Meiosis And Mitosis Flashcards
Interphase (mitosis)
- nuclear envelope + nucleolus still present
- chromatin/dna replicated (s phase)
- centrioles doubled (2 of them) (G phase)
Prophase (mitosis)
- nuclear envelope/nucleolus breaks down to give more space in the cell for genetic material to move around.
- chromatin/dna has condensed down into chromosomes (replicated chromosomes)
- centrioles have moved to pole of cell to start to make spindle fibres
Metaphase (mitosis)
- nuclear envelope/nucleolus still gone
- replicated chromosomes in equator of cell.
- Centrioles have fully formed spindle fibres attached, they attach to centromeres of replicated chromosomes.
Anaphase (mitosis)
- nuclear envelope/nucleolus still gone
- replicated chromosomes pulled apart into sister chromatids. Chromosomes form.
- spindle fibres contract and start to break down at the end of the anaphase.
Telophase (mitosis)
- nuclear envelope + nucleolus reform. 2 nuclei in 1 cell
- chromosomes decondense to chromatin
- spindle fibres fully break down.
Cytokinesis
division of the cytoplasm to form two separate daughter cells
What is meiosis?
A type of cell division that produces gametes
Ways that meiosis ensures genetic variation? (Not how yet)
- through the production of non-identical gametes
- as a consequence of independent assortment of chromosomes and crossing over of alleles between chromatids
Explain how meiosis gives rise to genetic variation in gametes?
Independent assortment gives rise to different/new combinations of chromosomes
Crossing over means sections of chromosomes are swapped over
What is crossing over and how does it create genetic variation?
- When sections of DNA are exchanged between non sister chromatids (chiasmata)
- This gives rise to a large amount of genetic variation
What is the locus of a gene?
The position of a gene on a chromosome
What is autosomal linkage?
When genes that are on the same autosome are not assorted independently (as they have a similar locus) so they become linked and stay together during original parental combination - meaning they are passed on to offspring all together.
What is the autosome?
All chromosomes except from the sex chromosomes
Why is colour blindness more common in males than females?
Colour blindness is caused by a recessive allele on the X chromosome
Males only need 1 recessive allele to be colour blind whereas females need 2
Why does males only having one X chromosome make it more likely they will inherit sex linked conditions?
Because it means they are hemizygous for any genes present on the X chromosome and therefore it will be expressed even if it is a recessive allele
What is sex linkage?
When genes are linked due to the locus of the allele that the gene codes for being found on a sex chromosome
What is a stem cell?
An undifferentiated cell that can give rise to specialised cells and divide to produce more stem cells
What is potency?
The ability to differentiate into more specialised cell types
What are totipotent stem cells?
Stem cells that can differentiate into any cell type
What are pluripotent stem cells?
Embryonic stem cells that can differentiate into any cell type found in an embryo, but not the cells forming placenta and umbilical cord
What are multipotent stem cells?
Adult stem cells which have lost some of the potency associated with the embryonic stem cells, no longer pluripotent
What are the differences between totipotent and pluripotent stem cells?
- totipotent can differentiate into any cell type
- pluripotent can only give rise to most (not all - cells in the body or totipotent cells)
- totipotent can give rise to an entire human, pluripotent cannot
What are the uses of embryonic stem cells, and where are they taken from?
Huge potential in development of therapeutic treatments of diseases
Taken from embryos, and can therefore be pluri or totipotent
What are the uses of adult stem cells, and where are they taken from?
Can be injected into damaged tissue to treat diseases such as leukaemia and also injuries - but must match blood type and antigen or there is a risk of rejection
Taken from bone marrow and brain
How do official regulatory authorities make decisions about the use of stem cells?
- review proposals for stem cell resarch
- licensing and monitoring of research centres
- providing guidelines and codes for practice
- monitoring new developments
- providing governments and other professional bodies with correct and up to date advice and info on stem cell research
What are the benefits of stem cells?
Potential to treat a range of diseases, can develop treatments from a patient’s own organs, adult stem cells are already successful.
What are the risks/issues with stem cells?
Stem cells could become infected in lab, can develop into cancer cells due to mutations, there are low numbers of donors
What are the social issues associated with the use of stem cells?
Lack of peer reviewed evidence to suggest success of stem cells, public must be educated sufficiently, embryonic stem cells collected before birth are expensive
What are the ethical objections to the use of stem cells?
Amounts to killing an unborn child/abortion/murder, intefering in nature, may have genetic consequences for future generations, pressure to harvest human eggs for medical use, money could be used for other types of research
What are the advantages of using stem cells from a patient rather than a donor?
Genetically identical cells to the patient therefore there is no risk of rejection.
No need to take immunosuppressant drugs, and less risk of infection
How do cells become specialised?
Differentiation, Stimulus (e.g. chemical) causes some genes to be switched on/expressed. This leads to transcription of active genes, the mRNA produced is then translate to create the protein. This protein modifies the cell/determines cell structure, causing it to become specialised.
What is a transcription factor?
A protein that controls the transcription of genes by binding to a specific region of DNA
What are the 2 types of transcription factor?
Transcription factors increasing rate of transcription = activators
Transcription factors decreasing rate of transcription = repressors
How do transcription factors control gene transcription?
Binding to promotor region of a gene
They interact with RNA polymerase, either assisting or preventing it
What is an operon?
A group or cluster of genes that are controlled by the same promoter
What is the lac operon?
Section of genes that controls production of enzyme lactase, which is ‘inducible’ and only synthesised when lactose is present
What happens when lactose is absent? (lac operon)
- regulatory gene is transcribed and translated to produce the lac repressor protein
- the protein binds to the operator region
- due to the presence of the promoter region, RNA polymerase is unable to bind
NO LACTASE PRODUCED
What happens when lactose is present? (lac operon)
- lactose is present so binds to lac repressor protein and distorts it
- this means it can no longer bind to the operator region
- RNA polymerase is able to bind to the promoter region
LACTASE IS PRODUCED
What is an organelle?
A component within a cell that carries out a specific task
What is a cell?
The basic functional and structure unit within a living organism
What is a tissue?
A group of cells that works together to perform a particular function
What is an organ?
Made from a group of different tissues working together to perform a particular function
What is an organ system?
Made from a group of organs with related functions, working together to perform body functions within the organism
What does monogenic mean?
Characteristics controlled by one gene
What does polygenic mean?
Characteristics controlled by several genes
What is polygenetic inheritance?
When a single characteristic is coded for by several genes at different loci. Gives rise to continuous variation
What is epigenetics?
The control of gene expression by factors other than an individual’s DNA sequence. This involves switching genes on and off without changing the actual code.
What are histones?
Proteins around which nuclear DNA is wrapped to form chromatin
How can chromatin be chemically modified to alter gene expression?
-methylation of DNA
-histone modification via acetylation of amino acid tails
What is DNA methylation and what change does it cause?
The direct addition of methyl groups (CH3) to DNA
Causes the inactivation of genes
How does DNA methylation cause the inactivation of genes?
Methylation suppresses the transcription of the affected gene by inhibiting binding of transcription factors and the enzymes needed for transcription
What can DNA methylation be affected by?
Environmental, lifestyle or age related factors
What is the acetylation of histones and what change does it cause?
The addition of acetyl groups (-COCH3) to lysine amino acids on histone proteins
It causes genes to be activated
How does histone acetylation cause gene expression?
On lysine amino acid, there is normally a positively charged R group which forms iocnic bonds with the phosphate backbone - so DNA coils tightly.
Adding an acetyl group removes the positive ion, removing a bond between the histone protein and DNA so it becomes less tightly wrapped.
Less tightly wrapped DNA means RNA polymerase and transcription factors can bind more easily and the gene is activated.
Can epigenetic changes be passed on?
Yes, via cell divison
EXAMPLE - during gamete production, DNA in parent cells usually undergoes de-methylation, but often methyl groups are not removed and therefore are present in DNA on sperm or egg cells
Epigenetic changes due to environmental factors therefore can be passed down to the next generation
Why does increasing the number of stem cell divisions increase the risk for cancer?
Greater number of cell divisions = more opportunities for errors in DNA replication
Therefore more mutations may occur, creating a greater chance of cancer.
Pluripotent differentiation heart example
Pluripotent stem cells can repair damaged heart tissue because pluripotent stem cells are undifferentiated stem cells which can differentiate into many different cells. Stem cells receive a signal from surrounding heart muscle cells which allows RNA polymerase to bind to the promoter. Genes are then activated in the stem cells and active genes are transcribed to mRNA which is translated to a protein. The proteins ensure that stem cells differentiate into heart cells.
What is the importance of meiosis in the production of gametes?
- It halves the chromosome number to produce a haploid nucleus so that once fertilisation has occurred, the diploid number of chromosomes is restored.
- it allows for genetic variation through independent assortment and crossing over.
