Cell Division, Cell Diversity And Cellular Organisation Flashcards
Why is cell division needed?
To grow and repair tissues in the organism
Stages of the cell cycle
interphase
G1=protein synthesis to make proteins that produce organelles then organelles replicate=cell grows
S=DNA is replicated
G2=cell continues to grow+replicated DNA is checked for errors, if there are any they’re repaired+other things needed for cell division are done
G0=cell leaves cell cycle (temporarily/permanently) eg. damaged DNA=x be used, differentiates=x divide again, become senescent=x carry out cell division (have divided too many times)
More senescent cells=link to age related disease arthritis
mitosis=nucleus divides
cytokinesis=cytoplasm divides= cells produced
Checkpoints of the cell cycle
During G1 phase - chromosomes are checked for damage. If damage is detected then the cell does not advance into the S phase until repairs have been made
During S phase - chromosomes are checked to ensure they have been replicated. If all the chromosomes haven’t been successfully replicated then the cell cycle stops
During G2 phase - an additional check for DNA damage occurs after the DNA has been replicated. 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
Why is it important to control the cell cycle?
It is essential that the DNA within new cells is accurate in order for them to carry out their function
When the DNA is replicated (during the S phase) errors can occur
There are several checkpoints throughout the cell cycle where the genetic information contained within the replicated DNA is checked for any possible errors
Specific proof-reading enzymes and repair enzymes are involved in this checking process
When possible enzymes will repair the error but in some cases the cell may destroy itself to prevent passing on harmful mutations
Stages of mitosis
prophase
- Chromosomes condense and are now visible when stained
The chromosomes consist of two identical chromatids called 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
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
metaphase
Centrosomes reach opposite poles
Spindle fibres (protein microtubules) continue to extend from centrosomes
Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles
Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres
This attachment involves specific proteins called kinetochores
Each sister chromatid is attached to a spindle fibre originating from opposite poles
anaphase
The sister chromatids separate at the centromere (the centromere divides in two)
Spindle fibres (protein microtubules) begin to shorten
The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)
telophase
Chromosomes arrive at opposite poles and begin to decondense
Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes
The spindle fibres break down
New nucleoli form within each nucleus
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Significance of mitosis in life cycles
MEIOSIS 1
prophase 1
- chromosomes condense+homologous chromosomes pair up=bivalents formed
- crossing over occurs bw bivalents
- nuclear envelope disintegrates+nucleolus dissapears
- centrioles migrate to opposite poles in cell+start forming spindle fibres
metaphase 1
- homologous pairs of chromosomes line up along equator
- orientation of each homologous pair is random+independent
- paternal+maternal chromosomes face opposite poles in each pair=independent assortment=genetic variation due to lots of combinations
- Each chromosome attaches to a spindle fibre by their centriole.
anaphase 1
- spindle fibres contract+shorten=homologous pairs of chromosomes are pulled to opposite poles+chromatids are still joined tgt
telophase 1
- chromosomes reach opposite poles of cell+uncoil
- Nuclear envelope forms around each set of chromosomes+nucleous starts to reform
- Cell undergoes cytokinesis=divides into 2 calls.
- Reduction from diploid to haploid
MEIOSIS 2
prophase 2
- Chromosomes condense = visible under microscope
- centrioles migrate to opposite poles (90 degrees from old one)+start forming spindle fibres
- nuclear envelope disintegrates+nucleolus disappears
metaphase 2
- chromosomes line up at equator
- each chromosome attaches to the spindle by their centromere
anaphase 2
- centromeres divide+separate each pair of chromatids
- spindle fibres contract+shorten=pulling chromatids to opposite poles of cell=4 groups of chromosomes
telophase 2
- chromatids reach opposite poles of cell as chromosomes=uncoil
- nuclear envelope reforms around each set of chromosomes+nucleolus reappears
- cytokinnesis occurs=4 cells produced which genetically diff from each other
Stages of meiosis
MEIOSIS 1
prophase 1
- chromosomes condense+homologous chromosomes pair up=bivalents formed
- crossing over occurs bw bivalents
- nuclear envelope disintegrates+nucleolus dissapears
- centrioles migrate to opposite poles in cell+start forming spindle fibres
metaphase 1
- homologous pairs of chromosomes line up along equator
- orientation of each homologous pair is random+independent
- paternal+maternal chromosomes face opposite poles in each pair=independent assortment=genetic variation due to lots of combinations
- Each chromosome attaches to a spindle fibre by their centriole.
anaphase 1
- spindle fibres contract+shorten=homologous pairs of chromosomes are pulled to opposite poles+chromatids are still joined tgt
telophase 1
- chromosomes reach opposite poles of cell+uncoil
- Nuclear envelope forms around each set of chromosomes+nucleous starts to reform
- Cell undergoes cytokinesis=divides into 2 calls.
- Reduction from diploid to haploid
MEIOSIS 2
prophase 2
- Chromosomes condense = visible under microscope
- centrioles migrate to opposite poles (90 degrees from old one)+start forming spindle fibres
- nuclear envelope disintegrates+nucleolus disappears
metaphase 2
- chromosomes line up at equator
- each chromosome attaches to the spindle by their centromere
anaphase 2
- centromeres divide+separate each pair of chromatids
- spindle fibres contract+shorten=pulling chromatids to opposite poles of cell=4 groups of chromosomes
telophase 2
- chromatids reach opposite poles of cell as chromosomes=uncoil
- nuclear envelope reforms around each set of chromosomes+nucleolus reappears
- cytokinnesis occurs=4 cells produced which genetically diff from eachother
Significance of meiosis in life cycles
Crossing over
Crossing over is the process by which non-sister chromatids exchange alleles
Process:
During meiosis I homologous chromosomes pair up and are in very close proximity to each other
The non-sister chromatids can cross over and get entangled
These crossing points are called chiasmata
The entanglement places stress on the DNA molecules
As a result of this a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes
There is usually at least one, if not more, chiasmata present in each bivalent during meiosis
Crossing over is more likely to occur further down the chromosome away from the centromere
Independent assortment
Independent assortment is 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
The different combinations of chromosomes in daughter cells increases genetic variation between gametes
In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle
Each pair can be arranged with either chromosome on top, this is completely random
The orientation of one homologous pair is independent / unaffected by the orientation of any other pair
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
Production of haploid cells=allows sexual reproduction to occur
Cell organisation
Specialised cells - These are cells with certain features that allow them to carry out a particular function.
Tissue - This is a group of similar cells working together to carry out a particular function.
Organ - This is a group of tissues working together to carry out a particular function.
Organ system - This is a group of organs working together to carry out a particular function.
Examples of specialised cells
ERYTHROCYTE
Flattened biconcave shape - This increases the surface area to volume ratio to allow diffusion of oxygen.
No nucleus or organelles - This provides more room for haemoglobin (the molecule that binds to oxygen).
Flexible - This is so they can fit through narrow capillaries.
NEUTROPHIL
Flexible cell membrane - This allows the cell to engulf pathogens.
Contain many lysosomes - These contain digestive enzymes to break down engulfed particles.
Multi-lobed nucleus - This allows cells to deform so they can squeeze through small gaps to reach sites of infection.
SPERM
Flagellum (tail) - This allows the cell to swim to the egg cell.
Many mitochondria - These supply the energy needed for movement.
Acrosome containing digestive enzymes - These digest the protective layers around the egg cell to allow the sperm cell to enter it.
SQUAMOUS EPITHELIAL
Very thin - This allows efficient diffusion of gases such as oxygen and carbon dioxide.
Permeable - This allows diffusion of gases.
CILIATED EPITHELIAL
Cilia (hair-like structures) - These beat to move pathogens and mucus away from the lung or egg cells towards the uterus.
PALISADE
Lots of chloroplasts - These absorb the light needed for photosynthesis.
Thin cell walls - This allows carbon dioxide to quickly diffuse into the cell.
Tall and thin shape - This allows many palisade cells to closely pack together to form a continuous layer near the surface of the leaf.
ROOT HAIR
Root hair structures - These increase the surface area for absorption.
Thin, permeable cell wall - This allows entry of water and ions.
Lots of mitochondria - These provide energy for active transport.
GUARD
Come in pairs - This allows a gap (stoma, plural: stomata) to form between them.
Change shape when light is present - Guard cells absorb water to become turgid, opening the stoma to allow entry of carbon dioxide.
Change shape when they lose water - Guard cells shrink and close the stoma to prevent water loss.
Thin outer walls and thick inner walls - This allows the cells to bend when they are turgid to open the stoma.
Examples of tissues
SQUMOUS EPITHELIUM
Squamous epithelium tissue provides a thin lining for many organs such as the lungs.
This tissue is made up of a single layer of squamous epithelial cells. Due to being only one cell thick, gases can quickly diffuse through the tissue.
CILIATED EPITHELIUM
Ciliated epithelium tissue lines organs such as the trachea where it can sweep mucus away from the lungs.
This tissue is made up of ciliated epithelial cells and goblet cells. The goblet cells release mucus to trap pathogens, whilst the ciliated epithelial cells use cilia to sweep the mucus away.
CARTILAGE
Cartilage is a type of connective tissue that acts as a cushion between bones and also provides support to organs such as the ears and nose.
This tissue is made up of chondrocyte cells fixed within an extracellular matrix.
MUSCLE
Muscle tissue is made up of muscle fibres (bundles of elongated cells). These fibres contract (shorten) and relax to move different parts of the body.
There are three types of muscle tissue:
Smooth - Found in the walls of organs.
Cardiac - Found in the heart.
Skeletal - Found attached to bones.
XYLEM
Xylem tissue is responsible for the transport of water and minerals within plants.
It is made up of dead xylem vessel cells which have no end walls and no organelles. This forms a continuous column through which water can flow. The walls of these cells are strengthened by a waterproof material known as lignin.
PHLOEM
Phloem tissue is responsible for the transport of sugars and amino acids within plants.
It is made up of columns of sieve tube elements and companion cells.
The sieve tube element cells are separated by sieve plates with holes so that sugars can pass through. Sieve tube elements contain very few organelles, allowing sugars to flow easily.
Companion cells contain many mitochondria to release energy.
Stem cells definition
undifferentiated cell, not adapted to any particular function+can differentiate to become a specialised (renewing sources undifferentiated cells)
- undergo cell division (mitosis) repeatedly until they become specialised
Levels of potency in stem cells
Totipotent stem cells - These can differentiate into any cell type and go on to form whole organisms.
Pluripotent stem cells - These can differentiate into most cell types, but cannot form whole organisms.
Multipotent stem cells - These can only differentiate into a few different cell types.
Unipotent stem cells - These can only differentiate into one type of cell.
Where to find stem cells
stem cell saurcesiinations
- EMBRYONiC STEMCELLS.
• round in early stages of emoryo development where the um disserentiate into celstosorma goetus
• First divisions, cell=totipotent outaster days damell-
pluripotent
- ADULT STEM CELLS
• Found in some adult tissue where they can replace gavity cells.
• Multipotent or unipotent
• stemiAcells in bone marrow replace enythro cyres (nac)+ neutrophils
- PLANT STEM CELLS
• Foundin moristematic tissue/ meristemsat tips og shoots or
4 roots+ in vascular cambiom (ou phloem + xgiem)
Punpotent
• Diggerentiate into xylem + phoem dells
Uses of stem cells
- repairing damaged tissues eg. heart disease (repairing damaged muscle tissue)
- treating neurological diseases eg. Alzheimer’s=loss of nerve cells —> transplanted stem cells help regenerate nerve cells+down symptoms
- Research into development of organisms=scientists can find how organisms grow+develop from a single cell
