Genetics of living systems Flashcards
mutation definition
a change in the sequence of bases in DNA
Gene mutations
Protein synthesis can be disrupted if mutations occur within a gene
The change in sequence is caused by the substitution/deletion/insertion of 1/more nucleotides within a gene
Point mutation - when only 1 nucleotide is affected
The substitution of a single nucleotide changes the codon in which it occurs
If the new codon codes for a different amino acid this will lead to a change in the primary structure of the protein
The degenerate nature of the genetic code may mean the new codon still codes for the same amino acid leading to no change in the protein synthesised
The position and involvement of the amino acid in R group interactions will determine the impact of the new amino acid on the function of the protein
The insertion/deletion of a nucleotide leads to a frameshift mutation. the addition/removal of a nucleotide shifts the reading frame of the sequence of bases and changes every codon from the point of mutation unless the number of nucleotides changed is a multiple of 3
the triplet code means the sequence of bases are transcribed consecutively in non-overlapping triplets (reading frame)
Effects of different mutations
No effect:
- there is no effect on the phenotype of an organism as normally functioning proteins are synthesised
Damaging:
- the phenotype of an organism is affected in a negative way as proteins are no longer synthesised or proteins synthesised are non-functional - this can interfere with essential processes
Beneficial:
- a protein is synthesised that results in a new and useful characteristic in the phenotype
mutagen definiton
A chemical/physical/biological agents that causes mutations
Causes of mutations
mutations can occur spontaneously - e.g during DNA replication
the rate of mutation is increased by mutagens
The loss of a purine base (depurination) or a pyrimidine base (depyrimidination) often occurs spontaneously - the absence of a base can lead to the insertion of an incorrect base through complementary base pairing during DNA replication
Free radicals (oxidising agents) can affect the structures of nucleotides and disrupt base pairing during DNA replication
Types of mutagens
PHYSICAL MUTAGENS
ionising radiations e.g X-Rays
- break 1/both DNA strands - some breaks can be repaired but mutations can occur in the process
CHEMICAL MUTAGENS:
E.g deaminating agents
- chemically alter bases in DNA, changing the base sequence e.g converting C to U
BIOLOGICAL AGENTS:
e.g alkylating agents
- methyl/ethyl groups are attached to bases resulting in the incorrect pairing of bases during replication
e.g base analogs:
- incorporated into DNA in place of the usual base during replication, changing the base sequence
e.g viruses
- viral DNA can insert itself into a genome, changing the base sequence
Silent mutations
- they don’t change any proteins/ the functions of any proteins synthesised
- they have no effect on the phenotype of an organism
- can occur in non-coding regions of DNA
- can code for the same amino acid (genetic code is degenerate)
- may result in changes to the primary structure but not change the overall structure/function of the proteins synthesised
Nonsense mutations
- result in a codon becoming a stop codon instead of coding for an amino acid
- a shortened protein is synthesised which is usually non-functional
- usually has a negative effect on phenotype
Missense mutations
- result in the incorporation of an incorrect amino acid into the primary structure when the protein is synthesised
- result depends on the role the Amino acid plays in the structure and the function of the protein synthesised
- can be beneficial/harmful/silent
conservative mutation - the amino acid being coded for has similar properties to the original so the effect of the mutation is less severe
Non - conservative - new amino acid coded for has different properties to the original
Gene mutations definition
occur in single genes/ sections of DNA
chromosome mutations definition
mutation affects the whole chromosome
Chromosome mutations
- can be caused by mutagens
- normally occurs during meiosis
- can be silent - often lead to developmental difficulties
CHANGES IN STRUCTURE INCLUDE:
- Deletion - a section of a chromosome breaks off and is lost within the cell
Duplication - sections are duplicated on a chromosome
Translocation - section of 1 chromosome breaks off and joins another non-homologous chromosome
Inversion - a section of 1 chromosome breaks off, is reversed, and joins back onto the chromosome
Housekeeping genes definition
code for enzymes which are necessary for reactions present in metabolic pathways and are constantly required
Tissue specific genes
code for protein-based hormones which are only required by certain cells at certain times to carry out a short lived response
Gene regulation
the entire genome of an organism is present in every prokaryotic/eukaryotic cell with a nucleus. this includes genes not required by that cell so the expression of genes and the synthesis of protein products has to be regulated
Genes can be turned on/off and the rate of product synthesis can be increased/decreased based on the demand
Bacteria can respond to changes in their environment due to gene regulation.
expressing genes only when the product is needed prevents resources being wasted
Gene regulation is required for cells to specialise and work in a coordinated way
Different ways that genes are regulates:
Transcriptional - genes can be turned on/off
Post transcriptional - mRNA can be modified which regulates translation and the types of proteins produced
Translational - translation can be stopped/started
Post translational - proteins can be modified after synthesis which changes their functions
Transcriptional control - chromatin remodelling
DNA has to be wound around histone proteins in eukaryotic cells to be packed into the nucleus
chromatin - the resulting DNA/histone complex
Heterochromatin - tightly wound DNA causing chromosomes to be visible during cell division
Euchromatin - loosely wound DNA present during interphase
The transcription of genes isn’t possible when DNA is tightly wound as RNA polymerase can’t access the genes
Genes in euchromatin can be transcribed
Protein synthesis occurs during interphase between cell divisions. this form of regulation ensures that proteins necessary for protein synthesis are synthesised in time and prevents protein synthesis occurring when cells are dividing
Transcriptional control - histone modification
DNA could around histones as they’re positively charged and DNA is negatively charged
Histones can be modified to increase/decrease the degree of packing
The addition of acetyl groups (acetylation) or phosphate groups (phosphorylation) reduces the positive charge on histones causing DNA to coil less tightly, allowing certain genes to be transcribed
The addition of methyl groups makes the histones more hydrophobic so they bind more tightly to each other causing DNA to coil more tightly and preventing transcription of genes
Epigenetics definition
used to describe the control of gene expression by the modification of DNA
Operon definition
A group of genes that are under the control of the same regulatory mechanism and are expressed at the same time
an efficient way of saving resources as if certain gene products aren’t needed, all of the genes involved in their production can be switched off
Transcriptional control - Lac Operon
Glucose is the preferred respiratory substrate of many bacteria
if glucose is in short supply, lactose can be used - different enzymes are needed to metabolise lactose
Lac operon - a group of 3 genes involved in the metabolism of lactose - lacZ, lacY and lacA.
They are structural genes as they come for 3 enzymes and are transcribed onto a single long molecule of mRNA
a regulatory gene lacI codes for a repressor protein that prevents the transcription of the structural genes in the absence of lactose
the repressor protein is constantly produced and binds to the operator - this prevents RNA polymerase binding and beginning transcription (down regulation)
promoter - the section of DNA that is the binding site for RNA polymerase
lactose binds to the repressor protein causing it to change shape so it can no longer bind to the operator. RNA polymerase can now bind to the promoter, the 3 structural genes are transcribed and the enzymes are synthesised
Transcriptional control - role of cAMP
the binding of RNA polymerase results in a slow rate of transcription that needs to be increased to produce the required quantity of enzymes to metabolise lactose efficiently
cAMP repressor protein (CRP) increases the rate of transcription when it binds to cAMP
if glucose is transported into a cell, the levels of cAMP decrease, reducing the transcription of the genes responsible for the metabolism of lactose
promoter definiton
region of DNA required to allow transcription of the gene to take place
operator definition
segment of DNA to which a repressor binds to inhibit the transcription of a gene
transcriptional factors
gene expression can be controlled at the transcriptional level by altering the rate of transcription of genes
this is controlled by transcription factors - proteins that bind to DNA and switch genes on/off by increasing/decreasing the rate of transcription
activators - factors that increase the rate of transcription
repressors - factors that decrease the rate of transcription
the shape of a transcription factor determines whether it can bind to DNA and can sometimes be altered by the binding of some molecules - this means the amount of certain molecules in an environment can control the synthesis of some proteins by affecting transcription factor binding
in eukaryotes transcription factors bind to specific DNA sites near the start of their target genes
regulatory gene
codes for an activator/repressor
structural genes
code for useful proteins e.g enzymes
control elements
include a promoter and an operator
promoter - DNA sequence located before the structural genes that RNA polymerase binds to
operator - a DNA sequence that transcription factors bind to
post-transcriptional control - RNA processing
Introns - don’t code for amino acids
Exons - code for amino acids
RNA PROCESSING:
- during transcription, introns and exons are both copied into pre-mRNA (a precursor molecule)
- pre-mRNA is modified forming mature mRNA before it can bind to a ribosome and code for the synthesis of the required protein
- a cap (modified nucleotide) is added to the 5’ end and a tail (a long chain of A nucleotides) is added to the 3’ end - these help to stabilise mRNA and delay degradation in the cytoplasm and the cap helps mRNA bind to ribosomes
- splicing occurs - introns are removed from pre-mRNA and exons are joined together in the nucleus
- mature mRNA leaves the nucleus for translation
post-transcriptional control - RNA editing
the nucleotide sequence of some mRNA molecules can be changed through base addition/deletion/substitution
- this results in the synthesis of different proteins, increasing the range of proteins that can be produced from a single mRNA molecule
Translational control
degradation of mRNA:
- the more resistant the molecule, the longer it will last in the cytoplasm and the greater quantity of protein synthesised
- binding of inhibitory proteins to mRNA prevents it binding to ribosomes and the synthesis of proteins
- activation of initiation factors - aid the binding of mRNA to ribosomes
Post translational control:
- addition of non-protein groups
- modifying amino acids and the formation of bonds
- folding/shortening proteins
- modification by cAMP
body plan definition
the general structure of an organism
proteins control the development of so that everything is in the right place
morphogenesis definition
the regulation of the pattern of anatomical development
why are fruit flies used in genetic studies
small
easy to keep
short life cycle
reproduce rapidly
mutations are easy to see under a low powered microscope
easy to raise in large numbers in a lab
Homeobox genes
DEFINITION - a group of genes which all contain a homeobox
homeobox:
definition - a section of DNA 180 base pairs long coding for a part of the protein 60 amino acids long that it highly conserved (similar) in plants, animals and fungi.
this section of DNA (homeodomain) binds to DNA and switches other genes on/off
homeobox genes are regulatory genes
homebox sequences code for the homeodomain which binds to specific sites on DNA, enabling the protein to work as a transcription factor
hox genes
definition - they are one group of homeobox genes that are only present in animals
responsible for the correct positioning of body parts
Hox genes are found in gene clusters - mammals have 4 clusters on different chromosomes
the order in which genes appear along the chromosome is the order in which their effects are expressed in an organism
code for proteins that control body plan development
somites
segments in the embryo
directed by hox genes to develop in a particular way depending on their position in the sequence
diploblastic
animals have 2 primary tissue layers
triploblastic
organisms have 3 primary tissue layers
radial symmetry
seen in diploblastic animals - they have no left or right side only a top and bottom
bilateral symmetry
seen in most animals
have a left and right side and top and bottom
Mitosis
results in cell division and proliferation
- increases the number of cells leading to growth
Apoptosis
programmed cell death
removes unwanted cells and tissues - shaped different body parts
cells undergoing apoptosis can also release chemical signals that stimulate mitosis and cell proliferation leading to the remodelling of tissues
during apoptosis the cell is brown down:
1) the enzymes inside the cell break down important cell components e.g proteins in the cytoplasm and DNA
2) as the cells contents are broken down, it begins to shrink and breaks up into fragments
3) the cell fragments are engulfed by phagocytes and digested
Factors affecting the expression of regulatory genes
can be influenced by the internal and external environment
stress - occurs when the homeostatic balance within an organism is upset
external factors:
- change in temp
- change in light intensity
- stress caused by a lack of nutrient availability - can result in gene expression that that prevents cells from undergoing mitosis
- attack by a pathogen - can lead to gene expression that results in apoptosis
Internal factors:
- release of hormones
- psychological stress
- DNA damage - if DNA damage is detected during the cell cycle this can result in the expression of genes which cause the cell cycle to be paused and trigger apoptosis
