2.1.6 - CELL DIVISION + CELLULAR ORGANISATION Flashcards
What does the cell cycle consist of?
Interphase and the M phase (mitosis and cytokinesis)
What is interphase divided into?
- Gap phase 1 (G₁)
- Synthesis
- Gap phase 2 (G₂)
What are the four stages in Mitosis?
- Prophase
- Metaphase
- Anaphase
- Telophase
What happens during interphase?
- Cell carries out normal functions, but prepares for division
- Cell’s DNA is unravelled + replicated to double genetic content
- Organelles also replicated so it has spare ones
- ATP content increased (to provide energy for cell division
What happens during prophase?
- Chromosomes condense, getting shorter and fatter
- Tiny bundles or protein (centrioles) start moving to opposite ends of the cell, forming a network of proteins fibres across it called the spindle
- Nuclear envelope breaks down + chromosomes free in the cytoplasm
What happens during metaphase?
- Chromosomes (each with two chromatids) line up along the middle of the cell and become attached to the spindle by their centromere
- At this metaphase checkpoint, the cell checks that all the chromosomes are attached to the spindle before mitosis can continue
What happens during anaphase?
- The centromeres divide, separating each pair of sister chromatids
- The spindles contract, pulling chromatids to opposite ends of the cell, centromere first
What happens during telophase?
- The chromatids reach the opposite poles on the spindle
- They uncoil and become long and thin again
- They’re now called chromosomes again
- A nuclear envelope forms around each group of chromosomes, so there are now two nuclei
What happens during cytokinesis?
- The cytoplasm divides
- In animal cells, a cleavage furrow forms to divide forms to divide the cell membrane
- There are now two daughter cells that are genetically identical to the original cell and to each other
- Cytokinesis usually begins in anaphase + ends in telophase
- SEPARATE TO MITOSIS
What is a centromere?
This joins two strands (chromatids) in the middle to make up a chromosome during interphase
What is mitosis?
Asexual reproduction of a cell. Results in two identical daughter cells
What is meiosis?
Sexual reproduction of a cell to produce gametes. Results in four haploid cells
What are gametes?
The sex cells (sperm cells in males and egg cells in females)
What is a reduction division?
Cells that divide by meiosis have the full number if chromosomes to start with, but the cells that are formed from meiosis have half the number in the end
Explain what is meant by haploid
Haploid cells are cells with half the normal number of chromosomes
Explain what is meant by diploid
A cell or organism that contains two complete sets of chromosomes (23 pairs of chromosomes or 46 chromosomes)
What are the two divisions of meiosis?
- Meiosis I (this is the reduction division - halves the chromosome number)
- Meiosis II
What are the stages of Meiosis I?
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
What are the stages of Meiosis II?
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
What happens during prophase I?
- Chromosomes condense (getting shorter and fatter)
- The chromosomes then arrange themselves into homologous pairs and crossing over occurs
- Centrioles start moving to opposite ends of the cell, forming spindle fibres
- The nuclear envelope breaks down
What are homologous pairs?
Pairs of chromosomes originating from each parent (one from the mother and one from the father)
(Imagine two chromosomes sitting next to each other in class so they get picked to be in the same group)
What is crossing over?
- Occurs between chromatids
- Chromatids from separate chromosomes (bit are in a pair) twist around each other and bits of chromatids swap over
- The chromatids still contain the same genes, but now have a different combination of alleles
What happens during metaphase I?
- The homologous pairs line up across the centre of the cell and attach to the spindle fibres by the centromeres
What happens during anaphase I?
- The spindles contract, separating the homologous pairs (one chromosome from each pair goes to each end of the cell)
What happens during telophase I?
A nuclear envelope forms around each group of chromosomes
What happens during prophase II?
- Chromosomes condense, getting shorter and fatter
- Tiny bundles or protein (centrioles) start moving to opposite ends of the cell, forming a network of proteins fibres across it called the spindle
- Nuclear envelope breaks down + chromosomes free in the cytoplasm
What happens during metaphase II?
- Chromosomes (each with two chromatids) line up along the middle of the cell and become attached to the spindle by their centromere
- At this metaphase checkpoint, the cell checks that all the chromosomes are attached to the spindle before mitosis can continue
What happens during anaphase II?
- The pairs of sister chromatids are separated - each new daughter cell inherits one chromatid from each chromosome
- Four (genetically different) haploid daughter cells are produced - these are the gametes
What happens during telophase II?
- The chromatids reach the opposite poles on the spindle
- They uncoil and become long and thin again
- They’re now called chromosomes again
- A nuclear envelope forms around each group of chromosomes, so there are now two nuclei
How many cells does meiosis produce?
Four
What happens to homologous pairs of chromosomes during meiosis I?
- They cross over during prophase I, where chromatids respective to each chromosome twist around each other and swap chromatids
- These chromosomes contain the same genes, but have a different combination of alleles
What two main events during meiosis lead to genetic variation?
- Crossing over of chromatids
- Independent assortment of chromosomes
What is independent assortment of chromosomes?
- Each homologous pair of chromosomes in our cells is made up of one chromosome from our mother and one chromosome from our father
- When the homologous pairs line up in metaphase I + are separated in anaphase I, it’s completely random which chromosome from each pair ends up in which daughter cell
- So the four daughter cells produced by meiosis have completely different combinations of those maternal and paternal chromosomes
- This ‘shuffling’ of chromosomes leads to genetic variation in any potential offspring
What are stem cells?
Unspecialised cells that can differentiate into different types of cell
What are stem cells used for in animals?
To replace damaged cells (e.g. to make new skin or blood cells)
What are stem cells used for in plants?
To make new shoots and roots throughout their lives
Stem cells can differentiate into various plant tissues including xylem and phloem
Stem cells in bone marrow can differentiate into other cell types. Name two of these cell types
- Erythrocytes (red blood cells)
- Neutrophils (white blood cells that help to fight infection)
Where are stem cells found in plants?
Meristems
What cell types can stem cells in the root and stem differentiate into?
- Xylem vessels
- Phloem sieve tubes
Describe how stem cells might be used to treat Alzheimer’s
- With Alzheimer’s, nerve cells in the brain die in increasing numbers
- This results in severe memory loss
- Researchers help to use stem cells to regrow healthy nerve cells in people with Alzheimer’s
Describe how stem cells might be used to treat Parkinson’s
- Patients with Parkinson’s suffer from tremors that they can’t control
- The disease causes the loss of a particular type of nerve cell found in the brain
- These cells release a chemical called dopamine, which is needed to control movement
- Transplanted stem cells may help to regenerate the dopamine-producing cells
Name four specialised animal cells
- Neutrophils
- Erythrocytes
- Epithelial cells
- Sperm cells
Name three specialised plant cells
- Palisade mesophyll cells
- Root hair cells
- Guard cells
Describe how neutrophils are specialised for their functions
- Defend the body against disease
- Flexible shape allows them to engulf foreign particles or pathogens
- The many lysosomes in their cytoplasm contain enzymes to break down the engulfed particles
Describe how erythrocytes are specialised for their functions
- Carry oxygen in the blood
- Biconcave disc shape provides a large surface area for gas exchange
- No nucleus so there’s more room for haemoglobin (protein that carries oxygen)
Describe how epithelial cells are specialised for their functions
- Epithelial cells cover the surfaces of organs
- Cells joined by interlinking cell membranes + a membrane at their base
- Ciliated epithelia (e.g. in the airways) have cilia that beat to move particles away
- Squamous epithelia (e.g. in the lungs) are very thin to allow efficient diffusion of gases
Describe how sperm cells are specialised for their functions
- Have a flagellum (tail) so they can swim to the egg
- They also have a lot of mitochondria to provide energy to swim
- Acrosome (head or part of the head) contains digestive enzymes to enable the sperm to penetrate egg surface
Describe how palisade mesophyll cells are specialised for their functions
- Present in leave + do most of photosynthesis
- Contain many chloroplasts, so they can absorb a lot of sunlight
- Thin walls so carbon dioxide can easily diffuse into the cell
Describe how root hair cells are specialised for their functions
- Absorb water and mineral ions from the soil
- Large surface area for absorption + thin, permeable cell wall for entry of water + ions
- Cytoplasm contains extra mitochondria to provide energy for active transport
Describe how guard cells are specialised for their functions
- Found in pairs, with a gap between them to form a stoma
- In the light, guard cells take up water + become turgid
- Their thin outer walls + thickened inner walls force them to bend outwards, opening the stomata (allows leaf to exchange gases for photosynthesis
Definite what is meant by a tissue.
A group of cells, plus any extracellular material secreted by them, that are specialised to work together to carry out a particular function
State and briefly describe four animal tissues
- Squamous epithelium - a single layer of flat cells lining a surface (found in many places like alveoli)
- Ciliated epithelium - a layer of cells covered in cilia (found on surfaced where things need to be moved - in trachea where cilia move mucus)
- Muscle tissue - made up of bubbles of elongated cells called muscle fibres | three types: smooth (found in stomach lining), cardiac (found in heart), skeletal (used to move)
- Cartilage - type of connective tissue found in joints - shapes and supports ears, nose and windpipe | formed when cells called chondroblasts secrete an extracellular matrix which they become trapped inside
State and briefly describe two plant tissues
- Xylem tissue - has two jobs: transport water around plant + supports the plant | contains hollow xylem vessel cells (dead) and living parenchyma cells
- Phloem tissue - transports sugars around plant | arranged in tubes + is made up of sieve cells,ompanion cells and some ordinary plant cells | each sieve cell has end walls with holes in them, so that sap can move easily through them
Define what is meant by an organ
A group of tissues that work together to perform a particular function
Name and explain one organ found in plants and one organ found in animals
- Leaves - contain palisade tissue for photosynthesis, as well as epidermal tissue (to prevent water loss from the leaf), and xylem + phloem tissues in the veins
- Lungs - contain squamous epithelial tissue (in the alveoli) + ciliated epithelial tissue (in the bronchi, etc). They also have elastic connective tissue + vascular tissue (in the blood vessels)
Define what is meant by an organ system
Organs work together to form organ systems - each system has a particular function
Name two organ systems and explain them
- Respiratory system - made up of organs, tissues and cells involved in breathing: lungs, trachea, larynx, nose, mouth and diaphragm
- Circulatory system - made up of the organs involved in blood supply: heart, arteries, veins and capillaries
Define totipotent
Can divide and differentiate into any type of cell
Define pluripotent
Can self-renew and differentiate into any type of cell except the cells that make up the placenta
Define multipotent
Can only differentiate and divide into a limited number of cell types
Define unipotent
Can only differentiate into a single type of cell e.g. cardiomyoblasts can only differentiate into cardiomyocytes
When can totipotent stem cells be used?
During the first few cell divisions of an embryo
What can pluripotent stem cells be used for?
To replace cells and treat human disorders like leukaemia and diabetes
What happens at the G₁ checkpoint?
- Chromosomes are checked for damage
- If damage is detected, the cell does not advance to S phase until the damage is repaired
What happens at the S checkpoint?
- Chromosomes are checked to ensure they’ve been replicated
- If they haven’t all be successfully replicated, the cell cycle stops
What happens at the G₂ checkpoint?
- An additional check for DNA damage occurs after DNA replication
- Cell cycle delayed until necessary repairs have been made
What happens at the metaphase checkpoint?
Final check determines whether the chromosomes are corrected attached to the spindle fibres before anaphase
What happens during G₁ phase in interphase?
- A signal is received telling the cell to divide again
- Cells make RNA, enzymes and other proteins required for growth
What happens during S phase in interphase?
- Relatively short
- DNA in the nucleus replicates - results in each chromosome consisting of two identical sister chromatids
What happens during G₂ phase in interphase?
- Cell continues to grow and the new DNA that has been synthesised is checked and any errors are repaired
