Cell Division (cell cycle, mitosis, meiosis, organisation/specialisation of cells, stem cells) Flashcards
Phases of the cell cycle in eukaryotes?
Interphase, mitotic (division) phase
What phase does a cell spend the most time in?
Interphase
What happens during interphase?
- DNA is replicated and checked for errors in the nucleus
- Protein synthesis occurs in the cytoplasm
- Mitochondria grow and divide, increasing in number in the cytoplasm
- Chloroplasts grow and divide in plant and algal cell cytoplasm, increasing in number
- The normal metabolic processes of cells occur (some, including cell respiration, also occur throughout cell division)
What are the three stages of interphase?
G1, S, G2
What happens during the G1 phase of interphase?
The first growth phase: proteins from which organelles are synthesised and produced and organelles replicate. The cell increases in size
What happens during the S phase of interphase?
Synthesis phase: DNA is replicated in the nucleus
What happens during the G2 phase of interphase?
The second growth phase: the cell continues to increase in size, energy stores are increased and the duplicated DNA is checked for errors
What is the mitotic phase of the cell cycle?
The period of cell division, it has two stages: mitosis and cytokinesis
What is cytokinesis?
The cytoplasm divides and two cells are produced
What happens in the phase G0?
This is the stage where the cell leaves the cell cycle (at the end of G1), either temporarily or permanently. This could be because of differentiation, damaged DNA, or the age of a cell (senescent cells - cells can only divide a certain number of times)
What are the control mechanisms in the cell cycle called? What do they ensure?
They’re called checkpoints, they ensure that each phase of interphase has been accurately completed before moving onto the next phase
What happens during the G1 checkpoint?
The checkpoint is at the end of the G1 phase, before entry into S phase. It checks for: cell size, nutrients, growth factors, and DNA damage
What happens during the G2 checkpoint?
This checkpoint is at the end of the G2 phase, before the start of the mitotic phase. It checks for: cell size, DNA replication and DNA damage
What happens during the spindle assembly / metaphase checkpoint (during mitosis)? Why is it important?
It checks to see if the chromosomes have been attached to the spindle. Its important because mitosis cannot proceed until this checkpoint has been passed
What class of enzyme brings about the passing of a checkpoint? What do they do?
Kinases - they catalyse the addition of a phosphate group to a protein (phosphorylation). This changes the tertiary structure of checkpoint proteins, activating them at certain points in the cell cycle
What disease is caused by cells dividing uncontrollably?
Cancer
What is the end product of mitosis?
Two genetically identical daughter cells
Is mitosis haploid or diploid?
Diploid
What type of cells does mitosis make?
Somatic (body) cells
Does crossing over occur in mitosis?
No
Reasons for mitosis occuring?
Growth, repair, replacement and asexual reproduction
What are chromatids?
A chromatid is one of two strands of a copied chromosome. Chromatids that are joined together at their centromeres are called sister chromatids. These chromatids are genetically identical. Chromatids are formed in both the cellular division processes of mitosis and meiosis
What are the 4 stages of mitosis?
Prophase, metaphase, anaphase and telophase
What happens during prophase?
Chromatin condenses into chromosomes, and the nuclear envelope, or membrane, breaks down. In animal cells, the centrioles near the nucleus begin to separate and move (using protein microtubules, which link the poles) to opposite poles (sides) of the cell
What happens during metaphase?
During metaphase, chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate, and then held in position
What happens during anaphase?
The centromeres holding together the pairs of chromatids in each chromosome divide during anaphase. The chromosomes are pulled to opposite poles of the cell by the shortening of spindle fibres
What happens during telophase?
In telophase, the chromatids have reached the poles and are now called chromosomes. The two new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them. The chromosomes start to uncoil and the nucleolus is formed
What is a cleavage furrow?
The formation around the middle of a cell during cytokinesis - the cell-surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming two cells
Cytokinesis in plants?
Plant cells have cell walls, so its not possible for a cleavage furrow to form. Vesicles from the golgi apparatus begin to assemble the the same place as where the metaphase plate was formed. The vesicles fuse with each other and the cell surface membrane, dividing the cell in two. New sections of the cell wall then form along the new sections of membrane
What is meiosis?
A form of cell division that forms gametes (haploid, therefore is known as a reduction division)
What is a zygote?
A fertilised egg
What are the stages of meiosis?
- Meiosis I: prophase I, metaphase I, anaphase I and telophase I
- Meiosis II: prophase II, metaphase II, anaphase II and telophase II
What happens during mitosis I?
The first division is the reduction 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
What happens during meiosis II?
The second division is similar to mitosis, and the pairs of chromatids present in each daughter cell are separated, forming two more cells
Products of meiosis
Four haploid daughter cells (gametes)
Does interphase happen before meiosis?
Yes, and the same processes precede it as mitosis
What happens during prophase I?
Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
What happens during metaphase I?
Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell (equator)
What happens during anaphase I?
Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
What happens during telophase I?
Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
What happens during prophase II?
Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
What happens during metaphase II?
Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) to align them along the cell equator
What happens during anaphase II?
Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
What happens during telophase II?
Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
What is crossing over?
The exchange of genes between homologous chromosomes, resulting in a mixture of parental characteristics in offspring (shuffling of genes)
When may interkinesis, a second growth phase, happen?
It may occur between meiosis I and II, no DNA replication occurs in this stage
Levels of organisation in multicellular organisms?
Specialised cells → Tissues → Organs → Organ systems → Whole organisms
How are erythrocytes adapted to their function?
Biconcave disc shape (increases SA:V), no nucleus or organelles (more room for haemoglobin, which binds and carries oxygen), flexible so can squeeze through capillaries
How are neutrophils adapted to their function?
Multi-lobed nucleus (easy to squeeze through gaps, get to site of infection), granular cytoplasm contains many lysosomes (these contain enzymes used to attack pathogens)
How are sperm cells adapted to their function?
Tail/flagellum (allows movement), contain many mitochondria (supplies energy needed to swim towards the egg), acrosome on its head contains digestive enzymes (released to digest the protective layers around the ovum/egg and allow the sperm to penetrate, leading to fertilisation)
How are palisade cells adapted to their function?
Contain chloroplasts (to absorb light for photosynthesis), shape of cells (rectangular box) allows for close packing to form a continuous layer, thin cell walls (increased rate of diffusion of CO₂), large vacuole (maintains turgor pressure), chloroplasts can move within the cell to obtain more light
How are root hair cells adapted to their function?
Root hairs increase surface area of the cell, maximising uptake of water and minerals from the soil
How are (pairs of) guard cells adapted to their function?
When they lose water, they become less swollen, as a result of osmotic forces, and they change shape and the stoma closes to prevent further water loss from the plant. The cell wall is thicker on one side so the cell does not change shape symmetrically as its volume changes
What are the four main categories of tissues found 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 other tissues together or as a transport medium
What is squamous epithelium?
A type of specialised animal tissue, made up of squamous epithelial cells, flat appearance, thin due to squat/flat cells (one cell thick), present when rapid diffusion across a surface is essential, forms the lining of lungs (allows rapid diffusion of oxygen into the blood)
What is ciliated epithelium
A type of specialised animal tissue, made up of ciliated epithelial cells, have cilia, line the trachea (allow mucus to be swept away from the lungs), goblet cells present release mucus (trap unwanted particles present in the air)
What is cartilage?
A connective tissue found in the outer ear, nose, and ends of (and between) bones. Contains fibres of the proteins elastin and collagen, its firm and flexible, composed of chondrocyte cells embedded in an extracellular matrix, prevents ends of bones from rubbing together and causing damage
What is muscle?
A tissue that needs to be able to shorten in length (contract) in order to move bones, which in turn move different parts of the body. Skeletal muscle fibres (muscles which are attached to bone) contain myofibrils, which contain contractile proteins
What is the epidermis?
A single layer of packed cells covering the surface of plants. Covered by a waxy, waterproof cuticle to reduce loss of water. Somata are present on the epidermis, allowing gas exchange
What is xylem tissue?
A type of vascular tissue responsible for the transport of water and minerals throughout plants. Composed of vessel elements, which are elongated, dead cells. Walls are strengthened with a waterproof material, called lignin, which provides plants with structural support
What is phloem tissue?
A type of vascular tissue in plants, responsible for the transport of organic nutrients, particularly sucrose, from leaves and stems where it is needed. It is composed of columns of sieve tube cells separated by perforated walls called sieve plates
What is an organ?
A collective of tissues that are adapted to perform a particular function in an organism (e.g. mammalian heart is adapted to pump blood around the body). Made up of muscle tissue and connective tissue. The leaf is a plant organ adapted for photosynthesis, and contains epidermis tissues and vascular tissue
What is an organ system?
A system composed of a number of organs working together to carry out a major function in the body
What does the digestive system do?
Takes in food, breaks down large insoluble molecules into smaller soluble ones, absorbs nutrients in the blood, retains water needed by the body, and removes any undigested material from the body
What does the cardiovascular system do?
Moves blood around the body to provide an effective transport system for the substances its carries
What does the gaseous exchange system do?
It brings air into the body so oxygen can be extracted for respiration, and carbon dioxide can be expelled
What are stem cells?
Unspecialised cells that are capable of division and differentiation to become other cells types
Two key qualities of stem cells?
- Self renewal - they can continuously divide and replicate
- Potency - they have the capacity to differentiate into specialised cell types
Link the behaviour of stem cells to the cell cycle…
When a cell differentiated and becomes specialised, it loses capacity to form alternative cell types and the ability to divide - it enters G0
Link differentiation to the cell cycle…
On differentiation, some of the genes in the genome genes are switched on and some are switched off
The cell cycle and division is carefully controlled. What happens when it is A) too slow, and B) too rapid?
A) ageing
B) tumours
Why is self-renewal important?
It maintains the stem cell pool
Why is cell differentiation important?
It replaces dead or damaged cells throughout your life
What is a totipotent stem cell?
A stem cell that can differentiate into any cell type, source: fertilised egg/zygote/8-16 cells of first mitotic divisions
What is a pluripotent stem cell?
A stem cell that can differentiate into all tissue types, but not whole organisms (origin of different tissue types). Source: present in early embryos/blastocysts
What is a multipotent stem cell?
A stem cell that can form a range of cells within a tissue, source: many regions, e.g. bone marrow, cord blood, adipose tissue, skin, testicles, intestine, brain, cardiac cells, bone marrow, and mesenchymal stem cells (MCSs, found in wisdom teeth)
What are haematopoietic stem cells?
Multipotent blood stem cells located in bone marrow (in colonies), can differentiate itself into several types of blood cell types (lymphocytes, monocytes, neutrophils, etc…). Cannot differentiate into brain cells, bone cells or other non -blood cell types
Why is scientific research of stem cells considered controversial?
Embryonic stems cells are used, and this takes away a potential life
What is the meristem of a plant? Give examples of this tissue
Where plant stem cells are found, apical meristems
How is the potency described of plant stem cells found in the cambium?
Multipotent
