA and mRNA?
DNA - deoxyribose sugar - double-stranded - thymine -longer RNA - ribose sugar - single stranded - uracil - shorter (usually fewer than 4000 nucleotides)
What are the similarities/differences between transcription and replication?
similarities: DNA molecule unwinds and free-floating nucleotides bind to strand creating a complementary sequence
differences:
- only a small part of DNA is used as a template for RNA, this coding region is a gene. DNA replication involves the whole gene
- the enzymes involved in joining the RNA nucleotides together are RNA polymerase rather than DNA polymerases
- only single-stranded RNA is produced in transcription and it is found in both the nucleus and cytoplasm whereas DNA formed by replication is double-stranded and found only in the nucleus
What are introns and exons and what is alternative splicing?
introns: non-coding DNA, removed before mature RNA leaves the nucleus
exons: parts of DNA that code for protein formation
ALTERNATIVE SPLICING: cells can recognise which introns need to be removed,determining which proteins will be produced. Hence the same gene can make different protein according to the introns removed/spliced
What is the universal genetic code?
it is the sequence of nucleotides in DNA/RNA that determines the specific amino acid sequence in the synthesis of proteins in nearly all organisms
Why are cell not the same if they all contain the same DNA content?
cells do not express all the genes at its genome at the same time and same rate, it ensures the cell does not waste energy and resources producing unwanted proteins, as well as ensuring the cell does not produce proteins whose action may interfere with the cell properly performing its specialised role
What is the make up of the human genome?
large sections are made up of non-coding DNA which is in turn made up of mostly repetitive sequence, their function not completely known.
less than 2% is coding DNA formed by exons called the exome
What is gene expression and gene regulation?
gene expression: gene being transcribed into mRNA and translated into a protein
gene regulation: processes within a cell that enable a gene to be expressed in specific cells and at specific times
How are genes turned on and off?
ADD DIAGRAM
genes are switched off and the corresponding proteins are not made when the DNA is packed into chromatin (nucleosome wound up around 8 histone proteins) as RNA polymerase is unable to access the DNA to begin transcription
genes are turned on when the DNA is in a relaxed, open configuration so it accessible to the RNA polymerase and other proteins required for manufacturing the encoded proteins
What is methylation?
the attachment of a methyl group to nucleotides or histone proteins that switches off gene expression because the methyl group projects out from DNA and blocks the RNA polymerase from binding or transcribing the gene. Also the methyl groups are positively charged which allows it to interact with the negatively charged phosphate groups of DNA, allowing the DNA to condense and hence restricting access by RNA polymerase
What is an example of DNA methylation?
X-inactivation, DNA methylation is used to silence one of the two X chromosomes in female mammals
in tortoise shell cats one X carries the orange fur allele and the other X carries the black allele. Depending which is inactivated early in embryonic development, patches of fur descending from the original cells only express the colour from the X chromosome that has not been inactivated
What is imprinting?
an epigenetic process where one allele of a gene is methylated and hence silenced. This lasts for the life time of the cell and is passed on
What is epigentics and epigenome?
epigenetic: the study of chemical modifications to gene function that are not dfue to DNA sequence changes i.e. DNA methylation
epigenome: chemical compounds that modify the genome
How do the products of other genes influence the phenotypic expression of genes? (regulatory proteins)
Regulatory proteins are products that regulate gene expression - RP that bind to DNA are transcription factors (TFs), most are activators but some are repressors
- activators bind to DNA to activate gene expression and enhance it, they normally recognise a 6-10 bp non-coding segment in the promoter region of eukaryotic genes. They enable DNA to unwind from histone proteins and expose the gene for transcription. They also assist the binding of RNA polymerase to promoters to begin trasncription
- repressors bind to DNA to switch off gene expression i.e. may bind to the promoter of a specific gene to block the RNA polymerase from binding and prevent transcription
What are regulatory proteins?
Regulatory proteins are products that regulate gene expression, one that serves as an activator for one gene may act as a repressor for another. It is dependent on the action of regulatory proteins with each other and their interactions with other proteins. They normally consist of multiple domains with separate functions
sequence specificity ensures the RP precisely target the gene whose expression they are meant to control
How can translation be regulated?
mRNA is prevented from being translated into a protein
- mRNA-binding proteins attach to specific mRNA sequences and block it from being translated. Upon binding the protein blocks the ribosome from assembling around the mRNA and translating it. Multiple genes of the same gene may be shut down
- in plants and animals microRNAs (miRNAs) - 20 nucleotide in length specifically transcribed and processed can repress translation by binding with a complementary sequence of mRN, forming a double-stranded RNA molecule that prevent translation. In some cases this is a trigger for the cell to digest and destroy the RNA molecules
How does E. coli control gene expression in mammals?
E.coli feed on glucose so lactose metabolism is turned off.
- 3 enzymes are translated into a single mRNA strand (lac operon)
- if lactose is present, the lac operon binds to the repressor protein, altering it shape so that it cannot bind to the operator (binding sites on a non-coding base sequence that signals the start of a gene called promoters). The three genes are transcribed into the three enzymes that are involved in the breakdown of lactose
- if lactose is absent/low, the repressor protein binds to the operator covering part of the promoter. RNA polymerase cannot bind to the promoter and transcription of the 3 genes is blocked
How does the environment influence the phenotypic expression of genes? (food and bacteria)
in food: stress and diet influence epigenetic changes i.e. agouti gene in a range of animals. If an organism is given a diet high in methyl groups, the gene methylates and give birth to normal offspring. When methylation is low, the gene has a high expression of its protein product and offspring are abnormal, obese and more prone to cancer and diabetes
in bacteria: the amino acid tryptophan can be synthesised by E. coli. If there are high leels of tryptophan transcription of the gene occurs and tryptophan is synthesised. When it is plentiful, tryptophan molecules bind to a regulatory protein repressor and activate it. This activated repressor blocks transcription of the enzymes and thus tryptophan is not synthesised by the cell, reducing unnecessary energy use
