1.6 genetic information is copied and passed on to daughter cells Flashcards
what are chromosomes made of?
DNA, which is wound around histone proteins
in a cell that is not dividing, the chromosomes are (unwound/wound) single structures and are (visible/not visible)?
unwound
not visible
CALLED CHROMATIN
what happens prior to cell division?
the DNA replicates so each chromosome consists of two identical DNA molecules
(the two identical molecules are joined at an area callrd the centromere)
where are the two identical chromatids joined at?
the centromere
what happens at the beginning of cell division?
the chromatin condenses
what does it mean that the chromatin condenses at the beginning of cell division?
the chromatin coils so tightly that the chromosomes become visible
what are the two identical parts of the chromosome called?
what are they joined by?
- chromatids
- joined by the centromere
what are the chromatids in one chromosome called?
sister chromatids
(they are genetically identical)
what is the name of the pairs of chromosomes in most body cells of most organisms?
homologous pairs
what are homologous pairs?
the chromosomes in pairs that do not have identical DNA to each other
- one of each pair has been inherited from the male parent and the other of each pair has been interited from the female parent
why are homologous pairs termed homologous?
because they are the same size and shape and carry the same genes in the same order at the same gene locus (position of the gene on the chromosome).
the alleles can be the same or can be different
what is the gene locus?
the position of the gene on the chromosome
what are alleles?
different forms of a gene
what is the name for cells where the chromosomes are paired?
diploid
what is the name for cells where the chromosomes are unpaired?
haploid
(e.g gametes)
by what can diploid cells divide by?
mitosis or meiosis
by what can haploid cells divide by?
only by mitosis
chromatin consists of ___ wrapped around histones?
DNA
DNA ___ prior to cell division?
replicates
sister ____ are genetically identical?
chromatids
what are the cell cycle stages?
- G1 (growth)
- S (synthesis)
- G2 (growth and preparation for mitosis)
- M (mitosis)
- C (cytokinesis)
G1, S and G2 phases are collectively called interphase
what phases are found is interphase?
G1, S and G2
what is the S phase in the cell cycle?
- where DNA replication takes place
- and only happens if the cell is going to proceed to mitosis or meiosis
when does the S phase only happen?
if the cell is going to proceed to mitosis or meiosis
in what phase in the cell cycle are cells that are carrying out their functions?
in the G1 phase for their lifespan
what is cytokinesis?
the division of cytoplasm to form two new cells, each with a nucleus
what is the cell doing in interphase?
- synthesising ATP
- FORMATION/SYNTHESIS OF organelles
- carrying out protein synthesis
- increasing in (cell) size
- replicating DNA
at G1, the DNA content of a cell is (constant/doubled/halved)?
constant
in the S phase, the DNA content of a cell is (constant/doubled/halved)?
doubles
because the DNA has replicated
in mitosis, the DNA content of a cell is (constant/doubled/halved)?
constant
the DNA content remains constant as thr chromosomes condense, align at the equator and are pulled to the pole inside the cell
in cytokinesis, the DNA content of a cell is (constant/doubled/halved)?
halves
as cytokinesis takes place and each new nucleus is now in a new cell
what are the 4 stages of mitosis?
- prophase
- metaphase
- anaphase
- telophase
what happens in prophase?
- chromosomes condense and become visible as 2 sister chromatids joined by a centromere
- centrioles migrate to the opposite sides of thr cell (the poles)
- the spindle begins to form in animal cells only, made by the centrioles
- the nucleur envelope disintegrates and the chromosomes scatter through the cell
- the nucleolus disappears
what happens in metaphase?
- the spindle completes
- the microtubules go from one side of thr cell to the other (from pole to pole)
- the chromosomes align at the equator of the cell and attach to the spindle fibres by the centromere
- each chromosome is attached to one spindle fibre
what happens in anaphase?
- the spindle fibres shorten pulling the chromatids to the poles of the cell by the centromeres
- the centrmere divides
- sister chromatids from one chromosome are pulled to opposite poles
- at this point the chromatids are called chromosomes again
what happens in telophase?
- the chromosomes reach the poles of the cell and uncoil
- the spindle fibres break down
- new nuclear membranes form around each set of chromosomes
how does mitosis differ in plant and animal cells in terms of centrioles and spindle fibres?
- plant cells lack centrioles, so although they have a spindle, it is not generated by centrioles
- in animal cells, the spindle degenerates at telophase but in plants it remains as the new cell wall is formed
how does mitosis differ in plant and animal cells at cytokinesis?
- at cytokinesis, animal cells form a cleavage furrow where the cytoplasm indents
- in plant cells a cell plate forms between thr two nuclei and this develops into the new cell wall
do the two daughter cells after mitosis have the same number of chromosomes as the parent cell?
yes
mitosis produces two new identical daughter cells, so the cells have the same chromosome number as the parent cell
what is mitosis used for?
- growth by making new cells
- repairing tissues by replacing damaged cells
- asexual reproduction; all the individuals produces are genetically identical
where does mitosis happen in animals?
- in a number of different tissues
where does mitosis happen in plants?
- it is confined to meristematic tisshes such as root and shoot tips and the cambium in stems
what is the length of the cell cycle controlled by?
genes
- they control the cell cycle by acting as a brake, preventing the cell cycle from repeating continually
- if these genes are damaged, the cell cycle can be shorter and mitosis is uncontrolled
what can uncontrolled mitosis lead to?
tumours forming and cancers
why is the root tip used in the practical of drawing cells from slides of root tp to show stages of mitosis instead of shoot tip?
the tissue is transparent and more easily locatable
- also the root tip is a growing region of the plant/meristem
- many cells undergoing mitosis here
what is the mitotic index?
the % of cells undergoing mitosis that cab be seen at any one time
(it is the number of cells visible in prophase, metaphase, anaphase and telophase all together, divided by the total number of cells in the field of view x100)
a higher mitotic index indicates a (faster/slower) growth rate?
faster
the mitotic index goes (up/down) the further back from the root tip the sampe is taken from?
down
what does it mean that meiosis is a reduction division?
that the chromosome number is reduced from diploidnto haploid
what is the product of meiosis?
a gamete
gametes fuse at fertilisation, restoring the diploid number
what does meiosis ensure?
that the chromosome number of individuals remains the same from generation to generation in sexually reproducing populations
how is the reduction of chromosome number achieved in meiosis?
by having two divisions of the chromosomes
(the DNA content of cells is reduced by half in the first division and again in the second division)
for each cell undergoing meiosis, how many daughter cells are produced?
4
why is each cell genetically different after meiosis?
because of crossing over and random/ independent assortment during meiosis
what does the random fusion of gametes mean about the zygotes and offspring?
they are all genetically different
what is a benefit of genetic variation from meiosis?
if environmental conditions change, some of these offspring will have variations that enable them to survive, increasing the chance of survival of the species
what happens in meiosis 1?
the cell is divided in 2
what are the stages of meiosis 1?
- prophase 1
- metaphase 1
- anaphase 1
- telophase 1
- cytokinesis
what happens in prophase 1?
- chromatin condenses to become shorter and thicker
- chromosomes becone visible as two sister chromatids joined by a centromere
- chromosomes come together in homologous pairs (bivalent)
- crossing over between the maternal and paternal chromosomes of the homologous pair takes place at chiasmata
- in animal cells, the centrioles migrate to opposite poles
- the spindle forms and the nuclear membrane disintegrates
in prophase 1, what do homologous chromosomes pair up to form?
bivalents
where does crossing over between the maternal and paternal chromosomes of thr homologous pair take place?
at chiasmata
what happens in metaphase 1?
- spindle forms
- the bivalents migrate to the equator of the cell and attach to the spindle by the centromeres
- the orientation of the maternal and paternal homologs towards the poles is random, this is random assortment
what happens in anaphase 1?
- the spindle contracts and pulls the chromosomes towards each pole
- each chromosome is still composed of two chromatids, although they are not genetically identical because of crossing over
what happens in telophase 1?
- the chromosomes reach the poles and the nuclear membranes reform
- each nucleus is haploid as the chromosomes are not paired
what happens in cytokinesis after telophase 1?
- forms two cells each with a haploid nucleus
what happens in meiosis II?
the daughter cells divide again in two
what are the stages in meiosis II?
- prophase II
- metaphase II
- anaphase II
- telophase II
- cytokinesis
what happens in prophase II?
- the chromosomes condense
- and become visible as two chromatids joined by a centromere
- the spindle forms and the nucleur membranes disintegrate
what happens in metaphase II?
- the chromosomes migrate to the equator and attach to the spindle by the centromere
- the chromatids are not identical, each one in the chromosome can orientate towards a pole randomly - random assortment
what happens in anaphase II?
- the centromeres divide as the chromatids are pulled to the poles by contraction of the spindle
- the chromatids are now called chromosomes
what happens in telophase II?
- the chromatids reach the poles
- the nuclear membranes reform
what happens in cytokinesis after telophase II?
each cell now divides, forming two daughter cells
in what phase in meiosis does random assortment occur?
metaphase
what are the differences between mitosis and meiosis?
- mitosis takes place in 1 division, meiosis has 2 consecutive divisions
- mitosis produces 2 genetically identical cells, as identical sister chromatids are separated. meiosis produces 4 cells that are not genetically identical because of crossing over in prophase I and random assortment in metaphase I and II
- meiosis can only happen in diploid cells as the homologous pairs form bivalents in prophase I. the diploid number is reduced to the haploid number. mitosis can happen in diploid or haploid cells as the sister chromatids from each chromosome are separated into new cells. haploid cells undergoing mitosis produce haploid cells, diploid cells undergoing mitosis produce diploid cells
when are chromosomes visible?
only after they condense at the onset of cell division
following DNA replication, how does a chromosome exist?
as two identical ‘sister’ chromatids, joined by a centromere
sister chromatids are ____?
genetically identical
what are homologous chromosomes?
chromosomes that are the same size and shape and carry the same genes but these may be different versions called alleles
1 chromosome of each pair comes from each parent
in humans what is the diploid (2n) number?
46 chromosomes
in what part of the cell cycle does the majority of the cell cycle in eukaryotes involve?
interphase
what happens to the quantity of DNA in interphase?
it doubles
(but the chromosome number remains the same, as chromosomes exist as two sister chromatids joined together at the centromere)
is the interphase metabolically very active?
yes
- DNA replication
- protein synthesis
occur, requiring much ATP
what does mitosis result in?
the production of two genetically identical cells
what happens in cytokinesis in animal cells?
- the membrane infolds via a cleavage furrow, until the two cell become separated
what happens in cytokinesis in plant cells?
the presence of the cellulose cell wall prevents the membrane infolding via a cleavage furrow, so instead a cell plate develops from the centre outwards, until the cell is divided into two
how do you calculate the percentage of cells undergoing mitosis?
(number of cells in prophase + metaphase + anaphase + telophase /total number of cells) x100
what does meiosis involve?
two consecutive cell divisions
where does meiosis occur?
in the reproductive organs of animals, plants and other protoctistans prior to sexual reproduction
in meiosis 1, what happens to cause genetic variation?
- in prophase 1
- homologous pairs come together to form bivalents
- and crossing over may occur
- in metaphase 1,
- bivalents align randomly at the equator
- = independent assortment
how many varients of chromosome assortment does random assortment give us?
2^n = 2^23
when trying to identify cells in meiosis 1 or II, what should u do?
count the number of cells :
- 2 cells usually indicates meiosis II
what is the significance of meiosis?
- generates genetic variation through crossing over (prophase 1) and random assortment (metaphase 1 and II)
- keeps the chromosome number constant: by producing haploid gametes that recombine during fertilisation, restoring the diploid number in the zygote
what is the difference in mitosis and meiosis between the number of cell divisions?
mitosis has 1 cell division
meiosis has 2 cell divisions
what is the difference in mitosis and meiosis between the cells produced?
mitosis produces genetically identical cells
meiosis produces genetically different cells
what is the difference in mitosis and meiosis between the number of cells produced?
mitosis cells are diploid
meiosis cells are haploid
what is the difference in mitosis and meiosis in terms of crossing over ?
mitosis has no crossing over
meiosis has crossing over occurring in prophase 1
what is the difference in mitosis and meiosis in terms of independent assortment?
mitosis has no independent assortment
meiosis has independent assortment occurring in metaphase 1 and II
what plant tissue could be observed to produce a drawing showing cells undergoing mitosis?
root TIP / shoot TIP / meristem
a sample of tissues from the leg muscle of a young spider mite was analysed. the mass of DNA in some of the cells was 6.8a.u whilst in other cells it was 3.4au. suggest explanations for this difference using your knowledge of the cell cycle [3]
- 3.4 units DNA - before replication/EARLY interphase
- 6.8 units of DNA /doubles due to DNA REPLICATION
- 6.8 units of DNA will be present during (late) interphase/prophase/metaphase/anaphase
name the organs in animals where meiosis occurs
testes/ovaries
what makes cancer?
uncontrolled rapid division of cells
The cell cycle is controlled by genes. If the genes that regulate the cell cycle are damaged, uncontrolled mitosis can occur. This rapid replication of cells can form tumours, leading to a disease called cancer. These genes that cause cancer are called oncogenes.
is interphase a ‘resting’ phase?
no the cells in interphase are biochemically very active
following their replication at interphase, chromosomes are referred to as chromatids until when?
telophase
when they reach the poles of the cell, and are referred to as chromosomes again
how do some chemicals prevent cancers?
they inhibit the cell cycle
what does meiosis result in?
the formation of four genetically distinct haploid gametes
chiasma (chiasmata = plural) definition
the site at which chromosomes exchange DNA in genetic crossing over
what is cancer and how do cancer drugs reduce tumour growth?
- cancer is the result of uncontrolled cell division
- cancer drugs reduce tumour growth by preventing mitosis and cell division from happening by interfering with the cell cycle
(it takes place when the genes that regulate mitosis are damaged)
when the tissues from the plant were removed, they were stained before being viewed under the microscope. explain how this makes the chromosomes visible? (tissues during prophase of mitosis)
stains the (DNA/ nucleic acid / histones)
what are two differences between stages anaphase I and anaphase II?
- in anaphase I, homologous pairs/ chromosomes pulled to opposite poles, whereas in anaphase II, the chromatids are being pulled to opposite poles
- in anaphase I the centromeres do not divide, in anaphase II they do divide
- anaphase I is diploid, anaphase II is haploid
why is genetic variation important?
- genetic variation allows the species to adapt/some of the offspring to survive
- in changeable environments
the cells labelled C are in interphase. explain why nuclei are only visible in some cells and, where nuclei are visible, why they appear to be of different sizes [2]
- plane/angle of section of through cell may not include a nucleus
- (where nucleus visible) may have been cut at different levels/planes
how does meiosis contribute to variation in offspring?
- independent assortment of chromosomes/chromatids
- crossing over
- produced haploid cells
the photograph shows all the chromosomes from a blood cell of a mammal.
explain why red blood cells could not have been used to produce the photograph of the chromosomes [1]
- they have no nucleus/chromosomes
X X V X V V
(the chromosomes in the picture)
use the photograph to deduce the sex of the mammal and explain your choice [1]
male
- not all homologous/different sex chromosomes/ has a Y chromosome
