2.3 + 2.4 - cell division Flashcards
how do chromosomes appear when the cell is not actively dividing
chromosomes are translucent to light and electrons
so we cannot see them under a microscope
how do chromosomes appear when the cell is actively dividing
chromosomes condense, becoming shorter and denser
they can now take up stain easily so can be seen under a light microscope
describe the role of histones during the condensation of chromosomes
-histones are positively charged basic proteins
-DNA winds around a histone to form nucleosomes (dense clusters)
-nucleosomes combine in a chain and coil, then supercoil to form chromatin
-this chromatin winds up to form a dense chromosome visible under a microscope
-in supercoiled areas genes are not able to be copied to make proteins
What is the function of mitosis
Growth, repair and asexual reproduction
Describe interphase
-a period of non-division where the cell increases in size, mass, carries out normal cellular activities, replicates DNA and organelles to have 2 copies of everything
what are the stages of the cell cycle
G1, G1 checkpoint, S, G2, G2 checkpoint, M, metaphase checkpoint, C
G1, S, G2 are part of interphase
what occurs during G1 phase
-organelles replicate
-size of cell increases and mass of cell increases
what occurs during S phase
-chromosomes replicate becoming double stranded chromatids
what occurs during G2 phase
-organelles replicate
-chromosomes are checked and repaired
-size and mass of cell increase
what chemical controls the cell cycle
-cyclins (protein)
-cyclins build up during interphase and attach to cyclin-dependent kinases (another protein)
-the cyclin-CDK complex phosphorylates other proteins, changing their shape in order to bring about the next phase of the cell cycle
describe prophase
-2 sister chromatids are joined together at the centromere
-chromosomes condense to become visible
-nucleolus and nuclear membrane break down
-the centrioles pull apart to begin to form spindle fibres
describe metaphase
-centrioles move to opposite poles of the cell forming spindle fibers which attach to the centromere of the chromosomes
-spindle fibers move the chromosomes to line up along the metaphase plate
describe anaphase
-the centromeres that originally linked the 2 chromatids split
-spindle fibres contract
-the chromatids from each pair are pulled to opposite poles of the cell
describe telophase
-spindle fibers break down
-nuclear envelopes form around the 2 sets of chromosomes
-nucleoli and centrioles reform
-chromosomes begin to unravel, becoming less dense and harder to see
describe cytokinesis in animal cells
-a ring of contractile fibers tighten around the centre of the cell until 2 cells have been separated
-2 genetically identical daughter cells are formed with a diploid nucleus (46 chromosomes)
describe cytokinesis in plant cells
-a cellulose cell wall builds up from the inside of the cell outwards
-2 genetically identical daughter cells are formed with a diploid nucleus
what are the advantages and disadvantages of asexual reproduction
advantages
-doesn’t rely on finding a mate
-less energy needed
-many offspring are produced
disadvantages
-no genetic variation in the species, so if conditions change becoming unfavorable the hole population is affected
name strategies for asexual reproduction
sporulation
fragmentation
budding
vegetative propagation
describe sporulation
the organism undergoes mitosis which produces asexual spores
these asexual spores grow into new organisms
describe fragmentation
organisms reproduce from fragments of themselves
describe budding
there is an outgrowth from the parent organism, which grows into a smaller identical organism, which eventually becomes detached and exists as a separate organism
describe vegetative propagation
the parent organism develops a structure that grows into an independent separate organism
the new organism can be grown from any part of the parent plant
the parent plant stores food from photosynthesis in the soil
definition of growth
a permanent increase in cell number, mass or size
what is dry mass and why is it used to measure growth
the mass of the body of an organism with all the water removed from it
it is used to measure growth because the water content of cells within an organism can vary greatly, so when measuring mass to monitor growth it can give misleading results
where does meiosis take place
for animals in the gonads (ovaries and testes)
when does crossing-over and independent assortment occur
in meiosis 1
Prophase 1 - crossing over
Metaphase 1 - independent assortment
describe what is meant by independent assortment
occurs during metaphase 1
when homologous pairs line up at the equator of the cell it is random which side of the equator the maternal and paternal chromosome from each homologous pair end up
describe what is meant by crossing-over
at the chiasmata, enzyme complexes cut and join parts of the maternal and paternal chromatids together
resulting in new combination of alleles
what are the stages of meiosis
interphase
prophase 1
metaphase 1
anaphase 1
telophase 1
sometimes cytokinesis
prophase 2
metaphase 2
anaphase 2
telophase 2
cytokinesis
describe the prophase 1 stage of meiosis
-2 daughter chromatids are joined together at the centrosmere
-chromosomes condense to become visible
-homologous chromosomes link together at the chiasmata
-crossing-over takes place
-nucleolus and nuclear membrane breaks down
-centrioles move to opposite poles of the cell, which starts forming spindle fibers
describe the metaphase 1 stage of meiosis
-spindle fibers move the pairs of homologous chromosomes to line up along the metaphase plate
-independent assortment
describe the anaphase 1 stage of meiosis
-the centromeres do not split
-spindle fibres contract
-the chiasmata between homologous chromosomes break
-which allows 1 chromosome from each homologous pair to be pulled to opposite poles of the cell
describe the telophase 1 stage of meiosis
-nuclear membranes reform around the 2 groups of chromosomes
-the chromosomes uncoil back into chromatids
describe the prophase 2 stage of meiosis
-2 daughter chromatids are joined together at the centromere
-chromosomes condense to become visible
-nucleolus and nuclear membrane break down
-the centrioles pull apart to begin to form spindle fibres
describe the metaphase 2 stage of meiosis
-spindle fibers move the chromosomes to line up along the metaphase plate/equator
describe the anaphase 2 stage of meiosis
-the centromeres that originally linked the 2 chromatids split
-spindle fibres contract
-the chromatids from each pair are pulled to opposite poles of the cell
describe the telophase 2 stage of meiosis
-spindle fibers break down
-nuclear envelopes form around the 2 sets of chromosomes
-nucleoli and centrioles reform
-chromosomes begin to unravel, becoming less dense and harder to see
describe what translocation is
-a chromosome mutation
-occurs when a piece from one pair of homologous chromosomes breaks off and reattaches to a chromosome from a completely different homologous pair
-some translocations are balanced and create no change to the phenotype
-meaning a piece from one chromosome is swapped with a section from a completely different homologous pair
-translocations which are unbalanced change the phenotype
-meaning one chromosome loses a piece and another chromosome from a different homologous pair gains it
what is non-disjunction of chromosomes
-a mutation that affects the whole chromosome
-during anaphase a chromosomes fail to separate into 2 chromatids
-so one of the cell has 2 copies of that chromosome and the other cell doesn’t have any copies of that chromosome
-can occur in meiosis and mitosis
what are the 3 effects non-disjunction
monosomy
polyploidy
aneuploidy
describe polyploidy
if an abnormal gamete which contains 2 copies of a type of chromosome fuses with a normal gamete the zygote with polyploidy will have 3 copies of a type of chromosome instead of 2
describe aneuploidy
where a cell lacks a chromosome or has more than 2 of a chromosome
describe monosomy
if an abnormal gamete which doesn’t contain a certain chromosome fuses with a normal gamete the zygote with monosomy will only have one chromosome from the homologous pair
state an example of polysomy
Down’s Syndrome
state an example of monosomy
Turner’s Syndrome
describe what causes Down’s syndrome
-when a zygote has polysomy
-non-disjunction of chromosome-21 means one of the gametes will contain 2 copies of a certain chromosome
-so after fertilisation with a normal gamete the zygote will have polysomy with 3 copies of chromosome-21
-the baby born with polysomy of chromosome-21 will have Down’s syndrome
what are the effects a baby with Down’s syndrome may have
-heart abnormalities
-severe learning difficulties
-lack of muscle tone
-visual problems
describe what causes Turner’s syndrome
-the when a zygote has monosomy of a sex chromosome
-the presence or absence of a Y chromosome determines the route for sexual development
-any embryo with at least 1 Y chromosome will develop male characteristics
-any embryo lacking a Y chromosome will develop female characteristics
-when there is non-disjunction of the male sex chromosome an egg may be fertilised by a sperm that has no sex chromosome
-the zygote will have monosomy with just 1 X chromosome from the ovum
-the genotype will be XO and the baby will have Turner’s syndrome
what are the effects of Turner’s syndrome
-the person will appear female
-but will be infertile
-will not go through puberty naturally
describe the process of spermatogenesis
-in the testes diploid primordial germ cells divide by mitosis several times to form diploid spermatogonia
-these spermatogonia grow large enough to be called primary spermatocytes
-the primary spermatocytes undergo meiosis, after the 1st meiotic division2 haploid secondary spermatocytes are produced
-the secondary spermatocytes undergo a 2nd meiotic division producing 4 halpoid spermatids
-the spermatids differentiate in the tubules of the testes to form spermatozoa
what is the name of the cell that produces ova
primordial germ cells
1 primordial germ cell makes 1 ovum
what is the name of the cell that produces spermatozoa
primordial germ cells
1 primordial germ cell produces 4 spermatozoa
where does spermatogenesis take place
in the testes
where does oogenesis take place
in the ovaries
compare the number of spermatozoa and ova produced from primordial cells
-each primordial cell in the testes produces many spermatozoa
-each primordial cell in an ovary produces 1 ovum
-because ova contain much more material than sperm, so require more energy to produce
describe the process of oogenesis
-the diploid primordial germ cell divides multiple times by mitosis to form diploid oogonia
-most of the oogonia degenerate, only 1 oogonium continues to grow and lots of storage material is used to make the cell very large compared to spermatocytes, this becomes a primary oocyte
-everything up to this stage occurs in the ovary of the fetus
-then starting at puberty once every menstrual cycle the primary oocyte undergoes meiosis, after the 1st meiotic division a diploid secondary oocyte is formed and a smaller polar body is formed
-the polar body degenerates
-after fertilisation, the secondary oocyte undergoes a 2nd meiotic division forming a haploid ovum and another polar body
-the first polar body undergoes its 2nd meiotic division forming 2 more polar bodies
-these 3 polar bodies degenerate
what is the function of polar bodies
-receive chromosomes during meiotic divisions
-degenerate and die as the ovum develops
what are the 3 adjectives to describe spermatozoa
many, mini, motile
what are the 3 adjectives to describe ova
few, fat, fixed
describe how the structures of spermatozoa are adapted to their function
acrosome - contains hydrolytic enzymes that digest through the zona pellucida so the sperm can fertilise the ovum
nucleus - haploid, chromosomes are highly condensed to reduce the amount of energy needed to transport them
middle piece - lots of tightly packed mitochondria to provide ATP for the movement if the flagellum
flagellum - propels the sperm to the ovum, contains microtubules which create a back and forth motion
describe how the structures of ova are adapted to their function
nucleus - haploid, receives chromosomes during meiotic divisions
cytoplasm - lots of food reserves to facilitate the growth of the zygote
zona pellucida - a clear jelly like layer, protective coating that sperm need to penetrate through for fertilisation, prevents polyspermy
cortical granules - release substances which cause the zona pellucida to harden, preventing more sperm from penetrating through the membrane
follicle cells - form a protective coating around the ovum
describe the process of the formation of pollen grains
-each anther contains 4 pollen sacs
-in each pollen sac there are many diploid microspore mother cells which divide by meiosis to produce 4 haploid microspores, which are the gametophyte generation
-each haploid nucleus within the microspores divide by mitosis, but there is no cytokinesis so there are 2 haploid nuclei within each pollen grain (microspore)
-one of these nuclei is a pollen tube nucleus, this produces a pollen tube which travels through the stigma, style, ovary then into the ovule
-the other nucleus is the generative nucleus which will fuse with the nucleus of the ovule
where are the female and male gametes of flowering plants produced
-the female gametes are produced in the ovules inside the ovaries
-the male gametes are produced in pollen grains inside the anthers
describe the process of the formation of egg cells in flowering plants
-the ovules are attached to the ovary by the placenta
-in ovules inside the ovary, the diploid megaspore mother cell divides by meiosis to produce 4 haploid megaspores
-3 of these haploid megaspores degenerate leaving 1 megaspore to continue developing
-the haploid megaspore undergoes 3 mitotic divisions that result in an embryo sac containing 3 anti-podal cellls, 2 polar nuclei, 1 egg cell, and 2 synergids
what is sexual reproduction in plants called
pollination
what are the 2 types of sexual animal reproduction
external reproduction and internal reproduction
what is external reproduction
-when the female and male gametes are released outside of the parental bodies, so the gametes can meet and fuse in the external environment
-only common is aquatic species
-because spermatozoa and ova are vulnerable to drying in the air
what is internal reproduction
-the male gamete is transferred directly to the female gamete
-fertilisation occurs inside the mother’s body
describe fertilisation in animals
-an ovum is released into the filopian tubes, it hasn’t fully completed meiosis so is a secondary oocyte
-when the head of the sperm come in contact with the zona pellucida the acrosome reaction is triggered
-enzymes from the acrosomes of the sperm are released, which digest through the follicle cells and zona pelluida
-multiple sperm are needed to provide enough digestive enzymes
-1 sperm head fuses with the membrane of the secondary oocyte
-this causes the secondary oocyte to complete its second meiotic division creating a haploid ovum nucleus
-ion channels within the cell membrane of the ovum open and close so that the inside of the cell becomes electrically positive instead of negative, this temporarily stops sperm fusing with the ovum
-cortical granules are released from the ovum which causes the cortical reaction, where the cortical granules combine with the zona pellucida to form a tough fertilisation membrane, which stops more sperm from fusing
-the head of the sperm enters the ovum, but the flagellum is left outside, the head absorbs water and swells, releasing its chromosomes to fuse with the chromosomes from the ovum, forming a diploid zygote
describe fertilisation in plants
-the pollen grain lands on the surface of the stigma
-the molecules on the surface of the pollen grain and stigma interact
-if they recognise each other as being a part of the same species but different plant, the pollen grain begins to germinate
-a pollen tube grows out from the tube cell of the pollen grain through the stigma into the style
-the tip of the pollen tube produces hydrolytic enzymes to digest through the style, the digested tissue acts as a nutrient source for the pollen tube
-the generative cell containing the generative nuclei move down the pollen tube to form 2 male nuclei
-the pollen tube grows through the ovary amd into an ovule where the 2 male nuclei are passed into the ovule for fertilisation
-double fertilisation happens, a male nucleus fuses with the 2 polar nuclei to form the endosperm nucleus which is triploid and the other other male nucleus fuses with the egg cell to form a diploid zygote
what is fertilisation in flowering plants called
pollination
what is the definition of totipotent
what can be described as totipotent
-a totipotent cell has the potential to form all the different types of cells needed to grow an organism
-a fertilised egg/zygote is totipotent
describe the stages that occur after fertilisation up until the formation of an embryo
-during the cleavage stage mitosis occurs, but there is not interphase between the mitotic divisions, this happens as the zygote travels down the oviduct into the uterus
-the cleavage stage produces a blastocyst, which is a mass of small, identical, undifferentiated cells in a hollow sphere
-these early cells are totipotent embryonic stem cells
-the outer layer of cells in the blastocyst form the placenta
-the inner layer of cells have lost some ability to differentiate so are pluripotent embryonic stem cells, but can still differentiate into most types of cells
-the blastocyst hatches, it breaks free of the outer layer and implants in the lining of the uterus
what does cell determination refer to
in the early development of an embryo cells are predetermined to become particular types of tissue
Mitotic index equation
Mitotic index = number of cells in mitosis / total number of cells
