2.6 - Cell Division Flashcards
What is the cell cycle?
- highly ordered sequence of events that takes place within the cell
- used by all somatic (body) cells
- results in the formation of 2 genetically identical daughter cells
What are the main phases of the cell cycle?
- interphase
- mitotic (division) phase
What is interphase?
A period of time where the cell is growing and carrying out its normal functions (eg producing enzymes + hormones)
Cell isn’t dividing, but actively preparing for cell division
What are some of the processes that occur during interphase?
- DNA is replicated and checked for errors in the nucleus
- protein synthesis occurs in cytoplasm
- mitochondria grow and divide, increasing in number in the cytoplasm
- chloroplasts grow and divide in the plant and algal cell cytoplasm, increasing in number
- the normal metabolic processes of a cell occur (some, eg respiration, occur during interphase and mitosis)
What are the stages of interphase?
G1: the first growth phase.
- proteins from which organelles are synthesised are produced, and organelles replicate
- the cell increases in size
S: synthesis phase
- DNA is replicated in nucleus
G2: second growth phase
- cell continues to increase in size
- energy stores are increased
- duplicated DNA is checked for errors
What is the mitotic phase?
The period of cell division.
Involves 2 stages:
- mitosis: the nucleus divides
- cytokinesis: the cytoplasm divides and 2 cells are produced.
What is the G0 phase?
The phase where the cell leaves the cycle, either temporarily or permanently.
Many reasons for this, such as:
- differentiation: a cell that becomes specialised to carry out a particular function is no longer able to divide. It will carry out this function indefinitely and won’t re enter the cell cycle
- damaged DNA: if the DNA of a cell becomes damaged, the cell can no longer divide and enters permanent cell arrest (G0)
A few types of cells can be stimulated to go back into the cell cycle and start dividing again, eg lymphocytes (white blood cells) in an immune response
What are senescent cells?
- the majority of normal cells can only divide a limited number of times, and will eventually become senescent and enter G0.
- As you age, the number of senescent cells increases.
- Growing numbers of senescent cells have been linked with diseases such as cancer and arthritis
Why is it important to regulate the cell cycle?
it’s vital to ensure that a cell only divides when:
- it has grown to the right size
- replicated DNA is error free/repaired
- chromosomes are in the right place during mitosis
this is to ensure that the two daughter cells produced are identical to the parent cell
What are checkpoints?
- the control mechanisms of the cell cycle
- they monitor and verify whether the processes at each phase of the cell cycle have been completed accurately before the next phase can be entered by the cell
They occur at the G1 phase, G2 phase and metaphase
What is the G1 checkpoint?
Occurs at the end of the G1 phase, before the S phase.
Checks for:
- cell size
- nutrients
- growth factors
- DNA damage
If the cell passes, it moves onto the S phase.
If it fails, it enters G0
What is the G2 checkpoint?
Occurs at the end of G2, before the start of the mitotic phase.
Checks for:
- cell size
- DNA replication (ensuring there are no errors)
- DNA damage
If the cell passes, it initiates the molecular processes that signal the start of mitosis.
What is the spindle assembly checkpoint?
- aka metaphase checkpoint
- occurs during metaphase
- checks that all chromosomes are attacked to the spindle fibres
- mitosis can’t proceed until this checkpoint is passed
What is mitosis?
- the division of the nucleus to produce 2 genetically identical nuclei, which will then form 2 identical daughter cells
- each new cell will have an exact copy of the DNA present in the parent cell and the same number of chromosomes
What is the importance of mitosis?
It is necessary when all the daughter cells must be identical.
This is the case during:
- growth
- replacement and repair of tissues
- asexual reproduction in plants, some animals and fungi
What are chromosomes and chromatids?
- a chromosome is a DNA molecule
- when DNA replicates, both strands are joined together in the middle by a centromere. this means there is still only 1 chromosome
- one half of the replicated DNA is called a chromatid.
- after DNA replication, there are the same number of chromosomes per nucleus but double the amount of chromatids
What are the stages of mitosis?
1) PROPHASE: chromosomes appear condensed, nuclear envelope is broken down
2) METAPHASE: thick, coiled chromosomes are lined up on the metaphase plate (equator). spindle fibres are attached to the chromosomes
3) ANAPHASE: chromosomes have separated and are moving towards the poles
4) TELOPHASE: the chromosomes are at the poles and are becoming more diffuse. the nuclear envelope is reforming.
What happens in prophase?
- chromatin fibres begin to coil and condense to form chromosomes that are visible under the light microscope
- the nucleolus and nuclear envelope are broken down
- in animal cells, centrioles migrate to opposite poles of the cell
What happens in metaphase?
- chromosomes line up along the middle of the cell along the METAPHASE PLATE
- they are attached to the spindle fibres at the centromere. The spindle fibres come from the centrioles
- metaphase checkpoint occurs, ensuring all chromosomes are attached to the spindle fibres
What happens during anaphase?
- the centromere holding the chromatids together in each chromosome is divided, meaning chromatids are separated
- spindle fibres contract, pulling chromatids to opposite poles of the cell
What happens during telophase?
- the chromatids have reached the opposite poles, and are now called chromosomes
- the two new sets of chromosomes become more diffuse (uncoil)
- the nuclear envelope beings to reform around the 2 new sets of chromosomes
- cytokinesis begins
What is cytokinesis?
- when cytoplasm divides, forming two genetically identical daughter cells
- usually begins before telophase is finished
- mechanism differs between animal and plant cells
What is the mechanism for cytokinesis in animals?
- the plasma membrane is pulled inwards by the cytoskeleton, forming a cleavage furrow, until it is close enough to fuse in the middle
- this divides the cytoplasm, forming two cells
What is the mechanism for cytokinesis in plants?
- plant cells have cell walls so they can’t form cleavage furrows
- vesicles from the Golgi apparatus assemble where the metaphase plate was formed
- the vesicles fuse with each other and the cell surface membrane, dividing the cell in 2
- new cellulose is laid down along the new sections of the membrane, reforming the cell wall
What are diploid cells?
Cells that have 2 chromosomes of each type, one inherited from each parent
Eg a human diploid cell has 46 chromosomes (23 pairs of chromosomes)
Mitosis produces diploid cells
What are haploid cells?
Cells that only have one chromosome of each type, and therefore only have half the genetic material of parent cells
Eg human haploid cells (gametes) only have 23 chromosomes
Meiosis produces haploid cells
What is meiosis?
A type of cell division that results in 4 non-genetically identical (genetically varied) daughter cells.
Each cell is a haploid - has half the number of chromosomes of the parent cell. This means meiosis is a reduction division
It is used to produce gametes
Why is meiosis important?
- makes gametes for sexual reproduction. In sexual reproduction, two gametes fuse to make a zygote
- increases genetic variation within a population
- maintains chromosome number in offspring of a species. Meiosis is a reduction division, so it halves the chromosome number. This means zygotes have the normal number.
What is a homologous pair?
A matching pair of chromosomes, one inherited from each parent.
- diploid cells have 2 full sets of genes (one from each parent), meaning they have different alleles of the same gene
- different alleles of a gene will have the same locus (position on a particular chromosome)
- this means each chromosome in a homologous pair has the same genes at the same loci
As a result, the chromosomes:
- will be the same length and size when in prophase
- the centromeres will be in the same positions
What are the stages of meiosis?
MEIOSIS I:
- prophase 1
- metaphase 1
- anaphase 1
- telophase 1
MEIOSIS II:
- prophase 2
- metaphase 2
- anaphase 2
- telophase 2
What is meiosis 1?
- the first division of meiosis
- a reduction division
- chromosomes of a homologous pair are separated into 2 cells
- each intermediate cell will only contain 1 full set of gene rather than 2, so the cells are haploid
What is meiosis 2?
- the second division in meiosis
- similar to mitosis
- pairs of chromatids are separated, forming 2 more cells
What happens during prophase 1?
- chromosomes condense
- nuclear envelope breaks down
- spindle fibres form from the centrioles
- homologous chromosomes pair up (forming bivalents)
- crossing over may occur
What is crossing over?
- during prophase I and metaphase I, homologous pairs of chromosomes pair up closely.
- chromatids twist around each other. The point where they join is called the chiasma.
- fragments of non-sister chromatids (ie fragments from the different chromosomes) swap over and alleles are exchanged.
- this forms recombinant chromatids
- sister chromatids are no longer genetically-identical
- this results in genetic variation.
What happens during metaphase 1?
- homologous pairs of chromosomes line up along the metaphase plate (rather than individual chromosomes like in mitosis)
- they attach to spindle fibres by their centromeres
- homologous pairs are arranged randomly due to independent assortment
What is independent assortment?
- the orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair
- the maternal or paternal chromosomes can end up facing either pole
- this can result in many different combinations of alleles facing the poles
- this results in genetic variation
What happens during anaphase 1?
- the spindle fibres contract
- homologous pairs are separated
- one chromosome from each homologous pair moves to each end of the cell
- each chromosome still consists of two chromatids
What happens during telophase 1?
- chromosomes assemble at each pole
- nuclear envelopes from around each set of chromosomes
- cytokinesis begins
What happens after telophase 1?
- cytokinesis is complete
- there is a short interphase where chromosomes uncoil
- each new nucleus contains half the original number of chromosomes (now haploid)
- each chromosome still consists of 2 chromatids
What happens during prophase 2?
- nuclear envelopes break down
- chromosomes coil and condense
- spindle fibres form
What happens during metaphase 2?
- individual chromosomes line up along the equator of the cell and are attached to spindle fibres bu their centromeres
- sister chromatids are no longer identical due to crossing over
- independent assortment happens again (of sister chromatids) resulting in more genetic variation
What happens during anaphase 2?
- centromeres divide
- the chromatids of each chromosome are pulled apart to opposite poles
What happens during telophase 2 and cytokinesis?
- nuclear envelopes form around each of the 4 haploid nuclei
- the two cells now divide to give 4 genetically different haploid cells
What are the stages in meiosis that can result in genetic variation?
- crossing over in prophase I
- independent assortment of homologous chromosomes in metaphase I
- independent assortment of chromatids in metaphase II
What are the levels of organisation in multicellular organisms?
From smallest to largest:
Specialised cells -> tissues -> organs -> organ systems -> whole organism
What are specialised cells?
Cells that have been differentiated to carry out a specific function.
What are some examples of specialised animal cells?
- erythrocytes
- neutrophils
- sperm cells
- squamous epithelial cells
- ciliated epithelial cells
What are erythrocytes?
- red blood cells
- found in animals
Specialisations:
- no nucleus, making it a biconcave shape which increases SA:V ratio. This increases oxygen absorption through plasma membrane
- lots of haemoglobin which binds to oxygen in the lungs and releases it when oxygen concentrations are low
- no nucleus = more space for haemoglobin
- elastic membrane, increasing flexibility and allowing erythrocytes to squeeze through capillaries
What are neutrophils?
- type of white blood cell
- found in animals
Specialisations:
- multi-lobed nucleus and changeable shape for better flexibility; they are able to squeeze through tiny gaps to reach pathogens and form pseudopodia to engulf microorganisms
- granular cytoplasm contains lysosomes that contain digestive enzymes, which are used to attack pathogens
What are sperm cells?
- male gametes
- function is to deliver genetic info to the female gamete, the ovum (egg)
- found in animal cells
Specialisations:
- acrosome on head of sperm contains digestive enzymes, which are released to digest the protective layers around the ovum and allow the sperm to penetrate, leading to fertilisation
- reduced cytoplasm to reduce mass for
- large spiral mitochondrion to provide energy needed to swim
- tail for movement
What are squamous epithelial cells?
- found in animals
- flattened cells
- very thin, so reduces distance for substances to diffuse through
What are ciliated epithelial cells?
- found in animals
- cells have ‘hair like structures’, cilia, which move in a rhythmic manner to sweep substances along
What are some examples of specialised plant cells?
- palisade cells
- root hair cells
- guard cells
What are palisade cells?
- found in the mesophyll of plants
- contain many chloroplasts for photosynthesis
- cells are closely packed to form a continuous layer
- thin cell walls to maximise rate of diffusion
- large vacuole to maintain turgor pressure
- chloroplasts can move within cytoplasm to absorb more light
What are root hair cells?
- present at surface of roots in plants
- have long extensions called root hairs to maximise SA of cell
- this maximises uptake of water and mineral ions from the soil
What are guard cells?
- found in pairs on the surface of leaves in plants
- form a small opening called stomata which open to allow CO2 to enter the cell and close to prevent excess water loss
- inner cell walls are thicker than outer one. When plant is turgid, this causes the cell to change shape asymmetrically, causing an opening (stomata) to form
What are tissues?
A collection of specialised cells that work together to perform a specific function/functions in an organism
What are the main categories of tissues in animals?
- 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 tissues together or as a transport medium
What is squamous epithelium?
- aka pavement epithelium
- tissue found in animals
- made of squamous epithelium cells
- very thin: one cell thick, squamous epithelium cells are very thin
- present when rapid diffusion across a surface is essential
- forms the lining of the lungs and allows rapid diffusion of oxygen in the blood
What is ciliated epithelium?
- tissue found in animals
- made of ciliated epithelial cells
- cilia move rhythmically to waft substances away
- goblet cells are also present, which secret mucus and trap microbes and particulates. The ciliated epithelial cells then waft the mucus away.
- found on surfaces where things need to be moved eg trachea
What is cartilage?
- connective tissue found in animals
- composed of chondrocyte cells embedded in an extracellular matrix of collagen fibres and elastin fibres
- firm and flexible
- found in nose, ear, rib cage, trachea, bronchi, ends of bones
- prevents ends of bones rubbing against each other and causing damage
What is muscle?
- tissue found in animals
- found in muscles
- made of bundles of elongated cells called muscle fibres
3 different types:
- smooth muscle: operates without conscious thought eg in intestin
- cardiac muscle: in heart
- skeletal: can contract and relax at will.
What is skeletal muscle?
- type of muscle tissue
- skeletal muscle fibres are attached to bone
- they contain myofibrils which contain contractile proteins
What is the epidermis in plants?
- plant tissue adapted to cover plant surfaces
- single layer of closely packed cells covering the surfaces of plants
- usually covered by waxy, waterproof cuticle to reduce water loss
- stomata found in epidermis to allow gas exchange of water, CO2 and oxygen
What is xylem tissue?
- vascular tissue found in plants
- transports water and mineral ions
- tissue is comprised of vessel elements, which are elongated dead cells.
- walls are strengthened by lignin which provides structural support for plants
What is phloem tissue?
- vascular tissue found in plants
- transports assimilates (sucrose and amino acids) from the leaves and stems to other parts of the plant
- composed of sieve tube elements, sieve plates and companion cells
What are organs?
A collection of tissues adapted to perform a particular function in an organism
Eg heart is made of muscle tissue and connective tissue
Leaf is made of epidermis tissues and vascular tissue
What are organ systems?
Large multicellular organisms have coordinated organ systems
Organ systems have a number of organs working to gatherer to carry out a major function in the body
Eg digestive system, cardiovascular system, gaseous exchange system
What are stem cells?
- undifferentiated cells
- renewing source of cells (bcs they are capable of mitosis)
- can be differentiated into a number of possible cell types
- give rise to other cells (ie every cell stems from them)
- can also produce more stem cells
What happens when stem cells become specialised?
they lose the ability to divide and enter the G0 phase
Why is it important to control the activity of stem cells?
- if they don’t divide fast enough, tissues aren’t efficiently replaced, leading to ageing.
- if they divide too quickly, they form tumours which can lead to the development of cancer
What is potency?
The ability of a stem cell to divide into different cell types.
The greater the number of cell types it can differentiate into, the greater the potency.
What are totipotent cells?
Can differentiate into any type of cell.
- can produce all cell types of the embryo
- can also form other extra-embryonic tissues eg umbilical cord
They can give rise to a whole organisms
Early embryonic cells are totipotent
What are pluripotent cells?
Have less potential than totipotent cells
- can give rise to any tissue type of an organism
- cannot produce extra-embryonic tissues eg placenta, umbilical cord
Therefore, they cannot give rise to whole organisms
They are present in early embryos and are the origin of different types of tissue in the organism.
What are multipotent cells?
Can only form a range of cells within a certain type of tissue
- eg haematopoetic stem cells are found in the bone marrow and give rise to different types of blood cells
Most stem cells in adults are multipotent
Can also be found in the umbilical cord and used to treat forms of genetic anaemia
How are neutrophils and erythrocytes derived?
- they have specific functions and therefore must specialise
- differentiate from multipotent haematopoetic stem cells in the bone marrow
- erythrocytes have a life span of 120 days and must be replaced constantly
- neutrophils have a life span of 6 hours and are mass produced when there is an infection
What are the sources of stem cells?
- embryos
- tissues eg bone marrow
- umbilical cords
Describe stem cells found in an embryo
- embryonic stem cells are present at a very early stage of embryo development
- they are totipotent
- after roughly 7 days, a mass of cells called a blastocyst is formed
- stem cells are now pluripotent, and remain in this state in the foetus until birth
Describe adult stem cells
- aka tissue stem cells
- present throughout life from birth
- found in specific areas eg bone marrow
- are multipotent (though there is growing evidence that they can be artificially triggered to become pluripotent)
Describe stem cells in the umbilical cord
- found in the umbilical cord
- are multipotent
Can be harvested from umbilical cord in newborn babies
This is advantageous as:
- there is a plentiful supply of umbilical cords
- invasive surgery is not needed
- they can be stored incase they are needed by the individual in the future
- tissues cultivated from these stem cells wouldn’t be rejected in a transplant.
How are xylem and phloem vessels produced?
- vascular cambium is a meristematic tissue found between phloem and xylem tissues
- it is lateral meristem
- cambium cells can differentiate into cells needed for xylem and phloem tissues
- this way, vascular tissue grows as the plant grows.
What are the potential uses of stem cells?
- treatment of type 1 diabetes: use stem cells to make functional pancreatic beta cells
- treatment of neurological diseases (eg Parkinson’s, Alzheimer’s) - use stem cells to grow nerve cells
- treatment of spinal injuries: inject neural cells made from stem cells into the spinal cord
- treatment of damaged tissues (eg burns): use stem cells to regenerate skin
- drug trials: use stem cells to create cell lines to test drugs on before live animals/humans
- developmental biology: used to research and study development of an organism
What are the sources of stem cells in plants?
Stem cells are found in meristematic tissue (meristems). This tissue is found wherever growth occurs. There are 3 different types:
- apical meristem: found at the tips of roots and shoots
- intercalary meristem: found at nodes, whenever there is a ‘branch’ (eg at the base of a leaf, places where trunk meets branch)
- arranged parallel to the sides of an organ (eg stem) and is responsible for the growth in diameter of a plant
The stem cells in plants are pluripotent
