Topic 3B/3C- Gametes and Fertilisation and differentiation and variation. Flashcards
(16 cards)
What is the definition of fertilisation?
The exact moment when the nuclei of both the mother and father parent cells fuse to form a zygote.
How is the egg cell specilised for its function?
It has a zona pellucida, which is a protective glycoprotein layer that sperm have to penetrate.
It has follicle cells which form a protective coating.
Describe how fertilisation occurs in the oviduct.
The sperm swim towards the egg cell in the oviduct.
Once one sperm comes in contact with the zona pellucida, the acrosome reaction occurs.
This is where the acrosome releases digestive enzymes that digest the zona pellucida. The sperm can make its way into the egg cell.
When the sperm cell fuses with the egg cell membrane, the cortical reaction is triggered.
Here, vesicles in the egg cell release cortical granules which cause the zona pellucida to thicken and become impenetrable to other sperm cells. This is to ensure than only one sperm fertilises the egg cell.
Then the sperm cells nuclei and the egg cells nuclei fuse together, and fertilisation occurs, forming a zygote.
Meiosis is a form of cell division which produces genetically different haploid daughter cells/gametes. How do these daughter cells become genetically different?
Crossing over of chromatids-
This is where two chromatids from different homologous pairs twist around each other and break. They rejoin onto the adjacent chromatid. This means that although the genes are the same on the homologous pair, the order of alleles are different are different, so the four new cells formed from the meiosis have different allele combinations.
Independent assortment-
The four daughter cells have different combinations of chromosomes. This includes different combinations of paternal and maternal DNA. When the gametes are actually produces, different combinations of those maternal/paternal chromosomes go into each cell.
How are some genes said to be linked?
Some genes lie on the same loci on a chromosome. This means that they cannot be separated during independent assortment, and they will be passed onto their offspring if this is the case. The only way they can be separated is during chromatid crossover, but this is less likely to happen if the two genes are very close together on the loci.
How are some characteristics sex-linked?
A characteristic is said to be sex linked when the locus for the allele which codes for it is on the sex chromosomes, X and Y. Most genes tend to be carried on the X chromosome, as it it much larger than the Y chromosome. If a gene is only carried on the X chromosome, it is said to be X-linked. Since males have the sex chromosomes XY, they are more likely to attain sex linked disorders such as colour blindness, as if they get only one recessive allele on their X chromosome, they will still express the trail for it, meaning they have a much higher probability of obtaining such diseases than women who would need to be homozygous recessive which is less likely to occur.
What are sex-linked disorders caused by?
Fault alleles found on sex chromosomes.
What are transcription factors?
Proteins which bind to DNA to determine whether a gene is activated or not by altering the rate at which the gene is transcribed.
Transcription factors which increase the rate of transcription are called activators, factors which decrease the rate of transcription are called repressors. They help or deter RNA polymerase from binding to DNA and transcribing it.
In eukaryotes, transcription factors tend to bind to the DNA sites which are near the start of their target gene. In prokaryotes, the control of gene expression involves often binding to structures called operons.
What is an operon?
What structures are included within an Operon?
An operon is a section of DNA that contains a cluster of structural genes, that are transcribed together.
POS- Promoter, Operator, Structural
Promotor- RNA polymerase binds here
Operator- Transcription factors bind here
Structural genes- code for useful proteins such as enzymes like lacZ lac Y lacA.
In the lac operon, what occurs when:
Lactose is not present
Lactose is present
When lactose is not present, a lac repressor which is produced by the regulatory gene lacI binds to the Operator, and prevent RNA polymerase from binding to the Promoter region, so the structural genes lac Z lac Y and lac A arent transcribed.
When lactose is present, it binds to the repressor coded for by lacI, and changes its shape. This shape change means that the repressor can no longer bind to the Operator, and RNA polymerase can therefore now bind to the promoter region and the structural genes lacZ lacY and lacAA can be transcribed.
woohoo
What are they benefits of stem cell therapy?
They could save many lives as well as improving the quality of peoples life.
Like if a person has had a heart attack, their cardiac muscle could be replaces using stem cells to do so, same with spinal tissue as well. Stem cells could eventually be used to grow replacement organs. They could be used to regenerate sight in blind people.
What are the two sources of stem cells?
Adult stem cells (from bone marrow)
The operator to remove such bone marrow involves anaesthetic, and a needle into the bone, it has very little risk, but the patient will like be in discomfort. The stem cells cannot give rise to as many cell types as embryonic stem cells but they still can give rise to some.
Embryonic stem cells (from embryos)
These are created during a process call IVF (in vitro fertilisation) which takes place outside the womb. Some egg and sperm cells are allowed to fertilise, to create a zygote, and these are left to grow for 4-5 days. At this point, the stem cells are removed from the embryos and the rest of the embryo is destroyed.
What are the ethical controversies associated with stem cell research?
Many believe that life begins at the moment of fertilisation, because the embryo created is viable and has the potential to grow into a fully grown human with a life. So in extension, this is killing. Some people suggest that embryonic stem cells should be taken from embryos which have been activates artificially to start dividing, because these embryos cannot live past a few days so if they are discarded, you are not killing something which is viable and has the potential to form a human with a life. People think scientists should just use adult stem cells, as they give rise to a human being. However, adult stem cells cannot give rise to all specialised cell types, so they are not as effective in treatment as embryonic stem cells are because they are totipotent, and can give rise to any cell type.
How does society help to manage ethics in cases such as stem cells?
Looking at proposals of research to decide whether such research should be allowed.
Producing guidelines and codes of practice.
Providing information and advice to government and professionals.
What is epigenetic control?
Is it hereditarry?
A process in eukaryotes which occurs in response to a change in environment, where genes can become activator or repressed.
Methylation of DNA represses genes. When methyl groups bind at CpG sites, the shaoe of the Dna changes, so that RNA polymerase cannot bind. So the DNA isnt transcribed and therefore it is repressed.
Histone modification involves the addition or removal of acetyl groups. If acetyl groups are added to the histone proteins, the DNA is less condensed, so RNA polymerase is able to bind to the DNA better, and the DNA can be transcribed, so it is activated. If acetyl groups are removed, the DNA will become more condensed, so there is less room for RNA polymerase to bind and transcribe it, so the DNA coding for the specific gene is not expressed/is repressed.
These changed changed can be hereditary. Whilst mostly the methyl and acetyl groups can be removed during the production of gametes, some remain. These means that offspring will have the same traits that the parents developed due to their environment, but except they’ll have it from birth.
