cell division Flashcards
define gene
small section of DNA which code for specific proteins
define gametes
haploid cells produced by the ovaries and testes
what is the longest part of the cell cycle?
interphase
what occurs during interphase?
- DNA replication
- ATP synthesis
- protein synthesis
- production of organelles
define centrioles
move to opposite poles of the cell and produce protein microtubule during prophase
define chromosome
made of DNA, protein and a small amount of RNA
define cytokenesis
constriction of the centre of the parent cell after telophase
in plant cells, a cell plate is formed first
the number of chromosomes present in a human somatic cell?
46
define metaphase
chromosomes arrange themselves at the equator during this phase of mitosis
define chromatin
this condenses during cell division to form chromosomes
the number of chromosomes present in a human gamete?
23
what is the name of the region where chromatids are joined?
centromere
what occurs during telophase?
- nuclear envelope reforms
- nucleoli reappear
define sister chromatids
identical copies of a chromosome, joined at the centromere
what occurs during anaphase?
spindle fibres contract and pull the sister chromatids to opposite poles
define fertilisation
haploid gametes fuse to form a zygote, restoring the diploid chromosome number
define chromatin
- when a cell is not dividing the DNA exists as chromatin within the nucleus
- chromatin is a complex of DNA and histone proteins found in the nucleus of eukaryotic cells
where do the chromosomes in each homologous pair come from?
one from the mother and one from the father
what are the four phases of mitosis?
- prophase
- metaphase
- anaphase
- telophase
outline what occurs during prophase
- chromatin condenses and becomes visible as two sister chromatids joined at the centromere
- centrioles move to opposite poles. protein microtubules form from each centriole and the spindle develops
- nuclear envelope disintegrates, the nucleolus disappears and chromosomes move towards the equator of the cell
- prophase is the longest stage of mitosis
outline what occurs during metaphase
chromosomes arrange themselves along the equator of the cell and becomes attached to spindle fibres and the centromere
outline what occurs at anaphase
- a very rapid stage where the spindle fibres contract and shorten
- centromere splits
- sister chromatids pulled apart
- chromatids then drawn towards opposite poles with the centromere leading
outline what occurs during telophase
- chromosomes uncoil and lengthen
- spindle fibres break down
- nucleoli and nuclear membranes reform
outline cytokenesis
- in animal cells, cytokinesis occurs by the constriction of the parent cell from the outside inwards, forming a cleavage furrow
- in plant cells, a cell plate forms across the equator of the parent cell from the inside outwards, and a new cellulose cell wall is laid down
identify the differences between mitosis in plant and animal cells
SHAPE:
animal cells - cells become elongated before mitosis
plant cells - no shape change
CENTRIOLES:
animal cells - present
plant cells - absent
CYTOKENESIS:
animal cells - cleavage furrow develops from the outside inwards, pinching the cytoplasm
plant cells - cell plate develops from the centre outwards forming a new cell
SPINDLE:
animal cells - degenerates at telephase
plant cells - remains throughout new cell wall formation
OCCURRENCE:
animal cells - in epithelia, bone marrow and other sites
plant cells - meristems
what is the significance of mitosis?
- gives genetic stability
- growth and repair of an organism
- replacement of dead cells
describe genetic stability arising from mitosis
- mitosis produces 2 daughter cells with the same number of chromosomes as the parent cell.
- each chromosome is an exact replica of one of the original
- mitosis produces cells that are genetically identical to the parent cell
- this gives genetic stability
where does mitosis take place in plants?
root tips and growing points of shoots
why does mitosis occur in human skin?
so that dead surface cells are replaced by genetically identical cells from below
describe asexual reproduction
- produces offspring that are genetically identical to the parent as a result of mitosis
- in certain flowering plants, large numbers of identical offspring are produced
- no genetic variation between offspring
- they are clones
advantage of asexual reproduction?
- offspring are genetically identical to parent, so they are able to survive successfully if environmental conditions are stable
- less time and energy is wasted as organisms do not need to find a reproductive partner
- the population can rapidly increase in number, providing the environmental conditions are stable
disadvantages of asexual reproduction?
lack of genetic variation would mean offspring are unable to adapt to a change in environmental conditions
what is the length of the cell cycle controlled by?
why is this important?
genes which ensure that mitosis happens where and when it is needed
- this allows for timely repair and replacement of tissues in adults and correct development in embryos
what are carcinogens?
radiation, certain chemicals, and some viruses can mutate DNA and affect the genes which control the cell cycle
what are proto-oncogenes?
genes which control the cell cycle, they tell the cell when to divide.
- if mutated, uncontrollable cell division can occur to form a tumour
what are the mutated form of proto-concogenes called?
oncogenes
what type of tumours form and describe them?
- benign - non-cancerous and wont invade nearby tissues
- malignant - cancerous and can spread to other tissues
describe meiosis
- used to produce gametes in sexually reproducing organisms
- meiosis produces four genetically different daughter cells
how many cell division stages are there in meiosis?
what are they? describe them.
2 cell division stages:
MEIOSIS I - chromosome number is halved
MEIOSIS II - the new haploid cells divide again to produce 4 genetically different daughter cells (identical to mitosis)
outline what occurs during prophase I
- chromosomes condense and become visible as two sister chromatids joined at the centromere
- homologous chromosomes pair up in a bivalent
- chromatids wrap around each other and then partially repel each other, but remain joined at points called at the chiasmata. Here, homologous chromosomes can exchange equivalent sections of DNA - this is called crossing over
- nuclear envelope disintegrates
- nucleolus disappears
- centrioles move to opposite poles and microtubules form the spindle
outline what occurs during metaphase I
- microtubules of spindle attach to centromere
- bivalents align randomly along the spindle at the equator (independent assortment of chromosomes takes place)
- this results in a random combination of paternal and maternal chromosomes in each daughter cell
outline what occurs during anaphase I
- spindle fibres contract and shorten, bivalents separate and chromosomes are pulled to opposite poles
- each pole receives one of each homologous pair of chromosomes
- there is a random mixture of maternal and paternal chromosomes as a result of independent assortment
outline what occurs during telophase I
- in some species, the nuclear envelope reforms around the haploid group of chromosomes, and the chromosomes unwind and so are no longer visible
- in many species, the chromosomes stay in their condensed form
outline what occurs during prophase II
centrioles migrate and new spindle develops at right angles to the previous ones
outline what occurs during metaphase II
- chromosomes align on the equator
- spindle fibres attach to centromere
- independent assortment happens because the chromatids of the chromosomes can face either pole
outline what occurs during anaphase II
- spindle fibres shorten
- centromeres split and sister chromatids are pulled to opposite poles
outline what occurs during telophase II
- chromosomes are uncoil and lengthen and are no longer visible under a microscope
- spindle breaks down
- nuclear membrane and nucleoli reform
- cytokinesis gives rise to 4 haploid daughter cells
why are sources of variation essential?
for a species to survive in a constantly changing environment
what are the 3 ways of creating genetic variation as a result of meiosis?
- crossing over
- independent assortment
- random fusion of haploid gametes
describe crossing over as a source of genetic variation
- during prophase I, chromosomes condense and homologous pairs form a bivalent; this is called synapsis
- the chromatids in the bivalent join at points called chiasmata
- parts of the chromosome may be exchanged at chiasmata
- this is crossing over
describe independent assortment as a source of genetic variation
- during metaphase I and II, the chromosomes distribute themselves randomly at the equator of the spindle
- the random distribution leads to independent assortment of chromosomes
describe random fusion of haploid gametes as a source of genetic variation
- at fertilisation, the diploid chromosome number is restored when the nuclei of haploid gametes fuse to form a zygote
- half of the chromosomes come from the mother and half from the father
what is the advantage of sexual reproduction?
- allows offspring to adapt to a changing environment, allows natural selection to occur
- spores produced by fungi and seeds by plants are dispersed widely to help reduce interspecific competition
suggest an organ present in a mammal which contains cells capable of dividing by both mitosis and meiosis
ovary
how does the mass of DNA change over time starting with a diploid cell that undergoes mitosis and meiosis
- the DNA content doubles as the DNA is replicated during interphase
- DNA content halves from 4 to 2 as a result of mitosis - DNA is divided into the nuclei of 2 daughter cells
- DNA content doubles as DNA is replicated during a second interphase
- DNA content halves from 4 to 2 during meiosis I as the chromosome number in each nucleus is halved
- DNA content halves during meiosis II, as DNA is divided into nuclei of 4 daughter cells
explain how mitosis maintains genetic stability (1 MARK)
2 genetically identical daughter cells produced, clones of parent cell
describe two ways in which the process of cell division differs between plant cells and animal cells ( 2 MARKS )
- cell wall grows from inside outwards
- no centrioles
explain two ways by which the process of meiosis contributes to genetic variation in an organism (2 MARKS)
- crossing over has occurred
- independent assortment of bivalents
describe two differences between metaphase II and metaphase I
- metaphase I has bivalents, whereas metaphase II does not
- in metaphase II, centromeres are on the equator whilst in metaphase I they are not
- in metaphase I, chromosomes either side of equator. however in metaphase II, chromosomes are on the equator
- in metaphase I, chiasmata are present but metaphase II has no chiasmata
describe two differences between anaphase I and anaphase II
- anaphase I is diploid and anaphase II is haploid
- in anaphase I, chromosomes are being pulled to opposite poles, whereas in anaphase II, chromatids are being pulled to opposite poles
- in anaphase I, centromeres do not divide and in anaphase II they do divide.
state how meiosis contributes to genetic variation in the species and explain why this is important
- chiasmata formation is taking place in prophase I
- independent assortment is occurring in metaphase I
- genetic variation allows the species to adapt in changeable environment
