D2.1 Cell and Nuclear Division Flashcards
what is cytokinesis
division of cytoplasm
happens in the final stage of nuclear division
what happens to mitochondria and chloroplasts in division
they are distributed evenly between the two cells
how does cytokinesis occur in animal cells
this happens with in tucking of the plasma membrane at the equator of the cell, pinching the cytoplasm in half
a ring of actin, myosin and other proteins form a contractile ring which pinches the membrane
how does cytokinesis occur in plant cells
the Golgi apparatus forms vesicles of new cell wall materials, which collect along the line of the equator ( cell plate)
vesicles then combine to form the new plasma membrane and cell walls
when can cytokinesis be unequal
in oogenesis ( egg production) a large egg is formed alongside several smaller cells.
Primary oocyte divides, one is a tiny polar body while the other is a secondary oocyte
When the secondary oocyte divides, another polar body is formed as well
the polar bodies then degenerate and their components are absorbed
What does the ovum require for the growing embryo
a large cytoplasm to provide nutrients and energy
why must nuclear division happen before cell division
to avoid production of cells with no nucleus
one will have a nucleus and the other will not
when does mitosis occur
they produce diploid cells so when
an organism grows
old cells are replaced
when an organism reproduces asexually
when does meiosis occur
produces haploid cells
occurs when sexual reproduction occurs, during the formation of gametes
When does DNA replication occur in the cell cycle
during interphase
produces double stranded chromosomes
what is each strand of a chromosome known as
chromatids, two in each chromosome
how are chromatids held together
held together by a centromere, a specialised DNA sequence that can be seen as a the constricted region of a chromosome
what happens to chromatids during nuclear division
each of the sister chromatids separate and move into different cells
what does each chromosome contain
one very long DNA molecule
what are the functions of the proteins in the chromosomes
some are enzymes involved in copying and repair reactions of DNA.
Bulk of then have a supporting and packaging role in DNA
what are nucleosomes
DNA double helix is looped around histone protein beads
Wrapped around 2 and a half times around 8 histone proteins
what is a chromatin
the nucleosomes are coiled up to form a chromatin fibre
what is a chromatid
the chromatin are coiled up again, looped around a scaffold protein and then supercoiled up again into the much condensed chromatid
what are histones
positively charged proteins containing a high concentration of amino acid molecules with additional basic functional groups
how are chromosomes moved during cell division
using microtubules and microtubule motors
what are microtubules and how do they assist in cell division
cytoskeletal fibres that can lengthen and shorten with polymerisation/ depolymerisation of tubulin
movement of chromosomes is achieved due to this and facilitated by motor proteins carrying them along the microtubules
What are the different stages of mitosis
Prophase
Metaphase
Anaphase
Telophase
steps of prophase
chromosomes present as long thin threads. They increasingly shorten and thicken by a process of supercoiling
In the end, chromosomes consist of two chromatids held together by a centromere
nucleolus gradually disappears
steps of metaphase
centrioles move to opposite ends of the cell
microtubules start to form a spindle, radiating out from centrioles
microtubules attach to centromeres and arrange them at the equator
steps of anaphase
centromeres divide
spindle fibres shorten and chromatids pulled by centromeres to opposite poles
Once separated. chromatids referred to as chromosomes
steps of telaphase
a nuclear membrane reforms around both groups of chromosomes at opposite ends of the cell
chromosome decondenses and becomes chromatin again
nucleolus reforms in each nucleus
follow by the division of the cytoplasm
features of meiosis
two divisions of the nucleus but only one replication of chromosomes
early in the meiosis one, homologous chromosomes pair up
no further interphase inbetween meiosis I and II
what is a bivalent
when homologous chromosomes pair up closely
members of a bivalent continue to shorten known as condensation
What happens during the coiling and shortening process in meiosis
chromatids can break. When non sister chromatids from homologous chromosomes break and rejoin, they can do so at corresponding sites, forming a cross structure known as chiasma
what is known as crossing over in meiosis
the creation of chiasmas that can happen at one or more places along the bivalent
it means genes have been exchanged between chromatids
how meiosis one end
with two cells each containing a single set of chromosomes, made of two chromatids
members of bivalents become attached by their centromeres to the fibres of the spindle at the equatorial plate of the cell. Spindle fibres pull the homologous chromosomes apart to opposite poles but the individual chromatids remain attached by the centromeres
what happens in meiosis II
centromeres of the chromosomes divide and the individual chromatids now move to opposite poles
leads to four cells, each with half the chromosome number of the original parent cell
what is non disjunction
there is failure of homologous chromosomes or sister chromatids to separate properly during meiosis, leading to some gametes with more than and some with less than the haploid number of chromosomes
what is the result of non disjunction
down syndrome where people have an extra chromosome
what are the two reasons the haploid cells produced by meiosis differ genetically
crossing over of segments, resulting in new combinations of genes on the chromosome
random orientation of chromosomes which happens because the way bivalents line up at the equator is completely random
what is the result of crossing over
due to formations of chiasmas, there can be new combinations of alleles on the chromosomes of the haploid cells
What is the result of random orientation
generates a huge amount of variation in the coded information carried out by different gametes in the fertilisation stage
bivalents separate and move to the nearest pole after lining up randomly
how do both animal and plants grow from a single cell
by repeated cell divisions to form an embryo
what is proliferation and when does it happen
a rapid increase in number of cells
happens after early stage of animal embryos and plant meristem tissues
what are meristems
they are a group of cells that retain the ability to divide by mitosis
they allow plants to grow at restricted points in the plant
features of meristem cells
small
thin cellulose walls
dense cytoplasmic contents
vacuoles are mostly absent
where are meristems found
at terminal growing points of stems and roots
or laterally
what are apical meristems
occur at the tips of stems and root, responsible for primary growth
Cell division and growth leads to the formation of stem/root tissue
new cells formed by division rapidly increase in size and then cells differentiate
what are lateral meristems
form from the cambium cells in the centre of vascular bundles, between the phloem and xylem tissue
it causes secondary growth leading to increased girth and length of stem
why is proliferation required inside animals
cells constantly need to be replaced in a routine process
stem cells under the skin
in the bottom/basal layer, stem cells remain undifferentiated and continue dividing throughout life
daughter cells differentiate and leave the basal layer
process maintained because basal cells are self renewing
proliferation in wound healing
after blood clotting, inflammation causes increases blood flow to the wound, enabling WBC and fibroblasts to travel rapidly to the damaged skin
WBC remove infection, and fibroblasts produce proteins( proliferation) to help wound closure
what are the three main phases in cell cycle
Interphase ( G1, S, G2)
division of nucleus by mitosis resulting in two identical cells
Division of cytoplasm and cell, Cytokinesis
What happens in G1 of interphase
the first phase of growth, there is synthesis of new organelles that take place in the cytoplasm
intense biochemical activity in the cytoplasm/organelles
accumulation of energy stores before nuclear division
what happen in S phase of interphase
period of synthesis of DNA, each chromosome makes a copy of itself
two identical structures are known as chromatids
what happens in G2 of interphase
the second phase of growth is a continuation of the biochemical activity and increase in amount of cytoplasm
how long does interphase take
longest part of the cell cycle, but time differs in mature cells and embryos. some cells never divide again
what is interphase
metabolically active period and growth involves synthesis of cell components including proteins and DNA. Mitochondria and chloroplasts are increased
what do chromosomes do during interphase
actively involved in protein synthesis
copies of particular genes are taken to the cytoplasm, ribosomes than synthesise proteins
what happens to chromosomes in interphase
they are dispersed and are referred to as chromatins
among the chromatins there are one or more nucleoli which synthesis ribosomes which then migrate out into the cytoplasm
cell cycle is regulated by molecular control system. What are key points of the system
key checkpoints where signals operate, along with stop points which have to be overridden
three checkpoints at G1, G2, mitosis
the molecular control signal in the cytoplasm is what
proteins known as kinases and cyclins
what are kinases
enzymes that activate/inactivate other proteins, always present in cytoplasm
how are kinases related to cyclins
kinases activated by specific cyclins and referred to as Cyclin Dependent Kinases (CDK)
what does cyclin do in the cytoplasm
cyclin concentration in the cytoplasm change constantly. As cyclin concentration increase, they combine with CDK molecules to form a complex that functions as a mitosis promoting factor ( MPF)
what happens when Mitosis Promoting factor accumulates
triggers chromosome condensation, fragmentation of the nuclear membrane and spindle formation( starts mitosis)
how is cyclin used as a checkpoint
specific level required to pass each checkpoint in the cycle
what happens to cyclin in anaphase
destruction of cyclin commences
what are proto-oncogenes
genes that encode proteins that stimulate cell division
mutations convert them to oncogenes, leading to increase in protein production, resulting in uncontrolled cell division, possibly leading to cancer
what is an example of cancer from oncogenes
seen in Ras gene which codes for Ras proteins. Oncogene codes for permanently activated Ras protein which triggers the kinase cascade in the absence of growth factor, resulting in uncontrolled cell division
mutations results from a dominant allele
What are tumour suppressor genes
encode for proteins that inhibit cell division or promote apoptosis if damaged DNA is copied
mutation of these result in lack of these proteins, resulting in uncontrolled cell division, possibly leading to cancer
what is an example of a mutation in tumour suppressor genes
seen in p53 gene which promotes protein synthesis of protein that triggers cell death
no p53 protein being produced means cell with damaged DNA continue to proliferate, leading to uncontrolled cell division
mutation in a recessive allele
what happens in all cancers
cells start to divide repeatedly without control or regulation
leads to rate of cell multiplication being much faster than the rate of cell death
what is the result of cancer
irregular mass of cells is formed, called a tumour
some can be benign, some can be malignant
what are malignant tumours
they are cancerous, secrete signals to trigger growth of blood and lymph vessels to serve tumour cells at the expense of other tissues
what is metastasis
the spread of cancer cells to other locations
cells can break away from primary tumour and carried to other parts
what is the result of cancer
cancerous cells ultimately take over the body at the expense of healthy cells, leading to malfunction and death
what is mitotic index
measures how many cells in a sample are in mitosis compared to total number of cells
used to differentiate between benign and malignant
what is the use of mitotic index
able to indicate rapidly dividing cell masses through high mitotic index, indicates tumour formation
used to investigate response to chemotherapy
