Cell Cycle And Mitosis Flashcards
What is the three stages of the cell cycle
Interphase, nuclear division and cytokinesis
What is the interphase
The longest stage in the cell cycle. Interphase is when the organelles double, the cell grows and then the DNA replicates.
Also known as the synthesis phase.
What is nuclear division?
Nuclear division can be either mitosis, creating two identical diploid cells or meiosis, creating four genetically different haploid cells.
Mitosis creates cells with identical DNA for growth and repair where as meiosis creates gametes.
What is cytokinesis?
This is the final stage. The division of the cytoplasm to create the new cells.
What are the four key facts of mitosis?
- involves one round of cell division
- genmetically identical cells are made
- diploid cells are made
- growth and repair where
What is prophase
In this stage the chromosomes condense and become visible. In animal cells, the centrioles separate and move to opposite poles of the cell.
The centrioles are responsible for creating spindle fibres which are released from both poles to create a spindle apparatus. The nuclear envelope also breaks down.
Plants have a spindle apparatus, but lack the centrioles.
What is the metaphase.
The chromosomes align along the equator of the cell between the two centrioles. The spindle fibres released from the poles now attatch to the centromere and chromatid.
Spindle fibres are a type of microtubule. The spindle fibres are collectively known as the spindle apparatus.
What is the Anaphase?
The spindle fibres start to retract and pull the centromere and chromatids they are bound to towards the opposite poles. This causes the centromere to divide in two and the individual chromatids and pulled to each opposite pole. These separated chromatids and now referred to as chromosomes.
This stage requires energy in the form of ATP which is provided by respiration in the mitochondria.
What is telophase?
The chromosomes are now at each pole of the cell and become longer and thinner again.
The DNA no longer needs to be condensed so the chromosomes are no longer visible as the chromatin spreads out.
The spindle fibres disintegrate and the nucleus starts to reform. The final stage in the cell cycle is when the cytoplasm splits in two to create the two new genetically identical cells.
What is the mitotic index
The mitotic index can be calculated by counting down how many cells are visible in the field of view and the number of cells tha are in the stage of mitosis.
Mitotic index = the number of cells in mitosis ÷ total number of cells
What is binary fission
The process of cell division (binary fission) in prokaryotic cells.
It is much simpler than mitosis in eukaryotic cells because Prokaryotic cells do not possess:
A nucleus
Chromosomes
Membrane-bound organelles
Spindle fibres
What are the DNA molecules like in Prokaryotic cells?
They have a single, circular DNA molecule and plasmids.
Plasmids are smaller, circular DNA molecules that are also replicated and inherited
The number of copies of plasmids that each cell inherits can vary, although this number is regulated
What is the process of binary fission?
The single, circular DNA molecule undergoes DNA replication
Any plasmids present undergo DNA replication
The parent cell divides into two cells, with the cytoplasm roughly halved between the two daughter cells
The two daughter cells each contain a single copy of the circular DNA molecule and a variable number of plasmids
How do viruses replicate
All viruses are parasitic, meaning they can only reproduce by infecting living cells (known as host cells)
Viruses replicate by injecting their nucleic acid into a host cell
What is the process of virus replication.
First, a virus uses attachment proteins on its surface to bind to complementary receptor proteins on the surface of a host cell
The virus then injects its DNA or RNA into the host cell
The host cell then uses its nucleic acid and protein-building machinery (ribosomes) to produce new viral particles
What are the two types of gene mutation?
- deletion
- substitution (nonsense, silence or missense)
What are the two types of chromotome mutation
- polyploidy
- non disfunctional
What is silent substitution?
Also the wrong nucleotide has been placed in the amino acid code.
The same amino acid is being coded for. Because the genetic cod is degenerate, there is no change in the polypeptide being produced.
What is missense substitution?
This type of mutation results in a different amino acid being coded for. This could affect the structure and function of the polypeptide chain.
What is nonsense substitution?
This results in a stop codon being coded for. This will prevent the rest of the polypeptide chain being produced.
What is polyploidy chromosome mutation?
When there is a chnage in the whole set of cchromosome. Caused by there being 3 sets of chromosomes rather than 2.
What is nondisfunctional chromosome mutation.
When indivitual homologous pairs of chromosomes fail to separate during mitosis. This usually results in the gamete having one more or one less chromosome
What is a mutagenic agent.
The mutation rate can be increased by mutagenic agents. Eg uv rays
These disrupt funcyion of DNA and interfere with transcription. Thry can help with diversity however some mutations can cause danger.
What is a cancer?
Cancers arise due to uncontrolled mitosis
Cancerous cells divide repeatedly and uncontrollably, forming a tumour (an irregular mass of cells)
Cancers start when changes occur in the genes that control cell division. A change in any gene is known as a mutation. If the mutated gene is one that causes cancer it is referred to as an oncogene
Stages of development of cancer
Malignant tumours interfere with the normal functioning of the organ / tissue in which they have started to grow (eg. they may block the intestines, lungs or blood vessels
Malignant tumour cells can break off the tumour and travel through the blood and / or lymphatic system to form secondary growths in other parts of the body
The spreading of cancers in this way is known as metastasis
Metastasis is very dangerous as it can be very difficult to detect, locate and remove secondary cancers
What is a gene
Small section of DNA that codes for one protein
What is locus
The position of a gene on a chromosome or DNA molecule
What is an allele
one of the many different forms of gene
What is a homologous pair of
pair of chromosomes (one maternal and one paternal) that have the same gene loci.
What is meiosis
Meiosis produces four genetically different daughter cells.
Meiosis involves two nuclear divisions and creates four haploid daughter cells from a single diploid parent cell.
What is a haploid and a diploid
Haploid - only one set of chromosomes.
Diploid - two sets of chromosomes
What is independant segregation
The homologous pairs of chromosomes line up opposite each other at the equator of the cell.
It is random which side of the equator the paternal and maternal chromosomes from each homologous pair lie.
These pairs are separated so one of each homologous pair ends up in the daughter cell.
This creates a large number of possible combinations in the daughter cells produced.
What are the two mechanisms that introduce variation
- independent segregation of homologous chromosomes
- crossing over between homologous chromosomes
What is crossing over
When homologous pairs line up opposite each other at the equator in meiosis 1, parts of the chromatids can become twisted around each other.
This puts tension on the chromatids, causing pairs of the chromatid to break.
The broken parts of the chromatid recombine with the another chromatid.
This results in new combinations of alleles.
First four stages of meiosis
- Interphase - DNA and organelles are replicated.
- Sister chromatids are attached by a centromere.
- Chromosomes lines up in homologous pairs along the equator (of the cell)- Independent segregation.
- Crossing over can occur. Alleles are exchanged between maternal and paternal chromosomes.
Last four stages of meiosis
- Spindle fibres attach to the centromeres, connecting each chromosome to one centriole.
- Spindle fibres contract, pulling one chromosome from each homologous pair to either side (of the cell), forming 2 new cells.
- Spindle fibres connect each centromere to both centrioles. When the fibres contract, sister chromatids are separated.
- 4 genetically identical gametes are created, each with 23 chromosomes.
What is chaisma
Chaisma forms a bivalent (connected to the homologous chromosomes). This starts during metaphase 1. This only occurs in the first round of mitosis.
Meiosis compared to mitosis:
Meiosis
Two nuclear divisions
Haploid cell (one set of chromosomes)
Introduces genetic variation
Mitosis
One nuclear division
Diploid cells
Creates genetically identical cells.
Genetic variation in gametes
There are 2n possible chromosome combination in gametes
It is random which egg and sperm will fuse in fertilisation. Therefore, variation is increased.
There is actually (2n ) 2 combinations of chromosomes because of random fertilisation
This is before crossing over is considered.
When do mutations in chromosomes arise
Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis.
Non disjunction - This is when the chromosomes or chromatids do not split equally during anaphase
How does polyploidy occur
Each homologous pair is doubled due to DNA replication in interphase.
All of the chromosomes fail to separate equally during metaphase in meiosis 1.
In meiosis 2, chromatids separate equally.
Instead of creating haploid cells, its created a diploid cell as the chromosomes failed to separate in meiosis one. If a diploid gamete fertilises with a normal haploid gamete (one copy of each homologous pair). There will be three copies of every chromosome.
How does nondisfunctional/aneuploidy occur
Each homologous pair is doubled due to DNA replication in interphase.
One chromosome does not separate equally.
Normal division in meiosis 2,m the chromatids seperate equally.
Two cells will be a haploid with an additional chromosome
Two haploid cells will be missing one chromosome
What causes natural selection
- Variation within a population - caused by a mutation
- Selection pressure causes some of the population to die
- Those with advantageous allele survive and reproduce
- An advantageous allele is passed on to the offspring
- Allele frequency therefore increases in the next generation.
What is the evidence for natural selection
- Organisms produce many more offspring than survive to be mature individuals
- The individuals in a species are not all identical
• random assortment of paternal & maternal chromosomes in meiosis
• crossing over of segments of paternal & maternal chromosomes
• random fusion of male & female gametes in sexual reproduction mutations - Natural selection results in offspring with favourable characteristics
- favourable characteristics expressed in their phenotypes of some offspring.
What is stabilising selection
• Stabilising Selection acts against both extremes in a range of phenotypes. As a result the variation about the mode is reduced.
• After selection, the mode is in the same position: This is the most advantageous phenotype. Stabilising selection has reduced the variation about this modal value.
What is directional selection
One of the extremes has the selective advantage. Occurs when there is a change in the environment.
The modal trait changes eg antibiotic resistance in bacteria.
What is a super bug
One bacterium contains a plasmid to be transferred
A a connection is formed and the plasmid is copied.
Both bacteria now contain plasmid.
The recipient may even integrate the plasmid into its chromosome.
Factors that cause varation - genemutatuion , independant segregation and crossing over
Gene Mutation - An error occurs during replication of the DNA in a gene. A change in the order of base alters the amino acid sequence in the protein coded for by the gene.
Independent segregation - During meiosis, maternal and paternal chromosomes are reshuffled. The chromosomes and therefore alleles of genes can combine in new ways.
Crossing over - During meiosis 1, sections of chromatids are interchanged. Blocks of genes are moved and linked alleles may separate and rejoin in new combinations.
Factors that cause variation - chromosome mutation, random fertilisation and environmental factors
Chromosome mutation - During cell division, sections of chromosomes are displaced, e.g., during anaphase. This can result in genes being deleted, duplicated or inversion of a sequence.
Random fertilisation - Each parent is genetically different and can produce huge numbers of gametes. Which gametes fuse at fertilisation is a matter of chance.
Environmental factors - The expression of genes may be affected by diet, disease or temperature during development. Mutagens may cause gene mutations in somatic cells.
Describe binary fission in bacteria
Replication of (circular) DNA;
- Replication of plasmids;
- Division of cytoplasm (to produce daughter cells
Describe the role of the spindle fibres and the behaviour of the
chromosomes during each of these phases
prophase, chromosomes condense;
- (In) prophase OR metaphase, centromeres attach to spindle fibres;
- (In) metaphase, chromosomes/pairs of chromatids at equator/centre of spindle/cell;
- (In) anaphase, centromeres divide;
- (In) anaphase, chromatids (from each pair) pulled to (opposite)
poles/ends (of cell); - (In) prophase/metaphase/anaphase, spindle fibres shorten;
Describe the appearance and behaviour of chromosomes during mitosis.
During prophase)
1. Chromosomes
coil / condense / shorten / thicken / become visible;
- (Chromosomes) appear as (two sister) chromatids joined at the
centromere;
(During metaphase)
3. Chromosomes line up on the equator / centre of the cell;
4. (Chromosomes) attached to spindle fibres;
5. By their centromere;
(During anaphase)
6. The centromere splits / divides;
7. (Sister) chromatids / chromosomes are pulled to opposite poles /
ends of the cell / separate;
(During telophase)
8. Chromatids / chromosomes
uncoil / unwind / become longer / thinner.
Compare what happens durinh cytokenisis in animal and plant cells
In animal cells - cell membrane is pulled inwards across centre of the cell. This separates the cytoplasm into two halvrs each containing a new nucleus
Plant cells - vesicles fuse to extend cell membranes across cytpp;assm. New cell walls develop
Why would we prepare a root tip squash with a stain
To distinguish chromosomes because would not have been visible without the stain
Explain how genetic variation is increased though meiosis
- homologous pairs pair up
-independant segregation
-crossing over
-new comninqation of alleles are created
and explain the factors that influence the duration of the cell cycle in multicellular organisms.
cell type; [1 mark]
• A rapidly-dividing cell (e.g. hair follicle, bone marrow) versus a slow-growing one (nerve cells, striated muscle cell); [1 mark]
• The organism; [l mark]
o fast-growing vs slow-growing
• The conditions; [l mark]
• e.g. a growth phase in a foetus vs an adult / a tropic
researcher pressed down on the sample tissue by applying pressure to the coverslip to…
Create a single / thin layer of cells/tissue; [l mark]
• So light could pass through the sample and give a clear image/ distinguish cells/nuclei/chromosomes;
researcher pressed down on the sample tissue by applying pressure to the coverslip to…
Create a single / thin layer of cells/tissue; [l mark]
• So light could pass through the sample and give a clear image/ distinguish cells/nuclei/chromosomes;
Explain the term metatesis
spreading of malignant OR cancerous tumours; [1 mark]
• By a piece of tumour breaking off and travelling in blood / lymph; [1 mark]
• To form a secondary tumour/growth (in other organs/tissues); [1 mark]
• Greatly reduced survival chances because of the difficulty of detecting and treating secondary tumours; [1 mark]
Explain what metatesis is
spreading of malignant OR cancerous tumours; [1 mark]
• By a piece of tumour breaking off and travelling in blood / lymph; [1 mark]
• To form a secondary tumour/growth (in other organs/tissues); [1 mark]
• Greatly reduced survival chances because of the difficulty of detecting and treating secondary tumours; [1 mark]
Compare and contrast the processes of binary fission and eukaryotic cell division.
Both replicate DNA / entire genome; [1 mark]
• Identical daughter cells produced; [1 mark]
No chromosomes
Plasmids
