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Science of reproduction > Chromosomes and gametes > Flashcards

Chromosomes and gametes Flashcards

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1
Q

What is the defining feature of all evolving living organisms?

A

The ability to reproduce

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2
Q

what is the 2nd principle fundamental to evolution?

A

Variation: the replicating system must undergo changes so they can adapt to the changing environment

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3
Q

What does the karyotype of human chromosomes tell us?

A

the no. and structure of chromosomes

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4
Q

For genes to be functional what must the DNA be able to?

A
  • replicate
  • separate its 2 copies at mitosis
  • maintain itself between generations
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5
Q

What is copy number variant?

A

Copy number variants (CNV) occur if there are one, three of more copies of alleles

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6
Q

How does gene transcription and translation take place?

A

1) Promotor and coding sequence transcribed into a gene product
2) Introns are removed from exon by splicing
3) mRNA exported out of nucleus
4) Translated into proteins in ribosomes i.e. complexes of tRNA and proteins
5) Proteins then folded into unique 3D structure that determines function

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7
Q

How can the same gene be tissue specific?

A

By having alternative promotors

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8
Q

What enzyme reacts to different substances in different tissues and how?

A

Aromatase is found in granulosa cells, breast tissue and placenta – they all have different promoters thus is each tissue the aromatase gene can respond to different substances

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9
Q

what are introns?

A

The non-coding regions in the gene that regulate gene expression

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10
Q

How are introns removed?

A

They are removed by the messenger RNA in splicing

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11
Q

What are exons?

A

The region in the gene that actually codes

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12
Q

How can one gene give rise to several products?

A

Alternative splicing of exons - products are known as Isoforms

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13
Q

How can the protein be modified once made?

A

Post-translational modification:
o phosphorylation
o Glycosylation i.e. adding on carbohydrates to protein, making protein more stable and soluble
o Often hormones secreted as “pro-hormones” and must be enzymatically processed to form the active hormone eg. pre-proGnRH

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14
Q

What can glycosylation do in hormones?

A

Glycosylation can affect the behaviour of the hormone

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15
Q

What are the DNA requirements for sexual reproduction?

A

Fusion of haploid cells (gametes) to create unique diploid progeny

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16
Q

How do somatic/diploid cells replicate?

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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17
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
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18
Q

What are the X and Y chromosomes?

A
  • 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.
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19
Q

What is a gamete?

A

A haploid cell specialised for sexual fusion

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20
Q

what is different about haploid cells?

A
  • They have 23 chromosomes in total

- Unlike other cells gametes go through cycles of diploidy & haploidy

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21
Q

How do gametes originate?

A

1) Gametes are formed from germ line cells: primordial germ cells that migrate into the gonad and then differentiate to either male or female gametes
2) The process producing oocytes: oogenesis (incorporated as part of folliculogenesis)
3) The process producing sperm: spermatogenesis
4) Undergo cycles of mitosis to increase numbers
5) Then undergo meiosis
6) Then combine at fertilisation

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22
Q

When do chromosomes replicate?

A

During the S-phase of the cell cycle

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23
Q

What are the steps of mitosis?

A

4 broad steps:

1) Prophase
2) Metaphase
3) Anaphase
4) Telophase

24
Q

What phase comes right before mitosis?

A

Interphase

25
Q

What is interphase?

A
  • Period of the cell cycle between cell divisions
  • Interphase is a time when the cell carries out its functions and grows
  • the DNA and organelles are duplicated
  • the genetic material is in the form of long, thin threads 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.
26
Q

What happens in prophase?

A
  • the chromatin condenses and forms chromosomes as DNA wraps around histones. The DNA then loops and twists to form a tightly compacted structure
  • DNA is in this condensed state, it cannot be replicated, and gene activity is shut down.
  • The sister chromatids are easier to separate without breaking.
  • The nuclear membrane also begins to break down.
  • In the cytoplasm the mitotic spindle forms. The mitotic spindle is made of microtubules associated with the centrioles.
  • The centrioles move away from each other toward opposite ends of the cell.
27
Q

What happens in metaphase?

A
  • the chromosomes attach to the mitotic spindles, forming a line (they line up one after the other) 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.
28
Q

What happens in anaphase?

A
  • Begins when the sister chromatids of each chromosome begin to separate (spindles on the microtubules shorten), splitting at the centromere.
  • The sister chromatids are considered chromosomes.
  • The spindle fibers pull the chromosomes toward opposite poles of the cell.
  • By the end equivalent collections of chromosomes are located at the two poles of the cell.
29
Q

What happens in telophase?

A
  • 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.
30
Q

What happens in cytokinesis?

A
  • division of the cytoplasm
  • begins at the end of mitosis, during telophase.
  • a band of microfilaments in the area where the chromosomes originally aligned contracts and forms a furrow. The furrow deepens, eventually pinching the cell in two. Each daughter cell is a diploid cell that is genetically identical to the parent cell.
31
Q

What happens in meiosis I?

A
  • The first meiotic division produces two cells, each with 23 chromosomes.
  • Each daughter cell contains one member of each homologous pair, with each chromosome consisting of two sister chromatids.
  • It is important that each daughter cell receive one of each kind of chromosome during meiosis I. If one of the daughter cells had two of chromosome 3 and no chromosome 6, it would not survive.
32
Q

What happens in prophase I?

A
  • The separation of homologous chromosomes occurs , members of homologous pairs line up next to one another by a phenomenon called synapsis (“bringing together”).
  • Matched homologous pairs become positioned at the midline of the cell and attach to spindle fibers. The pairing of homologous chromosomes helps ensure that the daughter cells will receive one member of each homologous pair.
33
Q

What happens during anaphase I?

A

The members of each homologous pair of chromosomes separate, and each homologue moves to opposite ends of the cell.

34
Q

What happens during telophase I?

A

cytokinesis begins, resulting in two daughter cells, each with one member of each chromosome pair. Each chromosome in each daughter cell still consists of two replicated sister chromatids.

35
Q

What is interkinesis?

A

Telophase I is followed by interkinesis, a brief interphase-like period. Interkinesis differs from mitotic interphase in that there is no replication of DNA during interkinesis.

36
Q

What is synapsis?

A
  • Pairing of homologous chromosomes to form a Tetrad in Prophase I
  • Genetic material from the homologous chromosomes is randomly swapped
  • This creates 4 unique chromatids hence increasing overall genetic diversity of the gametes.
37
Q

What happens in Meiosis II?

A
  • Each chromosome lines up in the center of the cell independently (as occurs in mitosis), and the sister chromatids making up each chromosome separate.
  • Separation of the sister chromatids occurs in both daughter cells that were produced in meiosis I.
  • This results in four cells, each containing one of each kind of chromosome.
  • The events of meiosis II are similar to those of mitosis, except that only 23 chromosomes are lining up in meiosis II compared with the 46 chromosomes in mitosis.
38
Q

What is main difference between mitosis and meiosis?

A
  • Mitosis: cell divides to produce 2 new ‘daughter’ cells that are identical to the original and diploid
  • Meiosis: similar to mitosis but more complex → results in production of ‘daughter’ cells that are non-identical and haploid
39
Q

why is meiosis advantageous?

A
  • random distribution of male and female homologous chromosomes
  • chromosomal crossing over occurs
40
Q

What are the 2 important functions of meiosis in sexual reproduction?

A
  • Meiosis keeps the number of chromosomes in a body cell constant from generation to generation.
  • Meiosis increases genetic variability in the population
41
Q

Compare mitosis and meiosis.

A

Mitosis:

  • Involves one cell division
  • Produces 2 diploid cells
  • Occurs in somatic cells
  • Results in growth and repair
  • No exchange of genetic material
  • Daughter cells are genetically similar

Meiosis:

  • 2 cell divisions
  • produces 4 haploid cells
  • only occurs in ovaries and testes during gamete formation
  • Parts of chromosomes are exchanged
  • Daughter cells are genetically different
  • Results in gametes
42
Q

How is genetic variability achieved?

A

Independent assortment and crossing over

43
Q

What happens in independent assortment?

A
  • 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.
44
Q

What happens in crossing over?

A
  • 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
45
Q

Does crossing over happen in sex chromosomes?

A

Sex chromosomes align but crossing over does not usually occur in X and Y chromosomes apart from at the pseudoautsomal regions (PAR) – PAR are at the end of the X and Y chromosomes

46
Q

Why doesn’t crossing over happen in sex chromosomes?

A
  • They are hemizygous to each other & so recombination proved harmful
  • it resulted in males without necessary genes formerly found the on the X chromosome, and females with unnecessary or even harmful genes previously only found on the Y chromosome.
  • PAR allows the X & Y chromosomes to pair and properly segregate during meiosis in males
47
Q

Why does x-inactivation happen?

A
  • X-inactivation occurs to prevent females from having twice as many gene products as males.
  • Choice of which is inactivated is random in placental mammals like humans.
48
Q

What is an aneuploidy?

A
  • A gain or loss of chromosomes from the normal 46 - it affects normal development and functioning.
  • 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.
49
Q

When do the majority of aneuploidies happen?

A
  • Majority occur from an error in maternal meiosis I because human oocytes are arrested in prophase I for decades
  • We know it’s maternal because of presence of polymorphic DNA markers on individual chromosomes that all distinction between maternally and paternally derived chromosomes.
50
Q

What needs to happen before the pairs of chromatids separate?

A
  • 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.
51
Q

What is non-disjunction?

A

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

52
Q

How may aneuploidy happen?

A

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.

53
Q

What type is the most common aneuploidy?

A

Trisomies

54
Q

What are the viable trisomies?

A
  • Trisomy 21 (aka Down’s syndrome)
  • Trisomy 18 (Edwards syndrome)
  • Trisomy 13 (Patau syndrome)
55
Q

What are the viable sex chromosome aneuplodies?

A
  • Turner syndrome (45, X monosomy) – caused by complete or partial absence of 2nd sex chromosome → phenotype=short stature, primary amenorrhea (classic Turners)
  • Klinefelter syndrome (47,XXY trisomy) – caused by presence of two X and one Y chromosome → variable phenotype=taller than average, small testes producing reduced testosterone, infertility
56
Q

What may affect the risk of trisomy’s?

A

Rick of trisomy’s increases as women age – maternal age effect

57
Q

What mechanisms contribute to the maternal age effect?

A
  • Recombination failure
  • Premature homologue separation
  • Premature sister chromatid separation due to loss of cohesion between sister centromeres

Science of reproduction (32 decks)

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