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16. Inherited Change Flashcards

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1
Q

What is a karyogram?

A

Visual arrangement of chromosomes. There are 22 pairs of homologous chromosomes (autosomes) and one pair of allosomes. The X and Y allosomes do not match (Y has some portions missing).

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2
Q

What is a homologous pair?

A

(In a diploid cell) two chromosomes with the same structure and genes (alleles differ) at the same loci as each other. Form a bivalent during the first meiotic division. In the original zygote, one of each pair comes from each parent.

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3
Q

How can homologous pairs be distinguished?

A

Shape, size, distinctive banding pattern when stained.

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4
Q

What is a locus?

A

Position at which a particular gene is found on a particular chromosome (always stays the same). Eg. CF gene found on chromosome 7.

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5
Q

What are haploid and diploid cells?

A

Possess one / two complete sets of chromosomes (n and 2n, respectively).

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6
Q

What are the two types of nuclear division?

A

Growth - diploid zygote divides by mitosis to form genetically identical cells, same chromosome number as parent cells.
Sexual reproduction - chromosome number halved, achieved by meiosis (reduction division).

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7
Q

What is the need for a reduction division?

A

If the gametes were not haploid, the chromosome number would double every generation.

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8
Q

Outline the stages of meiosis.

A 
Meiosis I (first division) - results in two haploid daughter nuclei.
Meiosis II - like mitosis, each haploid (still diploid in terms of chromatids) daughter nucleus divides again, resulting in four haploid nuclei.
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9
Q

Describe the stages of Meiosis I (prophase).

A
  • Early prophase I: centrosomes replicate, nuclear envelope intact, centromere has attached kinetochores, chromosomes start to appear.
  • Middle prophase I: homologous chromosomes pair up (bivalent) in a process called synapsis. Centrosomes moving towards opposite poles.
  • Late prophase I: nuclear envelope breaks up, nucleolus disappears. Crossing over may occur. Mitotic spindle fully formed.
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10
Q

What is crossing over?

A

Sections of chromatids in the bivalent ‘break’ and reconnect to a non-sister chromatid at a chiasma - maternal and paternal gene loci (and alleles) are swapped.

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11
Q

Describe the stages of Meiosis I (metaphase).

A

Bivalents line up randomly across the equator of the spindle, attached by their centromeres, via independent assortment.

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12
Q

What is independent assortment?

A

Random splitting of homologous chromosome pairs. After meiosis occurs, each haploid cell contains a mixture of chromosomes from the organism’s mother and father.

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13
Q

Describe the stages of Meiosis I (anaphase).

A

Centromeres do not divide - whole homologous chromosomes move to opposite poles, pulled by microtubules.

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14
Q

Describe the stages of Meiosis I (telophase).

A

Animal cells divide - nuclear envelope + nucleolus re-form, many plants go straight to Meiosis II.

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15
Q

Describe the stages of Meiosis II (prophase).

A

Nuclear envelope and nucleolus disappear, centrosomes and centrioles replicate + move to opposite poles of the cell.

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16
Q

Describe the stages of Meiosis II (metaphase).

A

Single chromosomes line up separately across the equator of the new spindle.

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17
Q

Describe the stages of Meiosis II (anaphase).

A

Centromeres divide and spindle microtubules pull sister chromatids to opposite poles.

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18
Q

Describe the stages of Meiosis II (telophase).

A

Like in mitosis, except four haploid daughter cells are formed.

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19
Q

How does meiosis increase variation?

A
  • Crossing over (maternal and paternal alleles swapped)
  • Independent assortment (daughter cells have a mix of maternal and paternal chromosomes)
  • Further: fertilisation (occurs randomly)
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20
Q

Define ‘gametogenesis’.

A 
Spermatogenesis = formation of male gametes
Oogenesis = formation of female gametes
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21
Q

Describe the process of spermatogenesis.

A

Occurs in tubules inside testes.

  • Diploid cells divide by mitosis -> many diploid spermatogonia
  • These grow -> diploid primary spermatocytes.
  • First meiotic division produces two haploid secondary spermatocytes.
  • Second division produces haploid spermatids, which mature -> spermatozoa.
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22
Q

Describe the process of oogenesis (1).

A

Produces fewer gametes and takes much longer. Occurs inside ovaries.

  • Diploid cells divide by mitosis -> oogonia
  • These begin to divide by meiosis but stop at prophase I (primary oocytes, still diploid).
  • All of this occurs before birth (roughly 400000).
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23
Q

Describe the process of oogenesis (2).

A

During puberty, some primary oocytes reach meiosis II (two haploid cells at this point).

  • The first division is uneven - one cell gets most of the cytoplasm, -> secondary oocyte, one -> polar body (dies).
  • One secondary is released into the oviduct each month - if fertilised, it continues division by meiosis -> ovum (one more polar body forms and dies).
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24
Q

How does fertilisation work?

A

Chromosomes of spermatozoan and ovum join -> diploid nucleus. Zygote divides repeatedly by mitosis -> embryo -> foetus.
Plants: male gamete from pollen grain fuses with female gamete inside an ovule.

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25
Q

How does gametogenesis work in flowering plants? (male gametes)

A

Inside the anthers, diploid pollen mother cells divide by meiosis -> four haploid cells.

  • The nucleus of each divides by mitosis but no cytokinesis occurs - each cell has two haploid nuclei (tube nucleus and generative nucleus).
  • These mature into pollen grains, surrounded by a wall made of tough exine and thinner intine.
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26
Q

How does gametogenesis work in flowering plants? (female gametes)

A

Inside each ovule, a large diploid spore mother cell develops.

  • Divides by meiosis -> four haploid cells, all but one degenerate.
  • The survivor develops into an embryo sac.
  • It grows and the nucleus divides by mitosis three times (8 nuclei) - one of these = female gamete.

In plants, gametes are formed indirectly - meiosis produces pollen grains and embryo sacs, gametes are formed inside by mitotic divisions.

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27
Q

Define ‘phenotype’.

A

Organism’s observable characteristics, resulting from interactions between the genotype and the environment.

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28
Q

What do the numbers of each type of gamete represent?

A

The relative chances of each genotype being inherited. Only probability - just a theoretical value.

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29
Q

How do you draw a genetic diagram?

A
  • Parental phenotypes and genotypes
  • Parental gametes
  • Punnett square
  • Offspring phenotypes and genotypes.
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30
Q

Define ‘codominance’.

A

Both alleles have an effect on the phenotype of a heterozygous organism (eg. HbA HbS).

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31
Q

Define ‘dominant’ and ‘recessive’ alleles.

A
  • Dominant alleles have identical phenotypic effects in a heterozygote and in a homozygote.
  • Recessive alleles only affect the phenotype when no dominant allele is present.
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32
Q

Define ‘F1 generation’.

A

Offspring resulting from a cross between a homozygous dominant and homozygous recessive genotype.

33
Q

Define ‘F2 generation’.

A

Offspring resulting from a cross between two F1 (heterozygous) genotypes.

34
Q

Define ‘test cross’.

A

Genetic cross where an organism showing a specific characteristic caused by a dominant allele is crossed with a homozygous recessive genotype; phenotypes of offspring help identify whether the parent is homozygous dominant or heterozygous.

35
Q

Describe an example of the presence of multiple alleles.

A

Blood type - A, AB, B, O (IA, IB, IO - former two codominant, latter recessive).
‘I’ refers to immunoglobulin - provides RBC with an antigen and the blood with an opposite antibody.

36
Q

How is sex inheritance determined?

A

Sex chromosomes - not always alike (X and Y), genes not always in the same position (not homologous). Y is shorter than X and has fewer genes.

37
Q

Define ‘sex-linked gene’.

A

Gene found on part of the X chromosome not matched by Y, therefore not found on the Y chromosome.

38
Q

Give an example of sex linkage.

A

Haemophilia (blood fails to clot properly).

  • X chromosome has a gene coding for the production of a protein for blood clotting (factor VIII).
  • Dominant H (normal), recessive h (lack of factor VIII).
  • Males who inherit one Xh copy will be sufferers.
39
Q

Define ‘monohybrid cross’ and ‘dihybrid cross’.

A

Monohybrid concerns inheritance of just one gene - dihybrid considers the inheritance of two genes at once.

40
Q

How do different types of gamete come about (dihybrid crosses)?

A
  • Metaphase I: homologous pairs line up independently, two pairs of chromosomes have two possible orientations.
  • End of meiosis II: Each orientation gives two types of gamete, so there are four possible types of gamete.
41
Q

Give an example of a dihybrid cross with two heterozygous organisms.

A

AaDd -> A d, a D / A D, a d (independent assortment)
A d, a D -> Ad, dA, Da, aD gametes
A D, a d -> AD, DA, ad, da gametes

42
Q

What are some common phenotypic ratios from dihybrid crosses?

A
  • 1:1:1:1 : typical between heterozygous and homozygous recessive where alleles show complete dominance.
  • 9:3:3:1 : two heterozygous alleles where alleles show complete dominance and genes are on different chromosomes.
43
Q

Give an example of two gene loci interacting to affect one phenotypic characteristic (chicken feathers).

A

Chicken feathers - I/i (white feathers) and C/c (coloured feathers).

  • carrying I = white (even with C)
  • iicc also = white
44
Q

Give an example of two gene loci interacting to affect one phenotypic characteristic (flower colour).

A 
Flower colour - A/a, B/b
- A_B_ = purple
- A_bb = pink
- aaB_ = white
- aabb = white
aa affects B/b locus - neither dominant B for purple nor recessive b for pink can be expressed in the absence of dominant A allele.
45
Q

Define ‘autosomal linkage’.

A

Presence of two gene loci on the same chromosome - inherited together and do not assort independently.

46
Q

Describe an example of autosomal linkage (Drosophila).

A

Fruit fly, normally with striped body and feathery arista.

  • E/e (e = recessive ebony allele)
  • A/a (a = recessive aristopedia antennae allele).
  • normal written as (EA)(EA) (indicating same chromosome).
47
Q

How would dihybrid crosses work with autosomal linkage?

A

Eg. crossing a heterozygous fruit fly with homozygous recessive results in a 1:1 ratio, rather than 1:1:1:1 - behaves as a monohybrid cross, indicating that the alleles which entered the cross together, stayed together.

48
Q

How is variation increased with autosomal linkage?

A

Crossing over.

49
Q

Define ‘recombinant’.

A

Result from crossing over - characteristics from the original parents are ‘recombined’. Many parental types are produced (1:1) and some recombinants too (1:1).
- eg. fruit flies - S/N and E/A 44% each, S/A and E/N 6% each.

50
Q

Define ‘cross-over value’.

A

Percentage of offspring belonging to recombinant classes. Measure of distance apart of gene loci on their chromosomes (smaller = closer together, less chance of crossing over).

51
Q

Define ‘chi-squared test’.

A

Statistical test comparing expected and observed results of test crosses, and whether or not there is a significant difference.

52
Q

What is the formula for the chi-squared test?

A

χ2 = ∑ ((O-E)^2 / E)

53
Q

How do you perform a chi-squared test?

A

1) Write out expected results using ratio
2) Record these and the corresponding observed results
3) Calculate differences, then square
4) Divide each by the respective E
5) Add all the values.

54
Q

How do you apply χ2 to a test cross?

A

Find the corresponding value under the 0.05 section of the table (critical value) and under the correct number of degrees of freedom (number of data classes minus 1) - if it is much lower, then the differences are likely to be due to chance (5% and higher).

55
Q

Define ‘mutation’.

A

Unpredictable change in the genetic material of an organism.

56
Q

Define ‘gene mutation’.

A

Change in DNA molecule structure caused by changes in base sequences. Produces a different allele of a gene.

57
Q

Define ‘chromosome mutation/aberration’.

A

Change in structure/number of whole chromosomes in a cell.

58
Q

How can mutations arise?

A

Random errors in replication etc. or via mutagens eg. ionising radiation, UV radiation, chemicals (eg. mustard gas).

59
Q

Describe the three types of gene mutation.

A
  • Base substitution: one base takes the place of another (silent mutation - no apparent effect except for introduction of a ‘stop’ triplet).
  • Base addition/deletion: one or more bases added/removed from the sequence (frame shift - alters every following triplet, may introduce premature ‘stop’).
60
Q

How is sickle cell anaemia caused?

A

Base substitution in β-globin base sequence - CTT replaced by CAT, 6th amino acid changes from Glu -> Val. HbS allele produced (Hb refers to locus).

61
Q

How does the sickle cell mutation affect cells?

A

No effect to oxyhaemoglobin but makes Hb much less soluble - molecules stick together to form long fibres in RBCs. They distort into sickle shapes, unable to transport oxygen and get stuck in small capillaries.
Results in severe anaemia (lack of oxygen transported to cells).

62
Q

How is albinism caused?

A

Autosomal recessive allele (different form affecting the eyes is sex-linked).
Mutation in gene for tyrosinase leads to its absence/inactivity in melanocytes - first two steps in metabolic pathway converting tyrosinase -> melanin cannot occur (tyrosine -> DOPA -> dopaquinone -> melanin).

63
Q

What does albinism do to the body?

A

Lack of melanin in eyes, skin and hair - pink/blue irises, red pupils, poor vision, rapid/jerky eye movements, avoidance of bright light.

64
Q

Describe the structure of tyrosinase.

A
  • Oxidase - two Cu atoms in active site which bind with O2.

- Transmembrane protein in melanosomes (organelles inside melanocytes) - mostly inside the organelle.

65
Q

How is Huntington’s caused?

A

Dominant allele - most sufferers are heterozygous with a 1/2 chance of passing on the allele.
Unstable segment in a gene on chromosome 4 (coding for huntingtin) causes many CAG repeats (stutter) - more of these = earlier onset.

66
Q

How does Huntington’s affect the body?

A

Mental deterioration (ventricles in brain enlarge), chorea (involuntary movements). Onset roughly middle age, allowing the allele to be passed on quite easily.

67
Q

Define ‘transcription factor’.

A

Protein that binds to a specific DNA sequence and controls info flow from DNA -> RNA by controlling mRNA formation.

68
Q

Define ‘structural gene’ and ‘regulatory gene’.

A
  • S: Gene coding for proteins required by a cell.

- R: Gene coding for proteins regulating expression of other genes.

69
Q

Define ‘repressible enzyme’ and ‘inducible enzyme’.

A
  • R: Synthesis prevented by binding a repressor protein to a specific site (operator) on a bacterium’s DNA.
  • I: Synthesis only occurs when substrate (inducer) is present - transcription only occurs when this interacts with a protein produced by a regulatory gene.
70
Q

Define ‘operon’.

A

Length of DNA making up a unit of gene expression in a bacterium. Consists of one or more structural genes, and control regions of DNA (recognised by products of regulatory genes).

71
Q

Describe the structure of the lac operon.

A

Three structural genes, length of DNA including operator and promoter regions (regulatory gene close to this).

  • lac Z (codes for β-galactosidase)
  • lac Y (codes for permease)
  • lac A (codes for transacetylase).
72
Q

Outline the role of the lac operon in E. coli.

A

β-galactosidase hydrolyses lactose -> glucose + galactose.

  • Number of these enzymes depends on lactose concentration in medium.
  • Since there is only one copy of the gene coding for this enzyme, altering concentration is done by regulating transcription.
73
Q

How does the lac operon alter transcription with no lactose present?

A
  • Regulatory gene codes for repressor protein.
  • Repressor binds to operator, close to lac Z.
  • RNA polymerase cannot bind to promoter in the presence of the bound repressor.
  • No transcription of the three genes.
74
Q

How does the lac operon alter transcription with lactose present?

A
  • Lactose taken up by bacterium, binds to repressor.
  • Shape of repressor distorted, cannot bind to DNA at operator region.
  • Transcription no longer inhibited - mRNA produced, genes are ‘switched on’ and transcribed together.
75
Q

How does E. coli use non-competitive inhibition to its advantage?

A

Repressor protein allosteric (two binding sites, binding at one changes shape and ability to bind at the other site).
- lactose and DNA binding sites are separate, allowing 3 enzymes to be produced only when lactose is available (saves energy and materials).

76
Q

What happens to E. coli when glucose is present in the medium?

A

Prefers glucose - lactose usage will be repressed by suppressing the lac operon using a different TF.

77
Q

How do transcription factors work in eukaryotes?

A
  • May bind to the promoter region, increase/decrease transcription etc.
  • Ensures genes are expressed in the correct cell/time/quantity.
  • Proportion of proteins as TFs increases as genome size increases.
78
Q

How do TFs affect eukaryotes?

A
  • Form part of protein complex binding to promoter region.
  • Activate appropriate genes in correct sequence.
  • Determination of sex in mammals.
  • Responding to environmental stimuli (eg. genes responding to temp.).
  • Products of pro-oncogenes and tumour suppressor genes regulate cell cycle, growth and apoptosis.
  • Used by hormones.
79
Q

How do gibberellins work in increasing amylase synthesis?

A

Cause breakdown of DELLA proteins by inhibiting binding of TF (phytochrome-interacting protein) to promoter region.
- When GA binds to receptor and enzyme, PIF can bind to a promoter region and cause transcription of gene coding for amylase (cannot occur when PIF binds to DELLA).

Biology (8 decks)

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