Module 6: Genes and evolution Flashcards
Why do cells switch on and off genes?
Cells in developing organisms need to be able to differentiate and specialise for different roles. This must occur in a specific, tightly controlled sequence.
This sequence is coded by transcription factors.
What are transcription factors and what do they do?
Proteins that bind to specific DNA sequences in order to control the rate at which particular genes are transcribed into mRNA by turning various different genes on and off in the correct order.
Homeobox gene sequences in plants, animals and fungi …
Are similar and highly conserved, meaning they have been maintained by natural selection.
Explain why homeobox genes are important.
They form the basic pattern of the body, they control the polarity of the organism, they control the segmentation of organisms such as insects into distinct body parts and they control the development of body parts such as wings + organs.
In this way, homeobox genes can be seen as ‘master genes’ that control which genes function at different stages of development.
Define mutation.
Changes in the sequence of nucleotides in DNA molecules
Describe the 5 types of mutation.
Insertion/deletion - where one or more nucleotide pairs are inserted or
deleted from the sequence. This type of mutation alters the sequence of nucleotides
after the insertion/deletion point known as a frameshift.
Point mutation occurs when one base pair is replaced by another.
A nonsense mutation - where translation is stopped early thus giving rise to a
truncated polypeptide due to premature introduction of a stop codon.
A missense mutation is a codon change which results in the production of a different
amino acid, thus resulting in altered tertiary structure of the protein.
A silent mutation is a codon change which does not affect the amino acid sequence
produced. Silent mutations are possible due to the degenerate nature of the genetic
code.
How can mutations have neutral effects?
Mutation could occur in the non-coding region of DNA (where it doesn’t code for an amino acid) or it could be a silent mutation.
Also the change in structure of the tertiary protein may have no effect on the organism.
Give an example of a beneficial and harmful mutation.
Beneficial - humans developed trichromatic vision.
Harmful - mutation in the CFTR protein which causes cystic fibrosis.
Give 3 examples of how gene expression can be controlled.
Transcriptional, post-transcriptional and post-translational levels.
Explain a the function of a named transcriptional level control.
Lac operon - a length of DNA composed of structural genes and control sites which controls the expression of beta-galactosidase responsible for hydrolysis of lactose in E.coli.
The operon consists of a promoter region which is the binding site for RNA polymerase to initiate transcription, operator region where the inhibitor binds and structural genes which give rise to 3 products, beta galactosidase, lactose permease and another enzyme. The inhibitor protein is coded for by a regulator gene, located outside the operon.
Explain transcriptional level.
In a case where the concentration of glucose is high and the concentration of lactose is low, the transcription of the structural genes is inhibited due to binding of the repressor to the operator region. However, in a case where the concentration of glucose is low and concentration of lactose is high, lactose binds to the repressor thus causing the shape of its DNA binding site to change, therefore making it ineffective. This means that it can no longer bind to the operator region therefore RNA polymerase is able to bind to the promotor region and transcription of the structural genes takes place.
Post transcriptional level.
Editing of the primary mRNA transcript, during which the non-coding regions called introns are removed, thus creating a mature transcript consisting only of protein-producing regions known as exons.
Post translational level.
Proteins such as adrenaline can be activated with the help of cyclic AMP. This occurs when adrenaline
binds to a complementary receptor, which activates the enzyme adenylate cyclase which
converts ATP to cyclic AMP which starts a cascade of enzyme reactions within the cell,
thus activating the protein.
Explain what happens during cell apoptosis.
Apoptosis is a form of programmed cell death which can act as a mechanism to control the
development of body plans such as fingers and toes. It is an integral part of tissue
development in both plants and animals. It is an ordered, controlled series of biochemical
events leading to cell death. It is the opposite of necrosis, which is cell death resulting from
damage and release of hydrolytic enzymes. It is a means of controlling the number of cells
and ensuring that it remains constant to prevent cancer.
During the process, enzymes break down the cytoskeleton of the cell, DNA and proteins. As
the contents of the cell are broken down, the cell begins to shrink and break up.
Subsequently, the cell fragments are engulfed by phagocytes and destroyed.
Phenotypic variation.
There are two types of variation, discontinuous and continuous.
Discontinuous variation is used to describe variation which can be assigned to a particular category, for instance shoe size or blood type. Whereas continuous variation is a type of variation where the differences between phenotypes are quantitative, for instance height or weight. Variation can be influenced by both environmental factors such as diet in animals and etiolation and chlorosis in plants and genetic factors.
Recessive – the characteristic is only expressed if there is no dominant allele present
* Homozygous – two identical alleles
* Heterozygous – two different alleles
* Codominance – both alleles contribute to the phenotype
Linkage is the phenomenon where genes for different characteristics are located at
different loci on the same chromosome and so are inherited together.
Monogenic inheritance – when a phenotype or trait is controlled by a single gene. For
instance, cystic fibrosis where the individuals with doubly recessive genotype are affected.
Dihybrid cross – inheritance of two genes
Sex linkage – expression of an allele dependent on the gender of the individual as the gene
is located on a sex chromosome, for instance, males are more likely to inherit an X chromosome linked condition because they only have a single copy of the X chromosome. An
example of sex linkage is haemophilia which is a recessive condition (hh).
Autosomal linkage – genes which are located on the same chromosome (which is not a sex
chromosome) and tend to be expressed together in the offspring
Codominance – when both alleles are expressed in a heterozygote, that is, both alleles
contribute towards the phenotype. Examples include blood type.
Epistasis – the interaction of different loci on the gene, one gene locus affects the other gene
locus. One gene loci can either mask or suppress the expression of another gene locus.
Recessive epistasis occurs when the presence of a recessive allele prevents the expression
of another allele at a second locus. Recessive epistasis gives the ratio of 9:3:4.
Dominant epistasis is when a dominant allele at one locus completely masks the alleles at
a second locus. Dominant epistasis gives a ratio of 12:3:1.
*Allele – alternative form of a
gene
*Locus – the specific position of a
gene on a chromosome, the two
alleles of a gene are found at the
same loci on the chromosome
pairs
*Phenotype – observable
characteristics of an organism
which are as a result of genotype
and environment
*Genotype – the alleles present
within cells of an organism, for a
particular trait or characteristic
*Dominant – only a single allele
is required for the characteristic to
be expressed, that allele is always
expressed in the phenotype
