chromosomes and gametes Flashcards
What is the two key features present in all living & evolving organisms
1.REPRODUCTION: Genes are passed down through reproduction.
- Variation: the replicating system must undergo changes
Each new generation in turn reproduces or dies out – selection of the fittest
What are the characteristics of the human karyotype
23 autosomes
1 pair of sex chromosomes
centrosomes: this is the constriction point for chromosomes (allows us to locate the gene)
p arm: short arm
q arm: long arm
For DNA to be functional, what are the requirements and how does this contrast to what is required for sexual reproduction?
- it must be able to separate its two copies during mitosis
- it must be able to maintain itself between generations.
- For sexual reproduction each parent passes on one copy of an allele (one version of a gene to each offspring). 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.
Describe the process of transcription and translation
Promotor and coding sequence transcribed into a gene product.
Introns are removed from exon by splicing.
mRNA exported out of nucleus
Translated into proteins in ribosomes i.e. complexes of tRNA and proteins
Proteins then folded into unique 3D structure that determines function
How can some genes be tissue specific
- One gene produces one protien but there are different promotors in different tissues which act upstream to drive transcription.
For example: CYP19 is a gene that codes for aromatase (which produces oestradiol from androgens) it has different promotors in different tissues
How can one gene give rise to several products
- The alternative splicing of exons. (ISOFORMS)
eg: one study found alternative variants of FSHR in testicular tissue
- Post translational modifications: phosphorylation, glycosylation (i.e. adding on carbohydrates to protein, making protein more stable and soluble)
eg: LH & FSH are modified by glycosylation. ( The amount of glycosylation tells us about the efficacy of FSH binding to its receptor)
study: in women who went through menopause - there was differential glycoysylation to FSHR- so that FSH bound more weakly)
Some hormones are secreted as pro hormones” and must be enzymatically processed to form the active hormone. eg. proinsulin to insulin and GnRH
What are the DNA requirements for sexual reproduction?
Mitosis the fusion of haploid cells (gametes) to create a unique diploid progeny. This uniqueness brought about by crossing over and independent sorting of chromosomes.
What occurs in somatic cell division
Somatic or diploid cells replicate by simple cell division
give identical progeny, usually have limited number of divisions,
eg hepatocytes, pancreas, skin cells
What are the advantages of sexual reproduction?
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
how did the X and Y chromosomes arise?
They arised from 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
eg: 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
how did the X and Y chromosomes arise?
They arised from 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
eg: 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
what are gametes and what is a special characteristic about them?
gametes are haploid cells that are capable of sexual fusion.
- special characteristic: they undergo cycles of being haploid cells or diploid cells.
2 haploid cells fuse to form a diploid cell
mitosis occurs to produce 4 diploid cells
each dipoloid cell undergoes meiosis to form a haploid cell
What cells do gametes originate from ?
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
Describe the stages of mitosis in detail
Interphase:
G1 phase: make more organelles. Synthesising proteins and enzymes to aid in DNA replication. Repair thymidine dimers- part of DNA repair mechanism
|
|
|
|
(G1/S phase checkpoint)
|
|
|
|
S phase: synthetic phase (going from 2n—-> 4n
DNA replication: Open the DNA and form a replication bubble, synthesises DNA based on what is present in the replication bubble - semi conservative model
* enzymes required for DNA replication: DNA polymerase I &II
|
|
|
|
G2 phase: focuses on cell growth to ensure its big enough to replicate
it does this by increasing cytoplasm
|
|
|G2/M checkpoint
–Ensures that there are no errors in DNA replication
Mitosis (M phase)
Prophase: DNA is loose – euchromatin, so we want to condense it. . 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 The nuclear envelope will dissolve. ). 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.
Formation of the centrioles, known as microtubule organisation centres these connect to the chromosomes to be able to separate them.
Metaphase: MTOC goes to opposites poles of the cell. These form the microtubules. Protien structure outside the centresome called the kinetochore which connects to the spindle. Chromosomes lined up on the metaphase plate.
- checkpoint: M checkpoint
Anaphase: the centromere is split and sister chromatids move to opposite poles.
Telophase: a cleavage furrow forms. A nuclear envelope reforms around the chromosomes. and the mitotic spindle disassembles. The chromosomes also become more threadlike in appearance.
Cytokinesis: cells pinch off to form 2 cells each with 2n (46 chromosomes in each cell)
Some cells go straight back into the cell cycle the proliferative cells.
Some cells go into G0 (quiescent) stage where they do not proliferate
Describe the process of meiosis
Meisois produces gametes to promote sexual reproduction
-In meiosis we are dealing with homologous chromosomes.
-Homologus chromosomes: we are not dealing with identical chromosomes but the pairing of chromosomes with different genes (alleles)
In prophase I homologous chromosomes align and we have genetic recombination — Key process in generating variation
In metaphase I
In anaphase sister chromatids do not seperate like in mitosis, the homologous pairs separate
