Cell division, diversity and organisation Flashcards
Cell division
-eukaryotic cells enter the cell cycle and divide by either mitosis or meiosis
-prokaryotic cells replicate through binary fission
- viruses do not undergo cell division as they are non living
What are the 3 stages of the cell cycle?
-Interphase(G1, s and G2)
-nuclear division(mitosis or meiosis)
-cytokinesis
Interphase
-longest stage in the cell cycle
Interphase :G1
-protein synthesis occurs to make proteins involved in synthesising organelles
-organelles replicate
-cell is checked that it is the correct size, has the correct nutrients, growth factors and that there is no damaged DNA.
-if cell does not pass these checks, replication will not continue
Interphase: s
-DNA is replicated
Interphase: G2
-cell continues to grow, energy stores increase and the newly replicated DNA is checked for copying errors
What are checkpoints?
-monitor and verify if each process of the cell cycle have been accurately completed.
G1 checkpoint
-end of the G1 phase, before entry into the S phase
-if cell does not satisfy requirements it will enter G0(Resting phase)
G2 checkpoint
-end of the G2 phase, before the start of the mitotic phase. cell needs to check if DNA has been replicated without errors
Mitosis
-created 2 identical diploid cells
-used for growth, tissue repair and asexual reproduction in plants, animal and fungi.
What are the 4 stages of mitosis
-prophase
-metaphase
-anaphase
-telophase
PMAT
Prophase in mitosis
-chromosomes condense and become visible
-in animals, the centrioles separate and move to opposite poles of the cell
-centrioles create spindle fibres which are released from both poles to create a spindle apparatus which will later attach to the centromere and chromatids on the chromosome
-plants have a spindle apparatus but lack centrioles
-nuclear envelope disintegrates and nucleus fades away
Metaphase in mitosis
-chromosomes align along the equator of the cell(metaphase plate)
-spindle fibres are released from the centrioles and attach to the centromere and the chromatids
-spindle assembly checkpoint occurs in this stage( it ensures every chromosome has attached to a spindle fibre before anaphase)
Anaphase in mitosis
-spindle fibres begin to shorten and move towards the centrioles and pull the centromere and chromatids they are bound to towards the opposite poles
-centromere divides and the individual chromatids are pulled to each opposite pole
-stage requires energy in the form of ATP which is provided by respiration in the mitochondria
Telophase in mitosis
-chromosomes are now at each pole of the cell and become longer and thinner again
-spindle fibres disintegrate and the nuclear membrane reforms
-the nucleolus also forms
Cytokinesis
- the cytoplasm splits into 2 genetically identical cells
-in animals a cleavage furrow forms in the middle of the cell and the cytoskeleton causes the cell membrane to draw inwards until the cell splits into two
-in plant cells, the cell membrane splits into 2 new cells due to the fusing of vesicles from the Golgi apparatus. cell wall forms new sections around the membrane to complete the division into two new cells
Observing mitsosis
- the stages of mitosis are visible under a light microscope in onion and garlic root tips
- a thin slice of the root tip is placed on a microscope slide and broken down with a needle
-a stain is added to make the chromosomes visible and the cover slip is pushed down. This is to squash the tip to achieve a single layer of cells so light can pass through
What is the mitotic index?
-it can be calculated by counting how many cells are visible in the field of view and the number of cells visible that are in the stage of mitosis
formula can be used to calculate the mitotic index:
the number of cells in mitosis/the total number of cells
X100
Meiosis
meiosis is 2 nuclear divisions resulting in 4 genetically different haploid daughter cells
haploid(n): one copy of each chromosome
diploid(2n):2 copies of each chromosome
Meiosis I
-pairs of homologous chromosomes are separated into 2 cells. Each cell will only contain one set full of genes so the cells are haploid
Meiosis II
-pairs of chromatids present in each daughter cell are separated, forming 2 or more cells. 4 haploid cells are produced in total
prophase 1
-chromosomes condense, nuclear envelope disintegrates, nucleolus forms and spindle formation begins
-homologous chromosomes pair up, forming bivalents and crossing over occurs
Metaphase 1
-homologous chromosomes line up in the metaphase plate
-orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair. Maternal and paternal chromosomes can end up facing either pole(independent assortment)
-results in many different combinations of alleles facing the pole.
-independent assortment of chromosomes in metaphase 1 results in genetic variation
Anaphase 1
-homologous chromosomes are pulled to opposite poles and the chromatids stay joined to each other
-sections of DNA on ‘sister’ chromatids break off and re join- resulting in an exchange of DNA. They break off at the chiasma
-recombinant chromatids are formed with genes being exchanged between chromatids
-combination of alleles on the recombinant chromatids will be different from the allele combination on either the original chromatids.
-genetic variation arises
Telophase 1
-same as telophase in mitosis
-chromosomes assemble at each pole and the nuclear membrane reforms
-chromosomes uncoil
-cell undergoes cytokinesis and divides into 2 cells. Cell becomes diploid to haploid
Prophase 2
-the chromosomes which have 2 chromatids condense and become visible again
-nuclear envelope breaks down and spindle formation begins
Metaphase 2
-individual chromosomes assemble on the metaphase plate
-chromosomes are no longer identical so there is independent assortment again and more genetic variation
Anaphase 2
-chromatids of the individual chromosomes are pulled to opposite poles after division of the centromeres(same as anaphase in mitosis)
Telophase 2
-chromatids assemble at the poles of telophase 2 as in telophase in mitosis
-chromosomes uncoil and form chromatin again
-nuclear envelope reforms and the nucleolus becomes visible
-cytokinesis results in the cell dividing into 4 haploid cells
-cells become genetically different due to crossing over and independent assortment
How are multicellular organisms organised?
Cells-tissue-organ-organ systems-entire organism
What are stem cells?
-stem cells are undifferentiated cells that can self renew and become specialised
-There are different types of stem cells:
Totipotent, Pluripotent, multipotent and unipotent
What is a totipotent cell?
-can divide and produce almost any type of body cell
-can translate only part of their DNA resulting in cell specialisation.
-can only occur for a limited time in embryos
What is a pluripotent cell?
-found in embryos and can become almost any type of cell
-can form tissue types but not whole organisms
What is a multipotent cell?
-can only form a range of cells within a certain type of tissue
-for example, in bone marrow, they can differentiate into a limited number of cells
What is a unipotent cell?
-can only differentiate into one type of cell
what are the sources for animal stem cells?(embryo)
-embryonic stem cells(present at an early stage of embryo development and are totipotent
-a mass of cells called a blastocyst is formed and cells are in a pluripotent state
-remain in this state as a fetus until birth
Where are the sources for animal stem cells? (tissue-adult stem cells)
-found in the bone marrow
-multipotent
Sources of plant stem cells
-present in the meristematic tissue. This tissue is found in the roots and the shoots
-also located between the phloem and xylem tissue and it is called the vascular cambium
-cells can differentiate into the cells present in the xylem and phloem tissue
What are the uses of stem cells?
-type 1 diabetes
-Parkinson’s disease
-Alzheimer’s disease
