Voice of the Genome Flashcards
Cells are fundamental units of …
All living organisms
What are tissues?
Group of similar cells that act together to perform a similar function
Give examples of tissues
- Mesophyll - plants - capable of photosynthesis
- Muscular tissue - animals - can contract to bring about force and motion
- Epidermal tissue - animal + plant - human skin or waxy covering of some plants
What are organ systems?
groups of organs that work together to perform a particular role
Give examples of organ systems .
Liver - produces bile
Small intestine - digests food and absorbs soluble food molecules
Stomach - digest food
Glands
Large intestine - absorbs water molecules from the remaining undigested food - allows it to produce faeces
What is the level of organisation in multicellular organism in ascending order of complexity?
Organelle, cell, tissue , organ , organ system, organism
Give two examples of eukaryotic cells
Plant cells
Animal cells
Describe the nucleus
A large organelle surrounded by a nuclear envelope (double membrane), which contains many pores.
The nucleus contains chromatin (which is made from DNA and proteins) and a structure called the nucleolus.
What is the function of a nucleus ?
The nucleus controls the cell’s activities (by controlling the transcription of DNA
DNA contains instructions to make proteins
The nucleus controls the cell’s activities (by controlling the transcription of DNA
DNA contains instructions to make proteins
Describe the lysosome?
A round organelle surrounded by a membrane, with no clear internal structure.
What is the function of lysosome?
Contains digestive enzymes. These are kept separate from the cytoplasm by the surrounding membrane
can be used to digest invading cells or to break down worn out components of the cell.
Describe ribosomes
A very small organelle that either floats free in the cytoplasm or is attached to the rough endoplasmic reticulum.
It’s made up of proteins and RNA.
It’s not surrounded by a membrane.
What is the functions of ribosomes?
The site where proteins are made.
Describe the Rough Endoplasmic Reticulum
A system of membranes enclosing a fluid-filled space. The surface is covered with ribosomes
What is the function of the Rough Endoplasmic Reticulum?
Folds and processes proteins that have been made at the ribosomes.
Describe the Smooth Endoplasmic Reticulum
A system of membranes enclosing a fluid-filled space. No ribosomes
What is the function of the Smooth Endoplasmic Reticulum?
Synthesis and processes lipids
Describe the Golgi Apparatus
A group of fluid-filled, membrane-bound, flattened sacs. Vesicles are often seen at the edges of the sacs.
What is the function of the Golgi apparatus ?
It processes and packages new lipids and proteins. It also makes lysosomes.
Describe the mitochondrian
They’re usually oval-shaped. They have a double membrane — the inner one is folded to form structures called cristae. Inside is the matrix, which contains enzymes involved in respiration.
What is the function of the mitochondria?
The site of aerobic respiration, where ATP is produced. They’re found in large numbers in cells that are very active and require a lot of energy.
Describe a centriole
Small, hollow cylinders, made of microtubules (tiny protein cylinders). Found in animal cells, but only some plant cells.
What is the function of a centriole?
Involved with the separation of chromosomes during cell division
Prokaryotic cells are
Single-celled organisms (e.g. bacteria).
In prokaryotic cells, how do cells arise from other cells.
Cells arise from other cells by binary fission in prokaryotic cells
Prokaryotic cells contain no…
membrane-bound organelles
Give an example of a prokaryotic cell
Bacteria cells
What does the cytoplasm of prokaryotic cells have?
no membrane-bound organelles.
It has ribosomes — but they’re smaller than those in a eukaryotic cell.
What is the size of ribosomes in prokaryotes?
70s
What is the flagellum?
The flagellum is a long, hair-like structure that rotates to make the prokaryotic cell move. Not all prokaryotes have a flagellum. Some have more than one.
Unlike a eukaryotic cell, a prokaryotic cell doesn’t have a nucleus, so….
The DNA floats free in the cytoplasm. It’s circular DNA, present as one long coiled-up strand. It’s not attached to any histone proteins
What are Plasmids ?
Plasmids are small loops of DNA that aren’t part of the main circular DNA molecule. Plasmids contain genes for things like antibiotic resistance, and can be passed between prokaryotes. Plasmids are not always present in prokaryotic cells. Some prokaryotic cells have several.
What are mesosomes?
Mesosomes are inward folds in the plasma membrane.
What is a capsule?
made up of secreted slime. It helps to protect bacteria from attack by cells of the immune system.
What is a pilli?
Short hair-like structures called pili. Pili help prokaryotes stick to other cells and can be used in the transfer of genetic material between cells.
What is the role of the cell wall and what is it made of?
The cell wall supports the cell and prevents it from changing shape.
It’s made of a polymer called murein. Murein is a glycoprotein (a protein with a carbohydrate attached).
Role of the plasma membrane is
To controls the movement of substances into and out of the cell.
Where are proteins made?
At the ribosomes .
What is the primary site of protein synthesis?
The rough Endoplasmic reticulum
What do the Endoplasmic reticulum membrane do?
It keeps the newly synthesised proteins separated from proteins produced by free ribosomes in the cytoplasm
How are proteins dispatched and where to?
Proteins are dispatch within vesicles from specialised regions to the Golgi Apparatus
What do the membrane transport vesicles leaving the Golgi have attached to it?
Have external molecules attached as guidance - extracellular enzymes (digestive enzymes)
After leaving the Golgi what happens to the membrane bound vesicles?
It moves to the cellular membrane where they fuse with it to release digestive enzymes
What is the role of the rough ER in the production of macromolecules?
Ribosomes on it assemble the protein destined for secretion
What is the role of the smooth ER in the production of macromolecules?
Synthesis of lipids e.g. steroid hormones , phospholipids, packages them into transport vesicles
What is the role of Golgi Apparatus in the production of macromolecules?
Receive transport vesicles. Modifies, stores and transports molecules for export around or from cells
What is the role of transport vesicles in the production of macromolecules?
These buds off the ER and move substances to Golgi apparatus
Describe the process of protein production and transport.
- Proteins are made at the ribosomes.
- The ribosomes on the rough endoplasmic reticulum (ER) make proteins that are excreted or attached to the cell membrane. The free ribosomes in the cytoplasm make proteins that stay in the cytoplasm.
- New proteins produced at the rough ER are folded and processed (e.g. sugar chains are added) in the rough ER.
- Then they’re transported from the ER to the Golgi apparatus in vesicles.
- At the Golgi apparatus, the proteins may undergo further processing (e.g. sugar chains are trimmed or more are added).
- The proteins enter more vesicles to be transported around the cell.
E.g. extracellular enzymes (like digestive enzymes) move to the
cell surface and are secreted.
Suggest why polypeptides requiring transport are synthesised by membrane bound ribosomes rather than free
So that they can be easily threaded through the ER membrane into the cisternal space of the ER
Here they are in place for subsequent modification packaging and export
How do we work out magnification?
magnification =
size of image / size of real object
What is magnification?
is how much bigger the image is than the specimen
What is resolution?
is how detailed the image is. More specifically, it’s how well a microscope distinguishes between two points that are close together.
How do light microscopes work?
- They use light to form an image.
- They have a maximum resolution of about 0.2 (μm). This means you can’t use a light microscope to view organelles smaller than 0.2 μm. That includes ribosomes, the endoplasmic reticulum and lysosomes. You may be able to make out mitochondria — but not in perfect detail. You can also see the nucleus.
- The maximum useful magnification of a light microscope is about × 1500.
How do electron microscope work?
- They use electrons to form an image
- They have a higher resolution than light microscopes so give a more detailed image (and can be used to look at more organelles).
- They have a maximum resolution of about 0.0002 micrometres (μm).
- The maximum useful magnification of an electron microscope is about × 1 500 000.
Mitosis is cell division that produces …
Genetically Identical Cells
In mitosis a parent cell divides to produce …
two genetically identical daughter cells
What is mitosis needed for ?
growth of multicellular organisms making sure its genetically consistent
repairing damaged tissues
asexual reproduction
The cell cycle consists periods of …
Interphase and mitotic division
What are the 3 phases of interphase ?
G1. S. G2
What happens in G1 and its internal checkpoint?
Cells grows and new organelles and proteins are made
DNA is checked to make sure there is no damage + the cells is checked to see if it the correct size and has enough energy reserve
What happens in S?
DNA synthesis
What happens in G2 and its internal checkpoint?
- Cells keeps growing and proteins needed for cell division are made
- Chromosomes are checked to make sure that they have all been replicated + none of the duplicated DNA is faulty
How are chromosomes affected during interphase?
The individual chromosomes are unravelled to access genetic material enabling new proteins to be synthesised
Describe a centriole and its function
- cylindrical shape
* produces spindle fibres
Describe what happens in Interphase
The cell’s DNA is unravelled and replicated, to double its genetic content. The organelles are also replicated so it has spare ones, and its ATP content is increased (ATP provides the energy needed for cell division)
Describe what happens in prophase
- Chromosomes condense and becomes more visible
- Nuclear membrane and nucleoulus begins to disintegrate
- Centrioles move up to opposite poles of the cells
- Spindle fibres form
Describe what happens in Metaphase
- Chromosomes line up along the equator
* Spindle fibres attach to centromeres of the chromosomes
Describe what happens in Anaphase
- Centromeres divide
- Spindle fibres attached to chromotids contract, moving to opposite poles of the cell
- Other spindle fibres lengthens pushing the poles apart + causing cells to elongate
Describe what happens in Telophase
- two nuclei form
- spindle fibres break down
- chromosomes decondense
Describe what happens in Cytokinesis
Cell surface membranes constricts around the centre of the cell wall forming two daughter cells
How can we observe Mitosis ?
Root Tips Can be Stained and Squashed
What is mitotic index?
the Proportion of Cells Undergoing Mitosis
How do you calculate mitotic index?
mitotic index = number of cells with visible chromosomes / total number of cells observed
When a plant root tip is constantly growing what can you expect ?
It to have a high mitotic index
Describe how you would carry out an experiment to observe mitosis in a root tip
- Cut 1 cm from the tip from a growing root (e.g. of an onion). It needs to be the tip because that’s where growth occurs (and so that’s where mitosis takes place).
- Prepare a boiling tube containing 1 M hydrochloric acid and put it in a water bath at 60 ̊C.
- Transfer the root tip into the boiling tube and incubate for about 5 minutes.
- Use a pipette to rinse the root tip well with cold water. Leave the tip to dry on a paper towel.
- Place the root tip on a microscope slide and cut 2 mm from the very tip of it. Get rid of the rest.
- Use a mounted needle to break the tip open and spread the cells out thinly.
- Add a small drop of stain and leave it for a few minutes. The stain will make the chromosomes easier to see under a microscope. toluidine blue
- Place a cover slip over the cells and push down firmly to squash the tissue. This will make the tissue thinner and allow light to pass through it. Don’t smear the cover slip sideways (or you’ll damage the chromosomes).
- Now you can look at all the stages of mitosis under a light microscope.
Name one way in which mitosis and meiosis are alike
In both, DNA is replicated before division
Meiosis produces,
Gametes for the purpose of sexual reproduction
What is the staring and end product of meiosis ?
4 haploid daughter cells formed from parent cells in 2 cycles of division
Describe what happens in preparation of meiosis
DNA uncondenses and replicates there are two identical copies of each chromosome, called chromatids.
The aim of meiosis I is to
Separate the homologous chromosomes into two intermediate cells
The aim of meiosis II is to
Pull the two chromotids apart but he the number of chromosomes remains the same
Describe the process of meiosis I
- DNA condenses and forms double armed chromosomes , joined by by centromeres
- chromosomes line up along the equator
- chromosomes have halved in number
Decribe the process of meiosis II
- The pairs of sister chromatids are separated.
* Four new daughter cells that are genetically different from each other are produced. These are the gametes.
How does the process of meiosis produce genetically varied gametes?
Crossing over
Independent assortment
How does crossing over work?
- The chromosomes of a homologous pair are arranged down the middle of the cell before the first cell division in meiosis I.
- Sections of DNA are swapped between the chromosomes in a pair
- Crossing over produces new combinations of alleles.
How does independent assortment work?
- The chromosomes in a homologous pair are separated in meiosis I in a random formation to produce two genetically different daughter cells.
- The combination of chromosomes in the two daughter cells is random.
- The process of separating chromosomes into random combinations is called independent assortments
- Independent segregation increases genetic variation.
Explain how the process of crossing over is going to alter the genotype of the gametes
Unexpected combinations of alleles for genes will occur that would not normally be present in gametes
Describe the importance of crossing over in the process of evolution
Crossing over provides one source of genetic variation amongst individuals in a population. This is important for providing the raw material upon which natural selection acts
What is a locus ?
is the location of genes on a chromosome.
The 1st-22nd pairs of chromosomes are
Homologous pairs
What are autosomes?
Chromosomes not concerned with sex determination
What is meant by genes being linked?
Any genes which are located on the same chromosomes
What is autosomal linkage ?
When two genes are located on the same autosomes and are likely to be inherited together
Why are autosomal linked genes more likely to be inherited together?
Because they are less likely to be separated during crossing over
What does it mean when there is a autosomal link between two genes?
There are only two combinations of alleles in the gametes
What are the sex chromosomes?
The X + Y chromosomes
Sex chromosomes are the only pair which are
Not fully homologous
Males are more likely to express a recessive allele on…
The X chromosomes in their phenotype
Haemophilia is a …
X linked recessive disorder which causes slow blood clotting and thus persistent bleeding
The dominant allele in haemophilia encodes for a?
Functioning factor 8 protein
The recessive allele in haemophilia encodes for a?
Non functioning factor 8 protein
If a male inherits one copy of the recessive allele for haemophilia….
They will have haemophilia because there is no dominant allele present on the Y chromosome
What are gametes?
The male and female sex cells
What is the joining of two gametes known as?
Fertilisation
What is the name of the male and female gametes?
Sperm cell
Egg cell / ova
How many chromosomes do humans have?
2 sets of 23 chromosomes
How much chromosomes does a gamete have?
23 - haploid cell
What is fertilisation ?
The exact moment when the nuclei of the sperm and egg cells fuse together to form a zygote.
What makes the offspring unique?
Combining genetic material from individuals
What structures make egg cells specialised?
Plasma membrane
Follicle form protecting coating
Zona Pellucida - Protective glycoprotein layer that sperms have to penetrate
Haploid Nucleus
What structures make sperm cells specialised?
Flagellum - swim towards egg cell
Acrosome - digestive enzymes to break down the egg cell’s zona pellucida + penetrate egg
Cell (plasma) membrane
Lots of mitochondria - respiration releases energy in the form ATP
Explain why sperms need to be motile ?
Once deposited in the vagina, sperm must be able to move through the uterus and into the oviducts to reach and fertilise the ovum. Sperm only live for about 48hrs so they must be able to move quickly to reach the ovum to attempt fertilisation before they die
Explain why a mature ovum needs to be so many times larger than a sperm?
A fertilised ovum must be able to sustain itself for 6-12 days as it travels down the oviduct and implants in the uterus. In able to do this, it contains its own nutrients and metabolites. The inclusion of these make it a much larger cell than sperm cells
Explain why the sperm cell has a large number of mitochondria?
The sperm is a motile cells, requiring propulsion to reach the ovum in order to fertilise it. Energy for movement ATP is generated in the large number of mitochondria present in the sperm cell
In mammals, where does fertilisation occur?
In the oviduct
Describe the pathway of the sperm cell once it reaches the oviduct. (7)
- The Sperm swims towards the egg cell in the oviduct
- Once the sperm makes contact with the zona pellucida of the egg cell, The acrosome reaction occurs - digestive enzymes are released via the sperm cells
- These enzymes digest the zona pellucida , so that the sperm can move through it to the cell membrane of the egg cell.
- The sperm head fuses with the cell membrane of the egg cell. This triggers the cortical reaction — the egg cell releases the contents of vesicles called cortical granules into the space between the cell membrane and the zona pellucida.
- The chemicals from the cortical granules make the zona pellucida thicken, which makes it impenetrable to other sperm. This makes sure that only one sperm fertilises the egg cell.
- Only the sperm nucleus enters the egg cell — its tail is discarded.
- The nucleus of the sperm fuses with the nucleus of the egg cell — this is fertilisation.
After a zygote is formed what happens ?
It immediately begins to divide via mitosis
Describe the process of fertilisation in mammals, following the acrosome reaction.
- The sperm head fuses with the cell membrane of the egg cell
- This triggers the cortical reaction, where the contents of the cortical granules are released from the egg cell
- The chemicals from the cortical granules make the zona pellucida thick and impenetrable to other sperm
- The sperm nucleus enters the egg cell and fuses with the egg cell nucleus — this is fertilisation
Explain the significance of the blocks that prevent entry of more than one sperm into the egg
It is necessary to prevent fertilisation of theft by more than one sperm because this would result in too many chromosomes in the zygote
Describe the significant event of occurring at capacitation of fertilisation
Changes in the surface of the sperm cell essential to enabling the acrosome reaction + sperm entry
Describe the significant event of occurring at the acrosome reaction of fertilisation
The release of enzymes from the acrosomes at the head of the sperm
These enzymes digests a pathway through the follicle cells and the zona pellucida
Describe the significant event of occurring at the fusion of the egg and sperm membranes fertilisation
Enables the sperm nucleus to enter the egg
The fusion causes a sudden depolarisation of the membrane that forms a fast block to prevent further entry of sperm cells
Describe the significant event of occurring at the cortical reaction of fertilisation
A permanent change in the egg surface that provides a slow permanent block to sperm entry
Involves the release of cortical granules which harden the egg outer layer
Describe the significant event of occurring at the fusion of egg and sperm nuclei in fertilisation
The fusion of the nuclei forms the diploid zygote and initiates the rapid cell division that follows fertilisation
What are stem cells ?
Undifferentiated cells found in multicellular organisms
What are the two type of stem cells ?
Adult + embryonic
Stem cells are characterised by ….
Self renewal
Potency
Adult stem cells are …
Multi potent - limited to certain types
Embryonic stem cells are
Totipotent - differentiated into any types of cell
Pluripotent - the ability of a stem cell to produce all the specialised cells in an organism
What is the process by which a cell becomes specialised?
Differentiation
What makes a stem cell?
Only has certain genes turned on
Becomes differentiated due to certain genes being turned on
Totipotent stem cells are only present in mammals in the…
The first few cell divisions of an embryo.
Stem Cells Become Specialised Through
Differential Gene Expression
Since Stem cells can develop into any specialised cell type….
It could be used to replace damaged tissues in a range of diseases.
What are the potential benefits of stem cell therapies?
They could save many lives — e.g. many people waiting for organ transplants die before a donor organ becomes available. Stem cells could be used to grow organs for those people awaiting transplants.
• They could improve the quality of life for many people — e.g. stem cells could be used to replace damaged cells in the eyes of people who are blind
Embryonic stem cells are
obtained from early embryos.
Embryos are created in a laboratory using in vitro fertilisation (IVF) — egg cells are fertilised by sperm outside the womb.
Once the embryos are approximately 4 to 5 days old, stem cells are removed from them and the rest of the embryo is destroyed.
Embryonic stem cells can develop into
all types of specialised cells.
Adult stem cells are
These are obtained from the body tissues of an adult. For example, adult stem cells are found in bone marrow
They can be obtained in a relatively simple operation — with very little risk involved, but quite a lot of discomfort. The donor is anaesthetised, a needle is inserted into the centre of a bone (usually the hip) and a small quantity of bone marrow is removed.
Adult stem cells aren’t as flexible as embryonic stem cells — they can only develop into a limited range of cells.
However, if a patient needs a stem cell transplant and their own adult stem cells can be used (from elsewhere in their body) there’s less risk of rejection.
Why does obtaining stem cells from embryos created by IVF raise ethical issues?
because the procedure results in the destruction of an embryo that’s viable (could become a fetus if placed in a womb).
A stem cell is ________________ and ___genes are on or expressed
A stem cell is undifferentiated and all genes are on or expressed
A differentiated cell has some genes ___or ___________ and some that a not.
A differentiated cell has some genes on or expressed and some that are not.
What is specialisation?
Gene switching or differential gene expression
What is produced when a gene is read?
mRNA strand instigating transciption
What is the promoter region?
Sequence of DNA 25-30 bp up from gene interested.
What is the TATA box?
A specific sequence of bases that has been identified that forms part of the promoter region.
Why is the promoter region important?
Provides binding site for protein which help RNA polymerase bind to DNA.
What is a transcription factor?
Protein that binds to promoter region
Where does RNA polymerase bind?
To an initiation site at the start of a gene with the help of transcription factor
What are enhancers?
the regions along the DNA that help or enhance the bindIng of transcription known as
What binds to enhancers?
Proteins known as activators
What happens when the activators binds?
The shape of dna changes - it bends
What does the activator bind with?
Mediator complex
When is it easier for RNA polymerase to lock into place?
Once the transcription factors have bound.
What is gene expression?
Switching genes on or off (controlled process)
What is the regulation of genes?
Increase in expression or decrease (Controlled process)
a cell even a _________ cell will not have all the genes it needs being expressed all the time so genes will be switched __ or not read.
a cell even a differentiated cell will not have all the genes it needs being expressed all the time so genes will be switched off or not read.
What is the transcription factor that helps RNA polymerase bind?
Basal transcription factor
What is example of transcription factors?
Activators that bind to enhancers
What are repressors?
Transcription factor that block the basal transcription factors and RNA polymerase binding to the promoter of DNA.
Where do repressors bind?
Region called silencer
Break down the word lac operon
Lac = lactose. Operon= set of genes with one promotor
What is Lac operon?
A set of genes with one promoter that encodes genes for the transport and metabolism of lactose in Ecoli + other bacteria
When is lac operon of ecoli expressed?
Only when lactose is present and glucose is absent.
How many genes are found in bacteria, E.coli?
3
What is theIr (bacteria) regulation / expression dependant on?
Presence of lactose
Absence of glucose
What does the proteins produced when the 3 genes are expressed help with?
Help breakdown Lactose or help with its uptake into the cell.
What do each gene in the operon result in?
Leads to the production of an enzyme that splits lactose into simple sugars -used as a fuel source for glycolysis
Encodes a membrane bound transporter that helps lactose get into the cell
What sits next to the promoter?
Sequence of bases - some overlap with the sequence of the promotors
Why is the operator important ?
The repressor will bind to the operator. It binds in a way that it blocks the promoter so the RNA polymerase can not bind.
What are the two regulators that turn the operator “on” and “off” in response to lactose and glucose levels?
The lac repressor and catabolite activator protein (CAP)
What is a lac repressor?
A protein that inhibits transcription of the lac operon
What does the lac repressor act as?
A lactose sensor.
What does a lac repressor do?
Blocks transcription of operon
When does the lac repressor stop working?
When lactose is present
How does the lac repressor sense the lactose indirectly?
Through its isomer allolactose
What is allolactose?
Rearanged version - isomer
What does allolactose bind to and what happens?
Binds to repressor and conformational changes occur.
For RNA polymerase to bind what is need?
The help of another protein = CAP (helps it bind to the promoter in ecoli)
What doe CAP act as?
Acts as a glucose sensor
What does CAP do?
Activates transcription of the operon
When does CAP work?
When glucoses levels are low
How does the CAP sense glucose indirectly?
Through the “hunger signal” molecule cAMP.
Describe what happens in lac operon in the presence of glucose and absence of lactose?
No transcription of lac operon
cAMP level is low, as glucose is present so CAP remains inactive and can’t bind to DNA
Lac operon remains bound to operator - prevents transcription via RNA polymerase
Describe what happens in lac operon in the presence of glucose and lactose?
Low level transcription of lac operon
cAMP level is low, as glucose is present so CAP remains inactive and can’t bind to DNA
Lac repressor is released from operator therefore allolactose is present
Describe what happens in lac operon in the absence of glucose and absence of lactose?
No transcription of lac operon
cAMP levels high, as glucose is absent. So CAP is active and bind to DNA
Lac repressor will also bind to operator (no allolactose) acting as a roadblock to RNA polymerase
Describe what happens in lac operon in the absence of glucose and presence of lactose?
Strong transcription of lac operon
cAMP levels high, as glucose is absent. So CAP is active and bind to DNA
Lac repressor released from operator (allolactose). Helps RNA polymerase bind to promoter
Changes in the environment can cause changes in ….
Gene expression
In eukaryotes, what can epigenetic control determine?
In eukaryotes, epigenetic control can determine whether certain genes are expressed, altering the phenotype.
How does Epigenetic control work?
It works by attaching or removing chemical groups to or from the DNA.
This alters how easy it is for the enzymes and other proteins needed for transcription to interact with and transcribe genes.
How does Epigenetic changes to gene expression play a role in lots of normal cellular processes?
They can also occur in response to changes in the environment — e.g. pollution and availability of food.
What are the two examples of epibenthic control?
Increased Methylation of DNA Represses a Gene
Modification of Histones
What is meant by methylation of DNA?
this is when a methyl group is attached to the DNA coding for a gene
Where does a methyl group always attach?
At a CpG site, which is where a
cytosine and guanine base are next to each other in the DNA.
How does increased methylation of DNA repress a gene?
Increased methylation changes the DNA structure, so that the proteins and enzymes needed for transcription can’t bind to the gene — so the gene is not expressed (i.e. it’s repressed or inactivated).
What are histones?
Histones are proteins that DNA wraps around to form chromatin, which makes up chromosomes.
Chromatin can be highly condensed or less condensed. How does its ability to condense affects the process of histone modification.
How condensed it is affects the accessibility of the DNA and whether or not the proteins and enzymes needed for transcription can bind to it.
What does Epigenetic modifications to histones include?
the addition or removal of acetyl groups
What happens when histones are acetylated?
When histones are acetylated, the chromatin is less condensed.
This means that the proteins involved in transcription can bind to the DNA, allowing genes to be transcribed (i.e. the genes are activated)
What happens when acetyl groups are removes from the histones?
When acetyl groups are removed from the histones, the chromatin becomes highly condensed and genes in the DNA can’t be transcribed because the transcription proteins can’t bind to them — the genes are repressed.
Can Epigenetic Changes Can be Passed On After Cell Division?
Yes
When a cell divides and replicates, epigenetic changes to its gene expression may be passed on to the resulting daughter cells. How in terms of removal of methyl group?
Methyl groups are usually removed from DNA during the production of gametes, but some escape the removal process and end up in the sperm or egg cells.
If epigenetic changes get passed on it would result in ….
certain genes that are activated or deactivated in the original cell will also be activated or deactivated in the daughter cells.
Variations in phenotypes can be
Continuous or discontinuous
What is continuous variation?
This is when the individuals in a population vary within a range — there are no distinct categories
Give examples of continuous variations
Height — you could be any height within a range.
Mass — you could be any mass within a range.
Skin colour — any shade from very dark to very pale.
What is discontinuous variation ?
This is when there are two or more distinct categories — each individual falls into only one
Give an example of discontinuous variation.
Blood group — you can be group A, group B, group AB or group O, but nothing else.
Variations in phenotype is influenced by
Variation of genotype
What makes individuals of the same species vary ?
Them having different genotypes
What is meant by the term monogenic?
The characteristics are controlled by only one gene - tend to show discontinuous variation
What is meant by term polygenic?
characteristics are controlled by a number of genes at different loci - show continuous variation
What are some examples of how the environment interacts with organisms genotype to produce its phenotype?
Height
MAOA
Cancer
Animal hair colour
How does height impact how the environment interacts with an organisms genotype to produce its phenotype?
Height is polygenic and affected by environmental factors, especially nutrition. E.g. tall parents usually have tall children, but if the children are undernourished they won’t grow to their maximum height (because protein is required for growth).
How does MAOA impact how the environment interacts with an organisms genotype to produce its phenotype?
Monoamine Oxidase A (MAOA) is an enzyme that breaks down monoamines (a type of chemical) in humans. Low levels of MAOA have been linked to mental health problems. MAOA production is controlled by a single gene (it’s monogenic), but taking anti-depressants or smoking tobacco can reduce the amount produced.
How does cancer impact how the environment interacts with an organisms genotype to produce its phenotype?
Cancer is the uncontrolled division of cells that leads to lumps of cells (tumours) forming. The risk of developing some cancers is affected by genes, but environmental factors such as diet can also influence the risk.
How does animal hair colour impact how the environment interacts with an organisms genotype to produce its phenotype?
Animal hair colour is polygenic, but the environment also plays a part in some animals. E.g. some arctic animals have dark hair in summer but white hair in winter. Environmental factors like decreasing temperature trigger this change but it couldn’t happen if the animal didn’t have the genes for it
