Chapter 6 - Cell Division Flashcards
Stages of the cell cycle
o interphase
o nuclear division (mitosis)
o cell division (cytokinesis)
Length of the cell cycle
variable depending on environmental conditions, the cell type and the organism
What is the movement from one phase to another triggered by
Chemical signals = cyclins
First phase
Interphase
What makes up interphase
o G1 phase
o S phase
o G2 phase
What happens in G1
o In this phase, at some point a signal is received telling the cell to divide again
o The gap between the previous cell division and the S phase is called the G1 phase
o Cells make the RNA, enzymes and other proteins required for growth during the G1 phase
What happens in S phase
o The S phase is relatively short
o The DNA in the nucleus replicates = resulting in each chromosome consisting of two identical sister chromatids
What happens in G2 phase
o the cell continues to grow and the new DNA is checked and any errors are usually repaired
o Other preparations for cell division are made = production of tubulin protein, which is used to make microtubules for the mitotic spindle
Where does cell growth stop
Mitosis
What is cytokineses
• Once the nucleus has divided, the whole cell divides + one nucleus moves into each cell to create two genetically identical daughter cells
• Animal cells = involves constriction of the cytoplasm between two nucleus
• Plant cells = new cell wall is formed
How is the cell cycle regulated
Checkpoints = • Specific proof-reading enzymes + repair enzymes are involved in this checking process
What happens if an error is identifies
enzymes will repair the error but in some cases the cell may destroy itself to prevent passing on harmful mutations
How many checkpoints
4
Where are the main checkpoints = detailed
- G1 phase - chromosomes are checked for damage.
o If damage is detected then the cell does not advance into the S phase until repairs have been made - S phase - chromosomes are checked to ensure they have been replicated.
o If all the chromosomes haven’t been successfully replicated then the cell cycle stops - G2 phase - an additional check for DNA damage occurs after the DNA has been replicated.
o The cell cycle will be delayed until any necessary repairs are made - During metaphase - the final check determines whether the chromosomes are correctly attached to the spindle fibres prior to anaphase
Where are the main checkpoints = numbers
G1
S
G2
Metaphase
Stages of mitosis
prophase + metaphase + anaphase + telophase
Define mitosis
• process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus
What happens in prophase
• Chromosomes condense and are now visible when stained
• chromosomes consist of two identical chromatids = sister chromatids (each containing one DNA molecule) that are joined together at the centromere
• The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)
• Spindle fibres (protein microtubules) begin to emerge from the centrosomes (consists of two centrioles in animal cells)
• The nuclear envelope (nuclear membrane) breaks down into small vesicles
• The nucleolus disappears
What happens in metaphase
• Centrosomes reach opposite poles
• Spindle fibres (protein microtubules) continue to extend from centrosomes
• Chromosomes line up at the equator of the spindle (the metaphase plate) so they are equidistant to the two centrosome poles
• Spindle fibres reach the chromosomes and attach to the centromeres
o attachment involves specific proteins = kinetochores
• Each sister chromatid is attached to a spindle fibre originating from opposite poles
What happens in anaphase
• The sister chromatids separate at the centromere = the centromere divides in two
• Spindle fibres begin to shorten
• The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)
What happens in telophase
• Chromosomes arrive at opposite poles and begin to decondense
• Nuclear envelopes begin to reform around each set of chromosomes
• spindle fibres break down
• New nucleoli form within each nucleus
Where does growth in plant occur
Meristems
Where are meristems found
just behind the protective root cap
How can root tip meristems be studies for mitosis
using the squash technique (root tips are stained and then gently squashed, spreading the cells out into a thin sheet and allowing individual cells undergoing mitosis to be clearly seen)
Method to view mitosis in plants
Method
• Garlic or onion (Allium cepa) root tips are most commonly used (the bulbs can be encouraged to grow roots by suspending them over water for a week or two)
• Remove the tips of the roots (about 1cm) and place in a suitable stain (eg. warm, acidified acetic orcein, which stains chromosomes a deep purple)
• The stained root tip is gently squashed on a glass slideusing a blunt instrument (eg. the handle of a mounting needle)
• Cells undergoing mitosis are seen
What does prophase look like
- big blob mess
What does metaphase and anaphase look like
Metaphase = kinda see the chromosome lines
Anaphase = pulling apart
Limitations of method
• It can be very difficult to distinguish between prophase and telophase in cells
• The size of cells or structures of tissues may appear inconsistent in different specimen slides
• Optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that can not be seen
• The treatment of specimens when preparing slides could alter the structure of cells
If there are multiple nuclei in one cell what stage is it in
Telophase
Significance of mitosis
- growth
- replacement of damaged cells and repair of tissue
- asexual reproduction
How is mitosis important in growth
• The two daughter cells produced are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell
• This enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms
• Growth may occur across the whole body of the organism or be confined to certain regions, such as in the meristems (growing points) of plants
How is mitosis important in replacement of cells and repair of tissue
• Damaged tissues can be repaired by mitosis followed by cell division
• As cells are constantly dying they need to be continually replaced by genetically identical cells
• In humans, for example, cell replacement occurs particularly rapidly in the skin and the lining of the gut
• Some animals can regenerate body parts, for example, zebrafish can regenerate fins and axolotls regenerate legs and their tail amongst other parts
How is mitosis important in asexual reproduction
• For unicellular organisms such as Amoeba, cell division results in the reproduction of a genetically identical offspring
• For multicellular organisms, new individuals grow from the parent organism (by cell division) and then detach (‘bud off’) from the parent in different ways
Define asexual reproduction
the production of new individuals of a species by a single parent organism – the offspring are genetically identical to the parent
Examples of organisms with asexual reproduction
Hydra and yeast and runners from strawberries
In S phase how is DNA copied
DNA sequences of high importance (genes that are active in all cells) are replicated first
Why is the S phase fast
Because DNA bases are exposed during replication thus more susceptible to mutagens
The quicker the s phase…
The lower chance of mutations occurring
Graph for mass of DNA at each stage
D
A
A
No DNA
224
17
Define meiosis
orm of nuclear division that results in the production of haploid cells from diploid cells - It produces gametes in plants and animals that are used in sexual reproduction
What happens in prophase 1
• DNA condenses = visible as chromosomes
• The chromosomes are arranged side by side in homologous pairs
• As the homologous chromosomes are very close together = crossing over of non sister chromatic = chiasma (chiasmata; plural)
• centrioles migrate to opposite poles
- spindle is formed
• nuclear envelope breaks down + nucleolus disintegrates
What does DNA look like in prophase 1
• DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere
What is a bivalent
o A pair of homologous chromosomes
What happens in metaphase 1
bivalents line up along the equator of the spindle = spindle fibres attached to the centromeres
The maternal and paternal chromosomes in each pair position themselves independently of the others = independent assortment
What is independent assortment
maternal and paternal chromosomes in each pair position themselves independently of the others
What does independent assortment mean
that the proportion of paternal or maternal chromosomes that end up on each side of the equator is due to chance
What happens in anaphase 1
homologous pairs of chromosomes are separated = microtubules pull whole chromosomes to opposite ends of the spindle
• The centromeres do not divide
What happens in telophase 1
chromosomes arrive at opposite poles
• Spindle fibres start to break down
• Nuclear envelopes form around the two groups of chromosomes + nucleoli reform
What do some plant cells do that’s different to normal cels
• Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
What are the sites where crossing over happens called
Chiasmata
What is independent assortment aided by
protein structure called the synaptonemal complex
What happens in cytokinesis 1
• division of the cytoplasm
• Cell organelles =. distributed between two developing cells
• The end product of cytokinesis in meiosis I is two haploid cells
What happens in cytokinesis 1 in animals
cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half
What happens in cytokinesis 1 in plant cells
vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate).
- vesicles merge with each other to form the new cell surface membrane + secrete a layer of calcium pectate which becomes the middle lamella.
- Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell
What is the end product of meiosis 1
two haploid cells
In plant cells, what do vesicles secrete
layer of calcium pectate
What does the layer of calcium pectate become
The middle lamella
What is difference between the second division of meiosis - before prophase 11
no interphase between meiosis I and meiosis II
What does having no interphase between meiosis I and meiosis II mean
DNA is not replicated
What happens in prophase II
o The nuclear envelope breaks down and chromosomes condense
o A spindle forms at a right angle to the old one
What happens in metaphase II
o Chromosomes line up in a single file along the equator of the spindle
What happens in anaphase II
o Centromeres divide and individual chromatids are pulled to opposite poles
o This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell
What happens in telophase II
o Nuclear membranes form around each group of chromosomes
What happens in cytokinesis II
o Cytoplasm divides as new cell surface membranes are formed creating four haploid cells
Draw a flow chart summarising meiosis
Significant of meiosis
increase the genetic diversity of gametes produced
Genetic diversity in meiosis
- crossing over
- independent assortment
- random fusion of gametes
- mutations ( not strictly meiosis)
What is crossing over
• process by which non-sister chromatids exchange alleles
Process of crossing over
o During meiosis I homologous chromosomes pair up and are in very close proximity to each other
o The non-sister chromatids can cross over and get entangled
o These crossing points are called chiasmata
o The entanglement places stress on the DNA molecules
o As a result of this a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
FORMS recombinant chromatids
Why is this swapping of alleles in crossing over significant
can result in a new combination of alleles on the two chromosomes
FORMS recombinant chromatids
How many chiasmata are there in each bivalent
At least 1
Where is crossing over most likely to happen
more likely to occur further down the chromosome away from the centromere
What is independent assortment
the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I
When does independent assortment happen
metaphase I
Process of independent assortment
o Each pair can be arranged with either chromosome on top, this is completely random
• The homologous chromosomes are then separated and pulled apart to different poles
• The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
How to work out the number of different possible chromosome combinations = independent assortment
2n can be used, where n corresponds to the number of chromosomes in a haploid cell
How does random fusion of gametes create genetic diversity
random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles
How do mutations increase genetic diversity
• random mutation that takes place during DNA replication can lead to the production of new alleles and increased genetic variation.
Function of erythrocytes
transport oxygen around the body and carbon dioxide to the lungs
Adaptations of erythrocytes (4)
o biconcave in shape which increases the surface area over which oxygen can be absorbed
o cytoplasm contains high amounts of the pigment haemoglobin which can readily bind to oxygen
o No nucleus = makes more space inside the cell for haemoglobin molecules for maximum oxygen-carrying capacity
o Elastic membrane allows cell to be flexible and change shape as it squeezes through narrow capillaries
Function of neutrophils
destroy pathogens by phagocytosis and the secretion of enzymes
Adaptations of neutrophils
o very flexible shape = allows them to squeeze through cell junctions in the capillary wall
o flexibility = form pseudopodia (cytoplasmic projections) that engulf microorganisms
o large number of lysosomes = digestive enzymes help to digest and destroy invading cells
o A flexible nuclear membrane = helps the cell to penetrate cell junctions.
Function of squamous epithelial cells
provide a surface covering or outer layer
aids rapid diffusion
Found on a variety of organs and structures e.g. blood vessels and alveoli
Adaptations of squamous epithelial cells
consists of a single layer of flattened cells on a basement membrane
one cell thick
o layer forms a thin cross-section which reduces the distance that substances have to move to pass through - it shortens the diffusion pathway
o It is permeable, allowing for the easy diffusion of gases
Function of ciliated epithelial cells
moving substances across the surface of a tissue
Adaptations of ciliated epithelial cells
o Have cilia = beat in a coordinated way to shift material along the surface of the epithelium tissue
o Goblet cells secrete mucus = trap dust, dirt and microorganisms - preventing them from entering vital organs where they may cause infection
Function of sperm cells
reproduction - to fuse with an egg, initiate the development of an embryo and pass on fathers genes
Adaptations of sperm cell
head contains a nucleus that contains half the normal number of chromosomes (haploid, no chromosome pairs)
o The acrosome in the head contains digestive enzymes that can break down the outer layer of an egg cell so that the haploid nucleus can enter to fuse with the egg’s nucleus
o The mid-piece is packed with mitochondria to release energy for the tail movement
o The tail rotates, propelling the sperm cell forwards and allowing it to move towards the egg
Function of palisade cell
carry out photosynthesis to produce glucose and oxygen
Adaptations of palisade cell
arge number of chloroplasts (the site of photosynthesis) are present in the cytoplasm to maximise the absorption of light for photosynthesis
o tall + thin shape of the cells allows light to penetrate deeper before encountering another cell wall (cell walls absorb/reflect light) and for many cells to be densely packed together
Function of root hair cell
absorption of water and mineral ions from soil
Adaptations of root hair cells
o Root hair = increase SA = rate of water uptake by osmosis is greater
o Thinner walls = water can move through easily (due to shorter diffusion distance)
o Permanent vacuole contains cell sap which is more concentrated than soil water, maintaining a water potential gradient
o Mitochondria for active transport of mineral ions
Function of guard cells
control the opening of the stomata to regulate water loss and gas exchange
Adaptations of guard cells
o Inner cell walls are thicker (those facing the air outside the leaf) while the outer cell walls are thinner (those facing adjacent epidermal cells).
difference in the thickness of the cell walls allows the cell to bend when turgid
o The cytoplasm has a high density of chloroplasts and mitochondria.
Tissues
group of cells that work together to perform a particular function
What forms the epithelial tissue
Epithelial cells = function = absorb food in small intestine
Organs
Made up of different tissue
Heart as an organ is made up of what
cardiac muscle tissue, blood vessel tissues and connective tissue
Organ systems
Different organs working together
Tissue / organ / organ system of epithelial cells
Tissue / organ / organ system of muscle cells
Tissue / organ / organ system of neurones
Tissue / organ / organ system of rod and cone cells
Function of xylem cells
transport water for tissue and dissolved ions
Adaptations of xylem cells
o No top and bottom walls between cells to form continuous hollow tubes through which water is drawn upwards towards the leaves by transpiration
o Cells are essentially dead, without organelles or cytoplasm, to allow free movement of water
o Outer walls are thickened with a substance called lignin, strengthening the tubes, which helps support the plant
Function of phloem cells
transport of dissolved sugars and amino acids
Adaptations of phloem cells
o Made of living cells which are supported by companion cells
o joined end-to-end and contain holes in the end cell walls (sieve plates) forming tubes that allow sugars and amino acids to flow easily through (by translocation)
o have very few subcellular structures to aid the flow of materials
Function of muscle cells
contraction for movement
Adaptations of muscle cells
have layers of protein filaments in them, these layers can slide over each other causing muscle contraction
o have a high density of mitochondria to provide sufficient energy (via respiration) for muscle contraction
o Skeletal muscle cells fuse together during development to form multinucleated cells that contract in unison
Three types of muscle in animals
skeletal, smooth and cardiac
Function of cartilage
provide support
prevents the ends of bones rubbing together
What is cartilage
strong and flexible connective tissue
Where is cartilage found
One place is in rings along the trachea, called Tracheal rings
found outer ear / nose / between bones
Stem cells
cell that can divide (by mitosis) an unlimited number of times
Potency
ability of stem cells to differentiate into more specialised cell types
Three types of potency
Totipotency
Pluripotent
Multipotency
o Totipotency
stem cells that can differentiate into any cell type found in an embryo, as well as extra-embryonic cells (the cells that make up the placenta).
o Pluripotency
embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into extra-embryonic cells (the cells that make up the placenta)
o Multipotency
adult stem cells that have lost some of the potency associated with embryonic stem cells and are no longer pluripotent
What are adult stem cells
small numbers of stem cells remain to produce new cells for the essential processes of growth, cell replacement and tissue repair
What are adult stem cells
Multi potent
Stem cells in bone marrow
Multi potent - only differentiate into blood cells (red blood cells, monocytes, neutrophils and lymphocytes)
Summary diagram - types of stem cells
erythropoiesis
• As red blood cells lack a nucleus, they cannot divide, meaning that new erythrocytes are constantly being formed from bone marrow stem cells in order to maintain the red blood cell count in the blood
Stages of erythropoiesis
Changes that occur when stem cells differentiate into neutrophils
o Indentations form in the nucleus, giving it a lobed structure
o Granules accumulate (these are lysosomes that contain hydrolytic enzymes)
Cambium
• xylem and phloem are formed from stem cells that are found in the tissue between them = tissue = cambium
What is cambium
A meristem
Meristem
any undifferentiated tissue in a plant that has the ability to give rise to new cells
Where in the cambium do the cells differentiate into xylem + where phloe,
stem cells at the inner edge of the cambium differentiate into xylem cells and the stem cells at the outer edge of the cambium differentiate into phloem cells
Changes that occur when differentiating into xylem
lose their cytoplasm, deposit lignin in their cell walls and lose their end cell walls
Changes that occur when differentiating into phloem
lose some of their cytoplasm and organelles, and develop sieve plates
What is cell differentiation from cambium triggered by
Hormones
Where are meristems found
Tips of roots / shoots
When are embryonic stem cells totipotent
if taken in the first 3-4 days after fertilisation
When are embryonic stem cells pluripotent
if taken on day 5
Uses of stem cells
used for research are often the waste (fertilised) embryos from in vitro fertilisation treatment
Why is there ethical objections to using embryonic stem cells as research
embryos have the potential to develop into human beings
Uses of multi potent adult stem cells
Transplant
Why is using multi potent stem cells not great
Problem caused by Alzheimer’s
- most common cause of dementia
+ memory loss + loss of other cognitive abilities
Treatment using stem cells - Alzheimer’s
Stem cells - grown into nerve cells = repair
Why is using stem cells in Alzheimer’s difficult
Damage to brain is widespread rather than localised
Problem caused by Parkinson’s
- shaking /stiffness / slowness of movement
What is Parkinson’s disease caused by
Death of dopamine
Treatment using stem cells - Parkinson’s
Stem cells can replace dopamine producing cells
Problem caused by age related muscular degeneration
- damage to macula of retina = cause loss of vision
