Cellular control Flashcards
what is gene expression?
the process by which specific genes are activated to produce a required protein
how is gene expression controlled?
not all genes in a cell are expressed- they are selectively switched on or off
because of this, different proteins are made and these proteins modify the cell- they determine the cell structure and control cell processes
gene expression can be controlled at the transcriptional, post-transcriptional and post-translational level
what happens at transcriptional level control?
the rate of transcription of genes is altered e.g. inc. transcription produces more mRNA which can be used to make more protein
this is controlled by transcription factors
factors that start transcription are called activators and those that stop transcription are called repressors
the shape of transcription factors determines whether it can bind to DNA or not and can sometimes be altered by the binding of some molecules e.g. certain hormones or sugars
this means the amount of some molecules can control the synthesis of some proteins by affecting transcription factor binding
in eukaryotes, transcription factors bind to specific DNA sites near the star of their target genes (histone modification)
in prokaryotes, control of gene expression often involves transcription factors binding to operons
What are transcription factors?
proteins that bind to DNA and control the rate of transcription by switching genes on and off
what is an operon?
a section of DNA that contains a cluster of structural genes that are all transcribed together, as well as control elements and sometimes a regulatory gene
the structural genes code for useful proteins such as enzymes
the control elements include a promoter- a DNA sequence located before the structural gene that RNA polymerase binds to
an operator- a DNA sequence that transcription factors bind to
the regulatory gene codes for an activator or repressor
What are introns and exons?
introns- sections of DNA that don’t code for amino acids
exons- sections of DNA that do code for amino acids
what happens at post-transcriptional level control?
control of gene expression can take place after genes have been transcribed
mRNA in eukaryotic cells is edited: that’s because genes in eukaryotic DNA contains introns and exons, they are both copied into mRNA
mRNA strands containing introns and exons are called primary mRNA transcripts
introns are removed from primary mRNA by the process called splicing
exons are joined together to form mature mRNA strands
this takes place in the nucleus and mature mRNA then leaves the nucleus for the next stage of protein synthesis (translation)
what happens at post-translational level control?
some proteins aren’t functional straight after they have been synthesised, they need to be activated to work
some molecules that control protein activation work by binding to cell membranes and triggering the production of cyclic AMP inside the cell
cAMP then activates proteins inside the cell by altering their 3D structure
e.g. altering 3D structure can change the active site of an enzyme making it become more or less active
what is a body plan?
the general structure of an organism
what are hox genes?
- Hox genes code for proteins that control body plan development
- similar genes are found in animals, plants and fungi, which means that body plan development is controlled in a similar way
what are homeobox sequences?
Homeobox sequences are regions within hox genes that have been highly conserved.
this means that these sequences have changed very little during the evolution of different organisms that possess these homeobox sequences
how do hox genes control development?
homeobox sequences code for a part of the protein called the homeodomain
the homeodomain binds to specific sites on DNA, enabling the protein to work as a transcription factor
the proteins bind to DNA at the start of developmental genes, activating or repressing transcription and so altering the production of proteins involved in the development of the body plan
what is apoptosis?
cells die and breakdown as a normal part of development
its a highly controlled process called apoptosis or programmed cell death
the cells is broken down in the following steps:
enzymes inside the cell breakdown cell components
the cell shrinks and begins to fragment
phagocytes engulf and digest the cell fragments
what is the role of mitosis and apoptosis in development?
mitosis and differentiation create the bulk of the body part and then apoptosis refines the parts by removing unwanted structures
e.g. when hands and feet first develop in humans the digits are connected. they only separate when cells in the connecting tissue undergo apoptosis
during development genes that control mitosis & genes that control apoptosis are switched on and off in appropriate cells
this means that some new cells are produced while some cells die and the correct body plan develops
what are responses of genes that regulate the cell cycle and apoptosis?
the genes that regulate progression through the cell cycle and apoptosis can respond to both internal and external stimuli
an internal stimulus could be DNA damage- if DNA damage is detected during the cell cycle, results in the expression of genes which cause the cycle to be paused and can even trigger apoptosis
an external stimulus such as stress caused by lack of nutrient availability- could result in gene expression that prevents cells from undergoing mitosis
