How many main phases are there in the cell cycle? What are they called?
Two, interphase and mitotic (division) phase.
What is interphase?
Long periods of growth and normal working separate divisions. Cells are not dividing continuously.
What does a cell do whilst it is in interphase?
Producing enzymes or hormones and actively preparing for cell division.
How does a cell actively prepare for cell division during interphase?
- DNA is replicated and checked for errors
- protein synthesis occurs in the cytoplasm
- mitochondria grow and divide, increasing in number in the cytoplasm
- chloroplasts grow and divide in plant and algal cells, increasing in number in the cytoplasm
- the normal metabolic processes of cells occur
What are the three stages of interphase?
Growth 1 (G1), Synthesis (S), Growth 2 (G2).
During the first stage of interphase, G1, what happens?
Proteins, from which organelles are synthesised, are produced and organelles replicate. The cell increses in size.
During the second stage of interphase, S, what happens?
DNA is replicated in the nucleus.
During the third stage of interphase, G2, what happens?
The cell continues to increase in size, energy stores are increased and the duplicated DNA is checked for errors.
What is the longest stage of the cell cycle?
Is interphase a stage in cell division?
No. It is the stage between cell divisions.
How many stages does cell division involve?
Two, mitosis and cytokinesis.
What happens during mitotis?
The cells nucleus divides
What happens during cytokinesis?
The cytoplasm divides and two cells are produced.
What is G0?
The phase when the cell leaves the cycle, either temporarily or permanently.
Explain the two reasons why a cell may leave the cycle.
- Differentiation- A cell the becomes specialised to carry out a particular function is no longer able to divide. It will carry out this function indefinitely and not enter the cell cycle again.
- The DNA of a cell may be damaged, in which case it is no longer viable. A damaged cell can no longer divide and enters a period of permanent cell arrest.
What does the term senescent mean?
It is a term used to dscribe a cell once it has divided a limited number of times.
Research has shown that as you age the number of senescent cells in your body increases. This has been found to be linked to...
...many age related diseases, such as cancer and arthritis.
Is it possible for any cells, once they enter G0, to start dividing again?
A few types of cells can be stimulated to go back into the cell cycle and start dividing again, for example white blood cells in an immune response.
What are three things that must be ensured before the cell divides? Why?
- that it has grown to the right side
- that the replicated DNA is error free
- that the chromosomes are in their correct positions
This ensures the fidelity of cell division- that two identical daughter cells are created from the parent cell
What are checkpoints? What are their role?
Control mechanisms of the cell cycle.
They monitor and verify whether the processes at each phase of the cell cycle have been accurately completed before the cell is allowed to progress into the next phase.
Name the three main checkpoints of the cell cycle.
G1 checkpoint, G2 checkpoint and the spindle assembly checkpoint.
Where does the G1 checkpoint occur? What does it do? If it is sucessful, what happens?
It is located at the end of the G1 phase, before entry into S phase. It checks for:
- Cell size
- growth factors
- DNA damage
If it satisfies the requirements, DNA replication begins. If not, it enters a resting state.
Where does the G2 checkpoint occur? What does it do? If it is sucessful, what happens?
It is located at the end of G2 phase before the start of the mitotic phase.
It checks for:
- cell size
- DNA replication
- DNA damage
If the checkpoint is passed, the cell initiates the molecular processes that signal the beginning of mitosis.
Where does the spindle assembly checkpoint occur? What does it do? If it is sucessful, what happens?
It is located in the mitosis phase of the cell cycle.
It checks for:
- chromosome attachment to spindle
If it is not passed, mitosis cannot proceed.
Mitosis is the term usually used to describe the entire process of cell division in eukaryotic cells. Is this accurate?
No. It actually refers to nuclear division.
What does mitosis ensure?
Mitosis ensures that both daughter cells produced when a parent cell divides are genetically identical.
In mitosis, how does the genetic information in one of the daughter cells compare to the genetic information in the parent cell?
Each new cell will have an exact copy of the DNA present in the parent cell and the same number of chromosomes.
Name some processes in the body that rely on mitosis to produce identical daughter cells.
- replacement of tissues
- repair of tissues
True or false? Mitosis is necessary for asexual reproduction
What is asexual reproduction?
The production of genetically identical offspring from one parent in multicellular organisms including plants, fungi, eukaryotic single-celled organisms and some animals.
How to prokaryotic organisms, including bacteria, reproduce?
They do not have a nucleus and they reproduce asexually by a different process known as binary fission.
What needs to happen before mitosis can occur?
All of the DNA in the nucleus must be replicated during interphase. Each DNA molecule (chromosome) is converted into two identical DNA molecules, called chromatids.
Explain the purpose of the centromere.
The two chromatids are joined together at the centromere. It is necessary to keep the chromatids together during mitosis so that they can be precisely manoeuvred and segregated equally, one each into the two new daughter cells.
How and when is chromatin formed?
Chromatin is formed during interphase. It is formed when DNA combines with proteins called histones to form a dense complex called chromatin.
What are the four stages of mitosis?
How can you view and identify the stages of mitosis?
Using a light microscope
Describe how dividing cells can be viewed under the microscope.
- Growing root tips of plants
- Treat with a chemical to separate the cells
- Squash on a microscope slide to form a single layer of cells
- Add stain to allow chromosomes to be clearly seen
In mitosis, what happens during prophase?
- Chromatin fibres begin to coil and condense to form chromosomes
- The nucleolus disappears
- The nuclear membrane begins to break down
- Spindle fibres form
- Two centrioles migrate to opposite poles of the cell
- Spindle fibres attach to specific areas on the centromeres
In mitosis, what happens during metaphase?
The chromosomes are moved by the spinde fibres to form a plane in the centre of the cell, called the metaphase plate, and held in position.
In mitosis, what happens during anaphase?
- the centromeres holding together the pairs of chromatids divide
- The chromatids are separated, pulled to opposite poles of the cell by the shortening spindle fibres
Why, during anaphase of mitosis, do the chromatids form a characteristic 'V' shape?
The characteristic 'V' shape of the chromatids moving towards the poles is a result of them being dragged by their centomeres through the liquid cytosol.
In mitosis, what happens during telophase?
- The chromatids have reached the poles and are now called chromosomes
- The nuclear envelope reforms around the two new sets of chromosomes
- Chromosomes start to uncoil
- Nucleolus is formed
- Cytokinesis begins
What is cytokinesis?
The actual division of the cell into two separate cells
What happens to an animal cell during cytokinesis?
A cleavage furrow forms around the middle of the cell. The cell-surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming two cells
What happens to a plant cell during cytokinesis?
Vesicles from the Golgi apparatus begin to assemble in the same place as where the metaphase plate was formed. The vesicles fuse with each other and the cell surface membrane, dividing the cell into two. New sections of cell wall then form along the new sections of membrane.
Why can't a plant cell form a cleavage furrow during cytokinesis?
Because plant cells have cell walls
What would happen if the dividing cell wall were formed before the daughter cells separated during the cytokinesis of a plant cell?
The cells would undergo osmotic lysis from the surrounding water.
Normal cells are diploid. What does this mean?
They have two chromosomes of each type, one inherited from each parent.
Fill in the gaps. During mitosis, the _______ divides _______ following DNA _______. This results in _______ genetically identical _______ daughter cells.
During mitosis, the nucleus divides once following DNA replication. This results in two genetically identical diploid daughter cells.
Explain why gametes must only contain half of the standard number of chromosomes in a cell.
In sexual reproduction two gametes, one from each parent, fuse to produce a fertilised egg. The zygote is the origin of all the cells that the organism develops. Gametes must therefore only contain half of the standard (diploid) number of chromosomes in a cell or the chromosome number of an organism woud double with every round of reproduction.
What process forms gametes?
How many divisions are there in meiosis?
Two, to produce four daughter cells
Each gamete is haploid. What does this mean?
Each gamete contains half of the chromosome number of the parent cell.
What is meant by the term reduction division?
That every gamete contains half of the chromosome number of the parent cell.
How is each characteristic of an organism coded?
By two copies of each gene, one form each parent.
Explain what is meant by the term homologous chromosomes?
Matching sets of chromosomes in the nucleus. Each nucleus of the organism's cells contains two full sets of genes, a pair of genes for each characteristic.
True or false? Each chromosome in a homologous pair has the same genes at the same loci.
Explain what is meant by the term allele.
An allele is a version of the same gene.
Why do some people have blue eyes whilst others have brown?
Genes for a particular characteristic may vary, leading to differences in the characteristic. The genes are the same in both the person with brown eyes and the person with blue as they both code for eye colour but the colour is different which means they have different alleles, versions of the same gene.
True or false? The different alleles of a gene will all have the same locus
What is the meaning of the word locus?
Why will homologous chromosomes be the same length and size when they are visible in prophase? What else will be the same?
They have the same number and types of genes. The centromeres will also be in the same positions.
In meiosis, what is meiosis 1?
The first division. It is the reductionist division when the pairs of homologous chromosomes are separated into two cells. Each intermediate cell will only contain one full set of genes instead of two, so the cells are haploid.
In meiosis, what is meiosis 2?
The second half of the division which is similar to mitosis. The pairs of chromatids present in each daughter cell are separated, forming two more cells. Four haploid daughter cells are produced in total.
Briefly explain what happens in meiosis 1 and meiosis 2.
During meiosis 1, what happens in prophase 1?
- chromosomes condense
- nuclear envelope disintegrates
- nucleolus disappeares
- spindle formation begins
- homologous chromosomes pair up to form bivalents
- crossing over occurs
During meiosis 1, what happens in metaphase 1?
The homologous pairs of chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate, and held in position
During meiosis 1, what happens in anaphase 1?
- Homologous chromosomes are pulled to opposite poles and chromstids stay joined to each other
- Sections of DNA from crossing over break off and rejoin
During meiosis 1, what happens in telophase 1?
The chromosomes assemble at each pole and the nuclear membrane reforms. Chromosomes uncoil.
The cell undergoes cytokinesis.
In meiosis 2, what happens in prophase 2?
The chromosomes (consisting of two chromatids) condense and become visible again. The nuclear envelope breaks down and spindle formation begins.
In meiosis 2, what happens in metaphase 2?
The individual chromosomes assemble on the metaphase plate
In meiosis 2, what happens in anaphase 2?
The chromatids of the individual chromosomes are pulled to opposite poles after division of the centromeres.
In meiosis 2, what happens in telophase 2?
The chromatids assemble at the poles. The chromosomes uncoil and form chromatin again. The nuclear envelope reforms and the nucleolus becomes visible.
Explain the term crossing over and how it results in genetic variation.
What is independent assortment?
What is the meaning of the word multicellular?
Made up of hundreds, thousands or millions of cells.
Fill in the blanks. Different cells within an organism are _________ for different roles and _________ into efficient biological _________, each with a particular _________.
Different cells within an organism are specialised for different roles and organised into efficient biological structures, each with a particular function.
The organisation of a multicellular organism can be summarised as...
specialised cells → tissue → organ → organ system → organism
What is the meaning of the term differentiated?
Specialised to carry out very specific functions.
How are erythrocytes specialised to carry out their function?
- flattened biconcave shape- increases their surface area to volume ratio which is essential to their role of transporting oxygen around the body
- no nuclei or other organelles- increases space available for haemoglobin
- flexible- able to squeeze through narrow capillaries
What are the dimentions of a red blood cell?
How are neutrophils (a type of white blood cell) specialised to carry out their function?
- play an important role in the immune system
- multi-lobed nucleus- makes it easier for them to squeeze through small gaps to get to the site of infections
- granular cytoplasm- contains many lysosomes that contain enzymes used to attack pathogens
What are the dimentions of neutrophils?
How are sperm cells specialised to carry out their function?
- male gametes- function is to deliver genetic information to the female gamete, the ovum
- tail or flagellum- capable of movement and contain many mitochondria to supply the energy needed to swim
- acrosome containing digestive enzymes- released to digest the protective layers around the ovum and allow the sperm to penetrate
What is the structure of a sperm cell?
How are palisade leaf cells specialised to carry out their function?
- contains chloroplasts- absorb large amounts of light for photosynthesis
- rectangular box shape- can be closely packed to form a continuous layer
- thin cell walls- increasing the rate of diffusion of carbon dioxide
- large vacuole- maintain turgor pressure
- chloroplasts can move within the cytoplasm- able to absorb more light
What are the dimentions of a palisade leaf cell?
How are root hair cells adapted to carry out their function?
- root hairs- increase the surface area of the cell which can maximise the uptake of water and minerals from the soil
- thin cell walls- allow for diffusion
- vacuole has a low water potential
What are the dimentions of a root hair cell?
How are guard cells specialised to carry out their function?
- form small openings called stomata, necessary for carbon dioxide to enter plants for photosynthesis
- when guard cells lose water and become less swollen as a result of osmotic forces, they change shape and the stomata closes to prevent further water loss from the plant
- cell wall is thicker on one side- cell doesn't change shape symmetrically as its volume changes
What is the structure of a guard cell? include dimentions.
What is a tissue?
A tissue is a collection of differentiated cells that have a specialised function or functions within the organism.
What are the four main categories of tissues in animals and what is each adapted for?
- nervous tissue- adapted to support the transmission of electrical impulses
- epithelial tissue- adapted to cover body surfaces, internal and external
- muscle tissue- adapted to contract
- connective tissue- adapted either to hold other tissues together or to act as a ransport medium
How is squamous epithelium tissue adapted to carry out its function?
- Made up of specialised squamous epithelial cells
- very thin- squat or flat cells that make it up and because it is only one cell thick
- present when rapid diffusion across a surface is essential.
- forms the lining of the lungs and allows rapid diffusiion of oxygen into the blood
How is ciliated epithelium tissue adapted to carry out its function?
- It is made up of ciliated epithelial cells and goblet cells
- 'hair-like' structures (cillia) move in a rhythmic manner to sweep mucus and dust away
- goblet cells- release mucus to trap any unwanted particles
Give an example of where you might find ciliated epithelium tissue and why it is important.
In the lining of the trachea which prevents any nwanted particles present in the air from reaching the alveoli once inside the lungs.
How is cartilage tissue adapted to carry out its function?
- connective tissue found in the outer ear, nose, and in between bones.
- contains fibres of the proteins elastin and collagen
- cartilage is a firm, flexible connective tissue composed of chondrocyte cells embedded in an extracellular matrix
- prevents the ends of bones rubbing
True or false? Many fish have whole skeletons made of cartilage, not bone
How is muscle tissue adapted to carry out its function?
- needs to be able to shorten in length in order to move bones, which in turn move the different parts of the body
- skeletal muscle fibres- attatched to bone. they contain myofibrils which contain contractile proteins
What are the two main tyoes of tissues in plants and what is each adapted to do?
- epidermis tissue- adapted to cover plant surfaces
- vascular tissue- adapted for transport of water and nutrients
How is epidermal tissue adapted to carry out its function?
The epidermis is a single layer of closely packed cells covering the surfaces of plants. It is usually covered by a waxy; waterproof cuticle to reduce the loss of water. contain stomata that can open and close to allow carbon dioxide in and out, and water vapour and oxygen in and out.
How is xylem tissue adapted to carry out its function?
- responsible for the transport of water and minerals throughout plants.
- vessel elements (elongated dead cells) which are strengthened with a waterproof material called lignin which provides structural support for plants
How is phloem tissue adapted to carry out its function?
- responsible for the transport of organic nutrients from leaves and stems where it is made by photosynthesis to all parts of the plant where it is needed
- composed of columns of sieve tube cells separated by perforated walls called sieve plates
What is an organ?
An organ is a collection of tissues that are adapted to perform a particular function in an organism
Fill in the gaps. The mammalian heart is an ______ that is adapted for ______ ______ around the body. It is made up of ______ tissue and ______ tissue.
The mammalian heart is an organ that is adapted for pumping blood around the body. It is made up of muscle tissue and connective tissue.
Fill in the gaps. The leaf is a ______ organ that is adapted for ___________. It contains _______ tissues and ______ tissue.
The leaf is a plant organ that is adapted for photosynthesis. It contains epidermis tissues and vascular tissue.
What is an organ system composed of?
A number of organs working together to carry out a major function in the body
What is the function of the digestive system?
It takes in food, breaks down the large insoluble molecules into small soluble ones, absorbs the nutrients into the blood, retains water needed by the body and removes any undigested material from the body
What is the function of the cardiovascular system?
Moves blood around the body to provide an effective transport system for the substances it carries.
What is the function of the gaseous exchange system?
It brings air into the body so oxygen can be extracted for respiration, and carbon dioxide can be expelled
The process of a cell becoming specialised is called differentiation.
True or false? Despite being differentiated in structure and function, all body cells within an organism have the same DNA
True or false? Differentiation involves the expression of some genes but not others in the cell's genome.
What do all cells in plants and animals begin as?
Undifferentiated cells called stem cells
What are stem cells?
Stem cells are undifferentiated cells that are not adapted to any particular funcition (they are unspecialised) and they have the potential to differentiate to become any one of the range of specialised cell types in the organism.
How many times are stem cells able to undergo cell division?
Many, many times
What are stem cells a necessary source for?
Growth, development and tissue repair.
What can no longer happen once a stem cell has become specialised?
The stem cell can no longer divide.
Why must the activity of stem cells be strictly controlled?
If they do not divide fast enough then tissues are not efficiently replaced leading to ageing. However, if there is uncontrolled division then they form masses of cells called tumours, which can lead to the development of cancer
What is the meaning of the word potency?
A stem cell's ability to differentiate into different cell types
Complete the following sentence. The greater the number of cell types a stem cell can differentiate in to...
...the greater its potency
What does it mean if a stem cell is totipotent? Where would you find totipotent cells?
These stem cells can differentiate into any type of cell. A fertilised egg, or zygote and the 8 or 16 cells from its first few mitotic divisions are totipotent cells. They have the ability to produce a whole organism and can produce extra-embryonic tissues like the amnion and umbilicus
What does it mean if a stem cell is pluripotent? Where would you find pluripotent cells?
These stem cells can form all tissue types but not whole organisms. They are present in early embryos and are the origin of the different types of tissue within an organism
What does it mean if a stem cell is multipotent? Where would you find multipotent cells?
These stem cells can only form a range of cells within a certain type of tissue. Haematopoetic stem cells in bone marrow are multipotent because this gives rise to the various types of blood cell
Why have multicellular organisms like animals and plants evolved from unicellular organisms?
Because groups of cells with different functions working together as one unit can make use of resources more efficiently than single cells operating on their own.
In multicellular organisms, why do cells need to specialise?
They must specialise in order to take on diffrenet roles in tissues and organs
Why do cells have different shapes and sizes and often contain different organelles?
They look very different because they have different roles and functions. They may be required to form barriers such as skin or be motile such as sperm cells
Erythrocytes and neutrophils are both present in the blood. Why do they look different?
Because they have different functions
Fill in the blanks. When cells ________ they become adapted to their _______ role. What form this adaptation takes is ________ on the _______ of the tissue, organ and organ system to which the cell belongs.
When cells differentiate they become adapted to their specific role. What form this adaptation takes is dependent on the function of the tissue, organ and organ system to which the cell belongs.
Where are all blood cells derived from?
Stem cells in the bone marrow.
Why do erythrocytes (red blood cells) have a short lifespan of around 120 days only?
Due to the lack of nucleus and organelles
Red blood cells need to be replaced constantly. How many need to be produced per day and where are they produced from?
The stem cell colonies in the bone marrow produce approximately three billion eythrocytes per kilogram of body mass per day to keep up with the demand
How long do neutrophils live for? How often are they replaced?
They live for about six hours and the colonies of stem cells in bone marrow produce in the region of 1.6 billion per kg per hour which increases during infection
What are embryonic stem cells and where are they found?
These cells are present at a very early stage of embryo development and are totipotent. After about seven days a mass of cells, called a blastocyst, has formed and the cells are now in a pluripotent state. They remain in this state in the fetus until birth
Describe what is meant by tissue (adult) stem cells.
They are present throughout life from birth. They are found in specific areas such as bone marrow. They are multipotent.
What are the advantages of umbilical cords of newborn babies being a source of stem cells?
- plentiful supply of umbilical cords
- invasive surgery is not needed
- can be stored for later use
- tissues cultured would not be rejected
Where can stem cells in plants be found?
In meristematic tissue (meristems)
Where are meristems found in plants?
Wherever growth is occuring in plants, for example at the tips of roots and shoots (apical meristems)
Sandwhiched between the phloem and xylem tissues and this is called the vascular cambium.
True or false? The pluripotent nature of stem cells in the meristems continues throughout the life of the plant.
Stem cells transplanted into specific areas have the potential to treat certain diseases including Parkinson's disease. Explain what the symtoms are, what it is caused by and what is currently available to treat it.
The symptoms include shaking and rigidity which are caused by the death of dopamine-producing cells in the brain. Drugs currently only delay the progress of the disease.
Stem cells transplanted into specific areas have the potential to treat certain diseases including Alzheimer's disease. Explain what the symtoms are, what it is caused by and what is currently available to treat it.
Brain cells are destroyed as a result of the build up of abnormal proteins. Drugs currently only alleviate the symptoms.
Why have stem cells become an important area of study in developmental biology?
It is their ability to divide indefinitely and differentiate into almost any cell within an organism as well as an organisms ability to grow and develop from a single cell and why things sometimes go wrong.
The embryos used originally were donated from those left over after fertility treatment. The UK law has more recently changed. What does it now say?
That embryos can be specifically created in the laboratory as a source of stem cells.
What currently happens when stem cells are removed from embryos?
The embryo gets destroyed.
What are the religious and moral objectiions to stem cell research?
Many people believe that life begins at conception and the destruction of embryos is, therefore, murder. There is a lack of consensus as to when the embryo itself has rights and also who owns the genetic material that is being used for research.
What are some of the ways that people are tying to get around the ethics of using embryonic stem cells for research?
- umbilical cord stem cells- but they are only multipotent not pluripotent
- adult tissue stem cells- but they do not divide as well and are more likely to mutate
- developments into induced pluripotent stem cells (iPSCs)- genetically modified adult stem cells that act like embryonic stem cells and so are pluripotent
Does the use of plant stem cells have the same ethical issues as animal cells?
What is one way of treating SCID?
Gene therapy using stem cells