Genetics Flashcards
Describe the structure of RNA
Single stranded. Made up of ribose nucleotides. U replaces T
How is the DNA sequence converted into proteins?
Transcription: reads DNA sequence and produces mRNA
Translation: reads mRNA and produces proteins
Outline the process of transcription.
- DNA uncoils into 2 strands with exposed bases, one is used as a template
- Free nucleotides line up next to their complementary base pairs, joined together by RNA polymerase
- mRNA is synthesised from the antisense strand
- U pairs with A
What happens to mRNA after transcription?
It moves out of the nucleus into the cytoplasm and attaches to a ribosome, ready for translation
Outline the process of translation.
- mRNA attaches to a ribosome
- Each tRNA carries a specific amino acid
- The anti-codon of tRNA attaches to complementary bases on the mRNA
- Amino acids bonded to tRNA forms peptide bonds, continuing to form a polypeptide chain until a stop codon is reached
Identify features of the genetic code.
- A triplet of bases code for a particular amino acid (e.g, 12 bases codes for 4 amino acids)
- Non overlapping = each triplet is only read once
(e.g, AAT + ACC + AGT gives 4 distinct amino acids) - Degenerate = more than one triplet codes for the same amino acid (64 possible triplets for 20 amino acids)
What is a gene?
A sequence of bases on a DNA molecule that codes for a specific sequence of amino acids in a polypeptide chain
What is substitution?
When one base is substituted for another
What is deletion?
When a base is deleted
What is insertion?
When an extra base is added
What is a silent mutation?
When substitution of a base still codes for the same amino acid, this is due to its degenerate nature
What is a nonsense mutation?
When a substitution of a base occurs leading to a premature ‘stop codon’ being coded for
What is a missense mutation?
When a change in base (substitution) leads to a different amino acid being coded for
What is a frameshift mutation?
An insertion or a deletion of a base and a nucleotide is lost from the DNA sequence
Define the term heterzygous
2 different alleles
Define the term homozygous
2 identical alleles
What is meant by incomplete dominance?
An allele whose characteristics is not completely expressed, the phenotype is a combination of 2 alleles
What is meant by genetic screening?
Determining if an individuals DNA contains a certain allele, usually one that results in a genetic disorder
Describe the process of chorionic villus sampling
- A sample of embryonic tissue is taken from the placenta at around 11-14 weeks of pregnancy and is screened for various disorders
Describe the process of amniocentesis
A sample of amniotic fluid is taken at around 15-20 weeks of pregnancy, collect foetal cells that have fallen off the placenta and sample the cells for diseases
Define the process of PIGD
Checking genes of embryos created through IVF for genetic diseases
What are some social and ethical issues surrounding prenatal genetic screening?
- procedures carry risk of harming foetus
- abortion
- high cost of bringing a child up with a condition
- emotional and mental stress on parents
What is a stem cell?
Undifferentiated cells, that can divide indefinitely and can turn into other specific cell types
What is the meaning of totipotent and pluripotent stem cells?
Totipotent = can develop into any cell type including the placenta and embryonic cells
Pluripotent = can develop into any cell type excluding placental cells
Give some uses of stem cells.
- Treat diseases
- Replace damaged tissue
Why is the use of some stem cells controversial?
- Involves killing embryos
- Risk of infection transplant
How do cells become specialised (gene expression)?
- Under the right conditions (stimulus), some genes are activated and others are inactivated
- mRNA is only transcribed from the active genes it is then translated into proteins
- These proteins modify the cell, they determine cell structure and control cell processes
- Changes to these cells causes differentiation
What is meant by epigenetics?
A heritable change in base sequence without change to the base sequence of DNA
How does DNA methylation affect gene transcription?
- Involves the addition of a -CH3 group at a CpG site (c and g base next to eachother)
- It changes DNA structure so that the proteins and enzymes needed for transcription can’t bind to the gene and transcribe it - the gene is repressed/inactivated
What are histones?
- Histones are proteins that DNA wraps around to form chromatin
- Chromatin can be highly condensed or less condensed - how condensed it is affects accessibility of the DNA and how easily it can be transcribed
How can histone modification affect gene expression?
- When histones are acetylated (-COCH3) the chromatin is less condensed, this means proteins in transcription can bind to the DNA
- When acetyl groups are removed, the chromatin becomes highly condensed and genes in the DNA cannot be transcribed
How are epigenetics passed on?
All cells that arise from the altered cells will possess the modifications
What are transcription factors?
Proteins that bind to DNA and activate or deactivate genes by increasing or decreasing rate of transcription
What are activators and repressors?
Activators = help RNA polymerase bind to the DNA and begin transcription
Repressors = prevent RNA polymerase binding and so stops transcription
What are control elements?
Promoter and operator
What is an operon?
A section of DNA that contains a cluster of structural genes (useful proteins) that are transcribed together
What is lac operon?
When glucose isn’t available for E.coli to respire, it can use lactose, the genes that produces these enzymes needed to respire lactose are found on an operon called lac operon
What 3 structural genes does lac operon have?
lacZ, lacY and lacA
What happens when lactose isn’t present in E.coli?
- The regulatory gene lacI produces the lac repressor, which binds to the operator site when there is no lactose present
- This blocks transcription so lacZ, Y and A aren’t transcribed
What happens when lactose is present in E.coli?
- When lactose is present, it binds to the repressor changing its shape
- RNA polymerase can now begin transcription of the structural genes - lacZ, Y and A are transcribed and produces proteins that help the digestion of lactose
What proteins help the bacteria digest lactose?
B-galactosidase and lactose permease
Scientists have suggested that epigenetic events are involved in regulating collagen synthesis in rats given alcohol.
i) Explain how epigenetic events might be involved in regulating collagen synthesis in rats given
alcohol.
- Exposure to alcohol may lead to histone modification such as adding acetyl groups
- Collagen gene more likely to be expressed
Describe how you could use a plant tissue culture technique to show totipotency in cotton
plant seedlings.
- Aseptic technique carried out
- Agar used
- Using a specific part of the seed
- Good supply of light
