Chromosomes and Gametes Flashcards
What are the fundamental principles of evolution?
- The ability to reproduce
- Variation
Each new generation in turn reproduces or dies out – selection of the fittest
What is a karyotype?
The number and appearance of human chromosomes
Describe the structure of a chromosome
Each chromosome has a constriction point called the centrosome
Divides chromosome into 2 sections / “arms” – short (p) and long (q)
What is the significance of the centromere location?
Gives a chromosome its characteristic shape
- used to describe location of the gene
What are the 3 roles of functional DNA?
- replicate
- separate its 2 copies at mitosis
- maintain itself between generations
What are the DNA requirements for sexual reproduction?
Each parent passes on one allele to each offspring
If alleles are heterozygous, trait phenotype either dominant / recessive
When do copy number variants occur?
Copy number variants (CNV) occur if more than one, three or more copies of alleles
Briefly outline transcription and translation if a gene
- Promoter + coding seq. transcribed into gene product
- Introns removed from exon by splicing
- mRNA exported out of nucleus
- Translated into proteins in ribosomes by complexes of tRNA & proteins
- Proteins folded into unique 3D structure - determines function
How can a single gene become tissue specific?
Same gene can be tissue specific by having alternative promoters
Give an example of a tissue specific gene
CYP19A1 (encodes aromatase)
uses different promoters in breast, ovary and brain
Aromatase converts androgens into oestrogen
What is the role of some non-coding regions in genes?
Most genes contain non-coding regions that regulate gene expression
How does one gene produce multiple gene products?
Via Alternative splicing of exons
- Products are known as Isoforms
How can a protein be modified after production?
Post-translational modification - e.g phosphorylation
Glycosylation
- adding on carbohydrates to protein
- making protein more stable and soluble
- LH/FSH is modified like this
How are hormones modified for action?
Often secreted as “prohormones” and are enzymatically processed to form active hormones
eg. prepro GnRH
Give an example of alternative splicing producing isoforms
3 alternatively spliced variants of human FSHR found in testicular tissue
– possible association with spermatogenic defects
What is the purpose of LH/FSH glycosylation?
Different no. of glycosylated elements added on to gonadotropins to form variations of the hormones.
Different glycosylated elements have different activities and will form different isoforms throughout reproductive life.
What is the significance of DNA in reproduction?
DNA needs to be able to be passed through generations
How do most cells in the body reproduce?
Most cells and many organisms replicate by doubling DNA and dividing to give 2 identical progeny or clones - asexual reproduction
What is the process by which human cells reproduce?
Mitosis:
Fusion of haploid cells (gametes) to create unique progeny.
Uniqueness brought about by crossing over and independent sorting of chromosomes
Describe the asexual mitosis of somatic cells
- Homologous chromosomes
- DNA replicated
- Sister chromatids form
- BIvalent chromosomes line up between 2 mitotic
spindles - Undergo two cell divisions
- gametes formed
What is the product of somatic cell division?
Somatic / diploid cells replicate by simple cell division
- give identical progeny
- limited no. of divisions,
e. g hepatocytes, pancreas, skin cells
What are the advantages of diploidy?
Prevents accumulation of genetic mutations
Increased genetic diversity
Maintenance occurs due to advantage of genetic variability
Variation in off-spring → survival of the fittest? More able to evolve + adapt to changing environment
Why is allelic variation not always bad?
Mitosis can cause allelic variation - but just as many good as bad
If 2 good ones occurred then both would proliferate
- out-survive others
Where are X and Y chromosomes thought to originate from?
Differentiated from pair of identical chromosomes (autosomes) ~300 million years ago
Ancestral mammals developed variation which made it male - gradually becoming Y and the other the X (sex locus (Y) = male)
How many working genes are found on each X and Y chromosome?
X chromosome → 1000 working genes
Y chromosome → 86 working genes
What is a gamete?
A haploid cell specialised for sexual fusion (egg + sperm)
How many chromosomes in a haploid cell?
23 in total
How do gametes reproduce?
Unlike other cells gametes go through cycles of diploidy & haploidy
Undergo cycles of mitosis to increase numbers
Then undergo meiosis
Then combine at fertilisation
How are gametes formed?
From germ line cells: primordial germ cells that migrate into gonad and differentiate to either male or female gametes
What is the process producing oocytes?
oogenesis (incorporated as part of folliculogenesis)
What is the process producing sperm?
spermatogenesis
Which phase of the cell cycle do chromsosomes duplicate?
Chromosomes replicate during S-phase of cell cycle
- still attached at centromere
What is a chromatid?
Each copy of replicated chromososme
‘sister’ as they’re identical
What is interphase?
Period of cell cycle between cell divisions - Not a “resting period”
What processes occurs during interphase?
Cell carries out functions and grows
DNA and organelles duplicate
Describe the structure of genetic material during interphase
Genetic material in long, thin threads called chromatin.
Twist randomly around one another like tangled strands of yarn.
What are the 4 stages of mitosis?
Prophase; Metaphase, Anaphase & Telophase
What occurs during prophase of the cell cycle?
nuclear & cytoplasmic changes.
Outline the nuclear changes that occur in prophase
Chromatin condenses
Forms chromosomes - DNA wraps around histones
DNA loops + twists to form tightly compacted structure
Condensed DNA can’t replicate - gene activity shut down
Sister chromatids easier to separate without breaking
Nuclear membrane also begins to break down.
What are the cytoplasmic changes of prophase?
Mitotic spindle forms - made of microtubules associated with centrioles
Centrioles duplicated during interphase, move away from each other toward opposite poles in prophase
