Chromosomes and Gametes Flashcards
(42 cards)
The defining feature of all evolving living organisms is
the ability to reproduce
Through reproduction we pass on our genes to a new generation
A second principle that is fundamental to evolution is
variation
The replicating system must undergo changes
The changes mean that you are better adapted to the environment.
Each new generation either reproduces or dies out – survival of the fittest.
Karyotype of Human Chromosomes
Humans can karyotype chromosomes – visualise
This can be exploited in reproductive technologies
Karyotype definition
number and appearance of human chromosomes
Centromere
Each chromosome has a constriction point called the centromere, which divides the chromosome into 2 sections or “arms” – short (p) and long (q).
The centromere location gives a chromosome the characteristic shape and is used to describe the location of the gene
There are 22 pairs of autosomes (inherit one from each parent.
1 pair of sex chromosomes = total of 46 chromosomes
For genes to be functional DNA must be able to
replicate
separate its 2 copies at mitosis
maintain itself between generations
DNA requirements for sexual reproduction are different
Each parent passes on one allele (i.e. one version of a particular gene) to each offspring
So if there are any abnormalities in the alleles, it can be passed on, BUT it can also be compensated for depending on what is inherited from each parent.
This is why in consanguineous relationships there is a loss of the ability to filter out common mutations.
Copy number variants (CNV) occur if there are one, three of more copies of alleles
If Alleles are heterozygous the phenotype of the trait can be dominant or recessive.
Gene Transcription and Translation
Another way we can induce differences is through the transcription and translation of these genes.
DNA has a promotor and coding sequence that is transcribed into a gene product -> RNA
Introns are then removed from exon by splicing
Exons will come together
mRNA is then exported out of nucleus
It is translated into proteins in ribosomes i.e. complexes of tRNA and proteins
Proteins are then folded into unique 3D structure that determines function
Tissue specific control of gene transcription
One gene controlled by different promoters in a tissue specific manor.
The same gene can be tissue specific by having alternative promoters
Eg the gene CYP19A1 codes for aromatase (androgens -> estrogens).
Aromatase is found in the granulosa cells in the ovary
But there is also the gene in breast, placental and adipose tissue.
The aromatase produced in the different tissues is exactly the same, this is because the coding region is the same (2-10)
BUT there is a splice site at the start of exon 2, this leads to different exon 1s being attached.
These different exons will have different promoters.
Eg: Ovary = promoter 2. The difference is that this promoter will respond to different hormones (FSH, insulin). This will then drive the exons 2-10 to make aromatase. SO… the aromatase is the same, but it responds to different hormones.
Breast = promoter 1.4 will respond to different growth factors.
In post menopausal women there is no ovarian function so they are not producing estrogen from their ovary. There is a risk of estrogen dependant breast cancer. This is due to abnormal activation of various promoters in the breast, this can drive the breast to produce estrogen which can drive the cancer. The breast gets androgens to make estrogen from the adrenal gland, and as you have abnormal activation of promoters there is a risk of breast cancer.
Gene to protein
One gene giving rise to several products
One gene can give rise to several products by alternative splicing of exons
These products are known as isoforms
A protein can be modified once it has been made by:
Post-translational modification eg phosphorylation,
eg. how are LH and FSH modified?
Glycosylation i.e. adding on carbohydrates to protein, making protein more stable and soluble
Often hormones are secreted as pro-hormones and then they must be enzymatically processed to form the active hormone.
Eg: pre-proGnRH and insulin
Describe alternate splicing
DNA is transcribed into RNA
It moves out of the nucleus
There is then splicing -> removal of introns
Depending on how the exons join, this can form a different protein
eg. 3 alternatively spliced variants of human FSHR found in testicular tissue – possible association with spermatogenic defects
Glycosylation of FSH and LH
Proteins can further be modified in the endoplasmic reticulum eg with glycosylation.
Adding on of various glycosyl groups
Eg can have tetra, di and tri glycosylated X
Each of these will behave differently
These proportions of glycosylation varies as you age, this can alter fertility.
DNA reproduction
DNA needs to be able to be passed through generations
Most cells and many organisms replicate by doubling DNA and dividing to give 2 identical progeny or clones = asexual reproduction
The name given to the duplication of the DNA in this process is: Mitosis
BUT
This is different for sexual reproduction, the DNA requirement is to half the number of the chromosomes.
Need fusion of haploid cells (gametes) to create unique diploid progeny
This uniqueness brought about by crossing over and independent sorting of chromosomes.
Somatic cells
Somatic or diploid cells replicate by simple cell division
give identical progeny, usually have limited number of divisions,
eg hepatocytes, pancreas, skin cells
Asexual vs Sexual Reproduction
The advantages of sexual reproduction are:
Prevents the accumulation of genetic mutations
Increase in genetic diversity
Maintenance occurs because of the advantage of genetic variability
Variation in off-spring → survival of the fittest? Better able to evolve and adapt to changing environment
X and Y chromosomes
Originally there wasn’t X and Y chromosomes.
It is thought to have differentiated from a pair of identical chromosomes (autosomes)..300 million years ago
Ancestral mammal developed a variation which made it male….gradually this chromosome became the Y and the other the X.
With evolution, genes advantageous to either sex became focussed on X or Y and those for ‘maleness’ close to SRY gene.
X chromosome → 1000 working genes
Y chromosome → 86 working genes
Recent comparisons of human and chimpanzee Y chromosomes shown that human Y chromosome has not lost any genes since divergence of human and chimpanzees 6-7 million years ago
Gametes
Gametes are haploid cells that are specialised for sexual fusion. They have 23 chromosomes.
Unlike other cells, gametes go through cycles of diploidy and haploidy:
Gametes are formed from germ line cells: primordial germ cells that migrate into the gonad and then differentiate to either male or female gametes
The process producing oocytes – oogenesis (incorporated as part of folliculogenesis)
The process producing sperm - spermatogenesis
Undergo cycles of mitosis to increase numbers
Then undergo meiosis
Then combine at fertilisation
Duplication of Chromatids
Chromosomes replicate during S-phase of cell cycle
They remain attached at the centromere
Each copy known as a chromatid → the 2 copies are identical to each other → “sister” chromatids
Exact copy of original chromosomes
Mitosis SUMMARY
Mitosis can be broadly divided into 4 stages:
Prophase, metaphase, anaphase and telophase.
Interphase
Interphase is where DNA is duplicated to sister chromatids. The centrioles are also duplicated. There is growth in preparation for cell division.
Interphase is the period of the cell cycle between cell divisions.
Interphase is not a “resting period,” as once thought. Instead, interphase is a time when the cell carries out its functions and grows.
If the cell is going to divide, interphase is a time of intense preparation for cell division. During interphase, the DNA and organelles are duplicated. Throughout interphase, the genetic material is in the form of long, thin threads that are often called chromatin. They twist randomly around one another like tangled strands of yarn. In this state, DNA can be synthesized (replicated) and genes can be active. At the start of interphase, during G1, each chromosome consists of a DNA molecule and proteins.
Prophase
Prophase Mitosis begins with prophase, a time when changes occur in the nucleus as well as the cytoplasm. In the nucleus, the chromatin condenses and forms chromosomes as DNA wraps around histones. The DNA then loops and twists to form a tightly compacted structure. When DNA is in this condensed state, it cannot be replicated, and gene activity is shut down. In this condensed state, the sister chromatids are easier to separate without breaking. At about this time, the nuclear membrane also begins to break down.
Outside the nucleus, in the cytoplasm, the mitotic spindle forms. The mitotic spindle is made of microtubules associated with the centrioles. During prophase, the centrioles, duplicated during interphase, move away from each other toward opposite ends of the cell.
Metaphase
Metaphase During the next stage of mitosis, metaphase, the chromosomes attach to the mitotic spindles, forming a line at what is called the equator (center) of the mitotic spindles. This alignment ensures each daughter cell receives one chromatid from each of the 46 chromosomes when the chromosomes separate at the centromere.
IMPORTANT: In mitosis, the chromosomes line up on the spindle one after the other.
Anaphase
Anaphase Anaphase begins when the sister chromatids of each chromosome begin to separate, splitting at the centromere. Now separate entities, the sister chromatids are considered chromosomes in their own right. The spindle fibers (microtubules condense) pull the chromosomes toward opposite poles of the cell. By the end of anaphase, equivalent collections of chromosomes are located at the two poles of the cell.
Telophase
Telophase During telophase, a nuclear envelope forms around each group of chromosomes at each pole, and the mitotic spindle disassembles. The chromosomes also become more threadlike in appearance.
