chromosomes and gametes Flashcards

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1
Q

What is the two key features present in all living & evolving organisms

A

1.REPRODUCTION: Genes are passed down through reproduction.

  1. Variation: the replicating system must undergo changes

Each new generation in turn reproduces or dies out – selection of the fittest

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2
Q

What are the characteristics of the human karyotype

A

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

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3
Q

For DNA to be functional, what are the requirements and how does this contrast to what is required for sexual reproduction?

A
  • 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.

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4
Q

Describe the process of transcription and translation

A

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

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5
Q

How can some genes be tissue specific

A
  1. 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
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6
Q

How can one gene give rise to several products

A
  1. 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

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7
Q

What are the DNA requirements for sexual reproduction?

A

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.

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8
Q

What occurs in somatic cell division

A

Somatic or diploid cells replicate by simple cell division
give identical progeny, usually have limited number of divisions,
eg hepatocytes, pancreas, skin cells

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9
Q

What are the advantages of sexual reproduction?

A

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

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10
Q

how did the X and Y chromosomes arise?

A

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

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11
Q

how did the X and Y chromosomes arise?

A

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

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12
Q

what are gametes and what is a special characteristic about them?

A

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

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13
Q

What cells do gametes originate from ?

A

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

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14
Q

Describe the stages of mitosis in detail

A

Interphase:
G1 phase: make more organelles. Synthesising proteins and enzymes to aid in DNA replication. Repair thymidine dimers- part of DNA repair mechanism
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(G1/S phase checkpoint)
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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
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G2 phase: focuses on cell growth to ensure its big enough to replicate
it does this by increasing cytoplasm

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|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

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15
Q

Describe the process of meiosis

A

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

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16
Q

what is main difference between mitosis and meiosis?

A

Mitosis:cell divides to prduce 2 new daughter cells that are identical to original and diploid Meiosis: _______________________
Why is meiosis advantageous?
random distribution of male and female homologous chromosomes
chromosomal crossing over occurs

17
Q

Compare mitosis and meiosis

A

Mitosis: *Involves one cell division
*Produces two diploid cells
*Occurs in somatic cells
*Results in growth and repair
*No exchange of genetic material
*Daughter cells are genetically similar

Meiosis: *Involves two cell divisions
*Produces up to four haploid cells
*Occurs only in ovaries and testes during the formation of gametes (egg and sperm)
*Results in gamete (egg and sperm) production
*Parts of chromosomes are exchanged in crossing over
*Daughter cells are genetically dissimilar

18
Q

How is genetic variability achieved?

A

Independent Assortment
homologous pairs of chromosomes line up at the equator (midpoint) of the spindle during metaphase I.
However, the orientation of the members of the pair is random with respect to which member is closer to which pole.

Crossing Over (Recombination)
Corresponding pieces of chromatids of maternal and paternal homologues (non-sister chromatids) are exchanged during synapsis when the homologues are aligned side by side.
Each of the affected chromatids has a mixture of maternal and paternal genetic information

19
Q

Where does crossing over occur?

A

Sex chromosomes align but crossing over does not usually occur in X and Y chromosomes apart from at the pseudoautsomal regions (PAR)
Why?
They are hemizygous to each other & so recombination proved harmful
PAR allows the X & Y chromosomes to pair and properly segregate during meiosis in males
X-inactivation occurs in which one of the copies of the X-chromosome is silenced to prevent females from having twice as many gene products as males.
Choice of which is inactivated is random in placental mammals like humans.

20
Q

Define aneuploidy and describe why it is a chromosomal abnormality?

A

A gain or loss of chromosomes from the normal 46 is called aneuploidy, affecting normal development and functioning.
Since each chromosome contains hundreds of genes, the addition or loss of a single chromosome disrupts the existing equilibrium of the cell leading to profound phenotypes.

21
Q

How are anueploidies detected and how do they occur?

A

Majority occur from an error in maternal meiosis I because human oocytes are arrested in prophase I for decades

-Aneupolidy is present in 6% of sperm from ostensibly normal men
-Aneuploidy in eggs is leading cause of infertility, miscarriage and congenital syndromes
-Majority are lethal
E.g. trisomies (47 chromosomes) account for 35% of spontaneous abortions/miscarriages.

Know it’s maternal because of presence of polymorphic DNA markers on individual chromosomes that all distinction between maternally and paternally derived chromosomes.
Before the pairs can separate, however, the crossovers between chromosomes must be resolved and meiosis-specific cohesins must be released from the arms of the sister chromatids. Failure to separate the pairs of chromosomes to different daughter cells is referred to as nondisjunction, and it is a major source of aneuploidy.

Arrested egg in meiosis I– anueoploidies can occur
Anueoploidies can occur in men – due to events of non dysjunction

22
Q

What is Non-dysjunction?

A

failure of homologous chromosome to separate during MI or sister chromatids to separate during MII, resulting in extra or missing chromosomes.

. A pair of chromosomes or sister chromatids may adhere so tightly to one another that they do not separate during anaphase. As a result, both go to the same daughter cell, and the other daughter cell receives none of this type of chromosome

23
Q

Provide examples of chromosomal anueploidies that are viable and those that are not viable

A

Trisomy 21 (aka Down’s syndrome, 1:750 births)
Trisomy 18 (Edwards syndrome)
Trisomy 13 (Patau syndrome)

50% of patients with primary amenorrhea as a result of premature ovarian insufficiency (POI) have an abnormal karyotype
Sex chromosome aneuploidy more viable, usually random event (not inherited):

Turner syndrome (45, X monosomy) » caused by complete or partial absence of 2nd sex chromosome (occurrence 1:2000 female births) → phenotype=short stature, primary amenorrhea (classic Turners)

Klinefelter syndrome (47,XXY trisomy) » caused by presence of two X and one Y chromosome (occurrence 1:500 male births) → variable phenotype=taller than average, small testes producing reduced testosterone, infertility

23
Q

Provide examples of chromosomal anueploidies that are viable and those that are not viable

A

Trisomy 21 (aka Down’s syndrome, 1:750 births)
Trisomy 18 (Edwards syndrome)
Trisomy 13 (Patau syndrome)

50% of patients with primary amenorrhea as a result of premature ovarian insufficiency (POI) have an abnormal karyotype
Sex chromosome aneuploidy more viable, usually random event (not inherited):

Turner syndrome (45, X monosomy) » caused by complete or partial absence of 2nd sex chromosome (occurrence 1:2000 female births) → phenotype=short stature, primary amenorrhea (classic Turners)

Klinefelter syndrome (47,XXY trisomy) » caused by presence of two X and one Y chromosome (occurrence 1:500 male births) → variable phenotype=taller than average, small testes producing reduced testosterone, infertility

24
Q

Does maternal age increase the risk of trisomy?

A

Multiple mechanism contribute to the maternal age effect –
Recombination failure
Premature homologue separation
Premature sister chromatid separation due to loss of cohesion between sister centromeres