Continuity of Cells Flashcards
G1 Checkpoint of the cell cycle.
The G1 checkpoint is checked off at the end of the G1 phase. Before DNA duplication can begin this checkpoint ensures that…
Cell has reached its critical size.
Enough nutrients are available for the next stage to take place.
Necessary growth (regulatory) factors are in place.
There is no DNA damage.
If the checkpoints are met the cell enters S phase and DNA synthesis takes place. If not, cell enters resting state or phase (G0).
G0 phase
Cells can remian in G0 for short time, long time or permanently.
What happens depends on a range of internal or external factors. Although cells in G0 have suspended activity, they may be very active in terms of carrying out normal cellular functions.
Highly differentiated cells in G0, such as neurons, do not return to the cell cycle whereas cells in epithelial tissue may not enter G0 stage at all.
Many lymphocytes are in G0 and only move out of G0 back into the cell cycle if triggered by the presence of specific antigens.
G2 Checkpoint
The G2 checkpoint is at the end of the G2 phase. The cell checks that the DNA duplication during the S phase has taken place accurately and there is no DNA damage. If there are no problems, the cell starts the process of mitosis.
What is cancer?
Cancer is caused by uncontrolled cell division/ uncontrolled mitosis. Cancer cells cannot enter G0 phase and cancer involves a breakdown in the ability of checkpoints in the cell cycle to regulate the process of cell division.
How does Vincristine treat cancer?
Vincristine is a miotic poison that inhibits the formation of microtubules, It binds onto the spindle fibres and prevents them from functioning properly, which stops them from contracting and pulling chromatids apart, preventing anaphase from taking place.
How does Flurocil treat cancer?
Flurocil is an antimetabolite that acts as an S phase inhibitor. It inhibits the enzyme involved in making the nucleotides that contain the base thymine, preventing DNA synthesis.
At what stage does the DNA content of a cell double?
S phase, returns to normal size during cytokenisis.
Histones
Chromosomes consist of an extended DNA molecule supported by special proteins called histones, which provide support for DNA when chromatin condenses to form visible chromosomes during nuclear division.
They are folded into compact stacks and DNA coils tightly around the stack to form a structure called a nucleosome. The arrangement supports and protects the DNA.
What is a karyotype?
Shows the chromosome arrangement in homologous pairs.
Each homologous pair carries different genes, so may differ in size from other cells.
In humans- homologous pairs numbered according to length, pair 1 contains the longest chromosomes.
Prophase mitosis
Chromatin in nucleus becomes more condensed.
During interphase some DNA remains unwound to facilitate protein synthesis. Condensed chromatin has greater strength, preventing damage during mitosis.
Chromosomes become more visible, nucleolus disappears.
In animal cells, centrioles move towards opposite poles of the cell- involved in formation of spindle fibres.
In plants, centrioles are not needed.
During late prophase the centrioles complete migration and spindle fibres form.
Nuclear membrane is broken down and each chromosome consists of 2 chromatids, joined together by a centromere.
Metaphase mitosis
Once chromosomes are aligned in position along the equator of cell, spindle fibres attach to centromeres.
Anaphase mitosis
Centromeres attaching 2 chromatids of each chromosome split- the contraction of spindle fibres pulls chromatids apart.
As anaphase progresses, chromosomes are pulled to opposite poles of the cell.
Telophase mitosis
Sister chromatids end up at opposite poles of the cell and are now referred to as chromosomes again.
Chromosomes decondense and become less visible, nucleolus reappears, nuclear membrane reforms around the 2 groups of chromosomes and the spindle disappears as it is broken down.
Cytokinesis mitosis
Cell divides to form 2 daughter cells, each with identical chromosome makeup that the parent cell had.
In animals, a cleavage furrow forms as the membrane invaginates and eventually splits.
In plants, a cell plate, precursor to a new cell wall is laid down along the centre of the cell. Golgi apparatus synthesises materials needed for the formation of the cell wall.
Mitosis produces 2 diploid daughter cells.
Meiosis is the process of reduction division, what is that?
Halves the number of chromosomes in gametes.
State the differences between meiosis and mitosis.
Meiosis only takes place in reproductive organs e.g ovaries.
Meiosis involves 2 divisions resulting in 4 daughter cells, mitosis involves one division resulting in 2 daughter cells.
In meiosis, chromosomes arranged in daughter cells are both different from each other and different from the parent cell
Prophase 1 meiosis
During interphase, homologoue chromosomes pair up to form bivalents.
Chromosomes condense and become visible, with each consisting of 2 chromatids.
Nucleolus becomes less obvious and nuclear membrane breaks down. Spindle fibres develop.
In animals, centrioles migrate to opposite poles.
Crossing over also takes place in prophase 1.
Metaphase 1 meiosis
Pairs of homologous chromosomes (bivalents) align along the equator and spindle fibres attach to centromeres.
Anaphase 1 meiosis
Mircrotubules of spindle contract, pulling chromosomes to opposite ends of the pole. This ensures that one chromosome from each pair ends up in the daughter cell. Separation of homologous chromosomes leads to daughter cells being haploid.
Telophase 1 meiosis
Chromosomes pulled to opposite ends of cell, nuclear membrane reforms. Chromosomes become invisible, nucleolus reappears and spindle is broken down.
Cytokenisis meiosis
Produces 2 daughter cells, each containing a haploid number of chromosomes
Meiosis 2
Very similar to mitosis.
Spindles are formed at right angles to the angle of division of meiosis 1. Each chromosome separates into 2 chromatids.
Cytokinesis occurs, each original daughter cell produces 2 new daughter cells. Therefore, meiosis produces 4 haploid cells, each different from each other, in 2 cell divisions.
Explain independent assortment
One chromosome from each homologous pair can enter a gamete. It can be either of 2 chromosomes from any pair. This is dependent on the random nature of how chromosomes line up at the equator at the start of metaphase 1.
As there are 23 chromosomes, there are millions of possible combinations for any one gamete.
This independent assortment produces variation in the next generation.
Explain crossing over
Sometimes 2 non sister chromatids break and exchange sections with each other. The points where chromosomes cross over are called chiasmata. Crossing over exchanges genes between chromosomes from 2 chromosomes in a homologous pair. This can work as genes will be the same but alleles may not be.
The consequence of this..
2 chromatids of same chromosme are no longer identical.
Some chrmatids may contain unique sequences of alleles that did not exist in the parental chromosome.
