6. Core Concepts - Genetic information is copied and passed on to daughter cells Flashcards
chromosome
made of DNA, which is wound around histone proteins
chromatin
in a cell that is not dividing
chromosomes are unwound single structures and are not visible
centromere
when do chromosomes become visable
At the beginning of cell division, the chromatin condenses. This means the chromatin coils so tightly that the chromosomes become visible.
chromatids
after replication, two identicle structures called chromatids are formed.
2 sister chromatids (genetically identicle) attached by the centromere make one chromosome
The chromatids in one chromosome are called sister chromatids and are genetically identical.
homologous pairs
chromosomes are in homologous pairs
do not have identicle DNA to each other
One of each pair has been inherited from the male parent the other of each pair has been inherited from the female parent
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 (different forms of a gene) can be the same or can be different.
diploid
Cells where the chromosomes are paired are termed diploid, these are body or somatic cells.
Diploid cells can divide by mitosis or meiosis.
haploid
Cells where the chromosomes are unpaired are termed haploid, these are gametes. Some organisms have body cells that are haploid.
Haploid cells can only divide by mitosis.
order of gene nucleus chromosome dna
alleles on homologous chromosomes
diagram of general cell cycle
interphase
G1, S and G2
S phase part of interphase
DNA replication takes place
only happens if the cell is going to proceed to mitosis or meiosis.
Cells that are carrying out their functions are in G1 phase for their lifespan.
mitosis
single parent cell divides to produce two genetically identical daughter cells
In interphase, the cell is:
synthesising ATP
making new organelles
carrying out protein synthesis
increasing in size
replicating DNA.
cytokinesis
Cytokinesis is the division of cytoplasm to form two new cells, each with a nucleus.
cell organelle distributed evenly
plant cell cytokinesis
golgi apparatus forms vesicles of new cell wall material
D
The graph below shows how the mass of DNA changes during the cell cycle.
Mitosis Prophase
the chromosomes condense and become visible as two sister chromatids joined by a centromere.
Centrioles migrate to the opposite sides of the cell (the poles).
The spindle begins to form in animal cells only, made by the centrioles.
The nuclear membrane disintegrates and the chromosomes scatter through the cell.
Mitosis Metaphase
the spindle completes, and the microtubules go from one side of the cell to the other (from pole to pole).
The chromosomes migrate to the equator of the cell and attach to the spindle fibres by the centromere.
Each chromosome is attached to one spindle fibre.
Mitosis Anaphase
the spindle fibres shorten pulling the chromatids to the poles of the cell by the centromeres.
The centromere divides.
Sister chromatids from one chromosome are pulled to opposite poles.
At this point the chromatids are called chromosomes again.
Mitosis 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.
Metaphase
A- centromere
B- spindle fibres
KLHI
J: Interphase
The cell is preparing for division. Chromosomes are not visible as they are in their uncondensed form, and the cell grows and duplicates its DNA.
K: Prophase
Chromosomes condense and become visible. The nuclear envelope starts breaking down, and the spindle fibers begin to form.
L: Metaphase
Chromosomes align along the metaphase plate (the center of the cell), attached to spindle fibers by their centromeres.
I: Anaphase
Sister chromatids are pulled apart to opposite poles of the cell as the spindle fibers shorten.
H: Telophase
Chromatids reach the poles, and nuclear envelopes begin to reform around the separated chromosomes. The cell starts to divide into two.
how is mitosis different
plant vs animals
- 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.
- At cytokinesis, animal cells form a cleavage furrow where the cytoplasm indents. In plant cells a cell plate forms between the two new nuclei and this develops into the new cell wall.
- Mitosis produces two new genetically identical daughter cells, so the cells have the same chromosome number as the ‘parent’ cell.
Uses of mitosis
- Mitosis is used for growth by making new cells, or for repairing tissues by replacing damaged cells. Mitosis happens in a number of different tissues in animals but in plants it is confined to meristematic tissue such as root and shoot tips and the cambium in stems.
- Mitosis is also used in asexual reproduction, all the individuals produced are genetically identical.
What controls the length of the cell cycle
The length of the cell cycle is controlled by genes. If these genes are damaged the cell cycle can be shorter and mitosis is uncontrolled. Uncontrolled mitosis can lead to tumours forming and cancers.
summary mitosis garlic experiment
microscope view of meristem
10x
microscope view of meristem
40x
X - ANAPHASE
Z - TELOPHASE
Y - METAPHASE
mitotic index
The mitotic index is the percentage of cells in a mitotic phase that can 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 multiplied by 100. A higher mitotic index indicates a faster growth rate.
The mitotic index goes down the further back from the root tip the sample is taken from. In the zone of elongation, the cells are getting longer, which also contributes to growth of the root.
The number of cells at each stage in mitosis can be expressed as a percentage of the total number of cells in the field of view, and if the duration of one cell cycle is known, the time taken for each phase can be calculated.
Meiosis is a reduction division
this means that the chromosome number is reduced from diploid (2n) to haploid (n).
product of meiosis
The product of meiosis is a gamete. Gametes fuse at fertilisation, restoring the diploid number.
meiosis ensures
Meiosis ensures that the chromosome number of individuals remains the same from generation to generation in sexually reproducing populations.
how is reduction achieved
meiosis reasons for genetic differences
Each cell is genetically different because of crossing over and random assortment during meiosis.
Random fusion of gametes means that the zygotes and offspring developing from them are all genetically different. This variation means that if environmental conditions change, some of those offspring will have variations that enable them to survive, increasing the chance of survival of the species.
crossing over
independent assortment
Mendel stated -
Inheritence of seed shape is independent of inheritence of seed colour
When gametes form the two alleles of any given gene segregate during meiosis independently of any two alleles of the other genes
Meiosis 1
Prophase 1
Prophase I: chromatin condenses and chromosomes become visible as two sister chromatids joined by a centromere. Homologous chromosomes pair up forming bivalents. Crossing over between the maternal and paternal chromosomes of the homologous pair takes place at chiasmata. In animal cells, the centrioles migrate to the poles. The spindle forms and the nuclear membrane disintegrates.
Meiosis 1
Metaphase 1
Metaphase I: 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.
Meiosis 1
Anaphase 1
Anaphase I: 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.
Meiosis 1
Telophase 1
Telophase I: the chromosomes reach the poles and the nuclear membranes reform. Each nucleus is haploid as the chromosomes are not paired.
Meiosis 1
Cytokinesis
Cytokineses: forms two cells each with a haploid nucleus.
diagram of crossing over during Prophase 1
Meiosis II: in this stage, the daughter cells divide again in two.
Prophase II: the chromosomes condense and become visible as two chromatids joined by a centromere. The spindle forms and the nuclear membranes disintegrate.
Meiosis II: in this stage, the daughter cells divide again in two.
Metaphase 2
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.
Meiosis II: in this stage, the daughter cells divide again in two.
Anaphase 2
Anaphase II: the centromeres divide as the chromatids are pulled to the poles by contraction of the spindle. The chromatids are now called chromosomes.
Meiosis II: in this stage, the daughter cells divide again in two.
Telophase 2
Telophase II: the chromosomes reach the poles and the nuclear membranes reform.
Meiosis II: in this stage, the daughter cells divide again in two.
Cytokinesis 2
Cytokinesis: each cell now divides, forming two daughter cells.
Prophase 1
Metaphase 1
Anaphase 2
Haploid
Telophase 1
Random assortment
meiosis 10x objective lens
Using the x40 objective lens focus on a pollen sac and the cells dividing, you will see something like the image below. Draw and label the dividing cells as shown.
Compare mitosis and meiosis
- Mitosis takes place in one division, meiosis has two consecutive divisions.
- Mitosis produces two genetically identical cells, as identical sister chromatids are separated. Meiosis produces four 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.
significance of mitosis
-daughter cells are genetically identicle, important for growth and development
-when repairing/replacing cells, important that exact copies are replaced
-basis of asexual reproduction, offspring are genetically identicle to parent
crossing over CLEGG
as the bivalent chromosomes shorten and thicken
chromatids break, the broken ends rejoin, some pieces may be swapped
chaisma - point where the 2 chromosomes join j
independant assortment CLEGG
of maternal and paternal homologous chromosomes
bivalents line up at the equator in a random order
which chromosome goes to which pole is random
site mitosis and meiosis in plant
mitosis = root tips
meiosis = anther
✅ What is Cancer?
Cancer is caused by uncontrolled cell division, leading to the formation of a tumour.
It occurs when mutations affect genes that control the cell cycle
🧬 Caner - Link to the Cell Cycle
Cancer cells:
Bypass checkpoints in the cell cycle (e.g., G1, G2, metaphase).
Divide more rapidly and frequently than normal cells.
Treatments often aim to reduce or stop cell division, especially:
Inhibiting DNA replication (S phase)
Disrupting mitosis (M phase)
💊 Cancer Treatments (Application Focus)
Chemotherapy and radiotherapy:
Target fast-dividing cells by:
Damaging DNA → triggers apoptosis
Inhibiting mitotic spindle formation (e.g. vincristine)
Also affect healthy fast-dividing cells (e.g. hair cells, lining of the gut) → side effects
Targeted therapies:
Aim to block specific receptors or proteins involved in signalling for cell division.
Fewer side effects than general chemotherapy.
🔋 Cancer - Link to Mitochondria (Application Questions)
Cancer cells often have more mitochondria or more active ones.
Why? They need lots of ATP to:
Support rapid cell division (synthesis of proteins, DNA, organelles)
Maintain their metabolism
In application questions: if a cell is dividing rapidly (like cancer cells), expect more mitochondria to support that activity.
