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Biology IB HL > 3.2 Chromosomes > Flashcards

3.2 Chromosomes Flashcards

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1
Q

Do prokaryotes possess a nucleus?

A

NO
Prokaryotes do not possess a nucleus – instead genetic material is found free in the cytoplasm in a region called the nucleoid

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2
Q

What does the genetic material of a prokaryote consist of?

A

The genetic material of a prokaryote consists of a single chromosome consisting of a circular DNA molecule (genophore)

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3
Q

What packaging does the DNA of a prokaryote have?

A

The DNA of prokaryotic cells is naked – meaning it is not associated with proteins for additional packaging

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4
Q

What other DNA molecules may a prokaryote contain?

A

In addition to the genophore, prokaryotic cells may possess additional circular DNA molecules called plasmids

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5
Q

What are plasmids?

A

Plasmids are small, circular DNA molecules that contain only a few genes and are capable of self-replication

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6
Q

Are plasmids present in eukaryotic cells?

A

Plasmids are present in some prokaryotic cells, but are not naturally present in eukaryotic cells

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7
Q

Can plasmids be exchanged?

A

Bacterial cells may exchange plasmids via their sex pili, in a process known as bacterial conjugation

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8
Q

What is the purpose of bacterial conjugation?

A

This exchange of genetic material allows bacteria to evolve new features within a generation (horizontal gene transfer)

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9
Q

What are plasmids ideal for and why?

A

As plasmids can self-replicate and autonomously synthesise proteins, they are ideal vectors for gene manipulation in labs

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10
Q

What does the genetic material of eukaryotes consist of?

A

The genetic material of eukaryotic cells consist of multiple linear molecules of DNA that are associated with histone proteins

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11
Q

What is the purpose of packaging DNA in eukaryotes?

A

The packaging of DNA with histone proteins results in a greatly compacted structure, allowing for more efficient storage

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12
Q

What is the first step of DNA organisation?

A

DNA is complexed with eight histone proteins (an octamer) to form a complex called a nucleosome

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13
Q

What are nucleosomes further linked by in DNA organisation?

A

Nucleosomes are linked by an additional histone protein (H1 histone) to form a string of chromatosomes

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14
Q

What happens to the chromatosomes?

A

These then coil to form a solenoid structure (~6 chromatosomes per turn) which is condensed to form a 30 nm fibre

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15
Q

What happens to the 30nm fibre?

A

These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin

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16
Q

What does chromatin form when it supercondenses?

A

Chromatin will then supercoil during cell division to form chromosomes that are visible (when stained) under microscope

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17
Q

What are chromosomes and when are they compacted?

A

Eukaryotic chromosomes are linear molecules of DNA that are compacted during cell division (mitosis or meiosis)

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18
Q

What divides chromosomes into two?

A

Each chromosome has a constriction point called a centromere, which divides the chromosome into two sections (or ‘arms’)

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19
Q

What are the two sections of the chromosome termed?

A

The shorter section is designated the p arm and the longer section is designated the q arm

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20
Q

How do chromosomes in eukaryotic species differ?

A

Eukaryotic species possess multiple chromosomes that may differ in both their size and the position of their centromere

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21
Q

What is a locus?

A

Each chromosome will carry specific genes and the position of a particular gene on a chromosome is called the locus

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22
Q

How to identify a locus?

A

The first point of reference is a number (or letter) which denotes the chromosome (e.g. 7q31 refers to chromosome 7)
The second point of reference is a letter (p or q) to denote which arm the locus is positioned on (e.g. 7q31 is on the q arm)
The third point of reference is a number corresponding to the G band location (e.g. 7q31 is at the longitudinal position 31)

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23
Q

What genetic material do sexually reproducing organisms inherit?

A

Sexually reproducing organisms inherit their genetic sequences from both parents

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24
Q

Therefore how many copies of a chromosome will an organism possess?

A

This means that these organisms will possess two copies of each chromosome (one of maternal origin ; one of paternal origin)

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25
Q

What are the maternal and paternal chromosomes termed?

A

These maternal and paternal chromosome pairs are called homologous chromosomes

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26
Q

What do homologous chromosomes share?

A

The same structural features (e.g. same size, same banding patterns, same centromere positions)

The same genes at the same loci positions (while the genes are the same, alleles may be different)

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27
Q

What must be done before reproduction?

A

Homologous chromosomes must be separated in gametes (via meiosis) prior to reproduction, in order to prevent chromosome numbers continually doubling with each generation

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28
Q

What does it mean if an organism is diploid?

A

As sexually reproducing organisms receive genetic material from both parents, they have two sets of chromosomes (diploid)

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29
Q

What can sex cells be termed as?

A

To reproduce in turn, these organisms must create sex cells (gametes) with half the number of chromosomes (haploid)

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30
Q

Why are gametes haploid?

A

When two haploid gametes fuse, the resulting diploid cell (zygote) can grow and develop into a new organism

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31
Q

What do diploid nuclei possess?

A

Nuclei possessing pairs of homologous chromosomes are diploid (symbolised by 2n)

These nuclei will possess two gene copies (alleles) for each trait

32
Q

What will all somatic body cells posses?

A

All somatic (body) cells in the organism will be diploid, with new diploid cells created via mitosis

33
Q

What are haploid nuclei?

A

Nuclei possessing only one set of chromosomes are haploid (symbolised by n)

34
Q

What will haploid nuclei contain?

A

These nuclei will possess a single gene copy (allele) for each trait

35
Q

What cells will be haploid?

A

All sex cells (gametes) in the organism will be haploid, and are derived from diploid cells via meiosis

36
Q

Where are haploid cells also present?

A

Haploid cells are also present in bacteria (asexual) and fungi (except when reproducing)

37
Q

How is sex determined in humans?

A

In humans, sex is determined by a pair of chromosomes called the sex chromosomes (or heterosomes)

38
Q

What are the sex chromosomes of females?

A

Females possess two copies of a large X chromosome (XX)

39
Q

What are the sex chromosomes of males?

A

Males possess one copy of an X chromosome and one copy of a much shorter Y chromosome (XY)

40
Q

What does the y chromosome contain?

A

The Y chromosome contains the genes for developing male sex characteristics (specifically the SRY gene)

41
Q

What will an absence of a y chromosome cause?

A

In its absence of a Y chromosome, female sex organs will develop

42
Q

In what gender are there homologous chromosomes?

A

The sex chromosomes are homologous in females (XX) but are not homologous in males (XY)

43
Q

Therefore what parent is responsible for determining sex of the offspring?

A

Hence the father is always responsible for determining the sex of offspring:

44
Q

What gender will the offspring be if the sperm has an x-chromosome?

A

If the male sperm contains an X chromosome, the growing embryo will develop into a girl

45
Q

What gender will the offspring be if the sperm has an y-chromosome?

A

If the male sperm contains a Y chromosome, the growing embryo will develop into a boy

46
Q

What chromosome will the egg cell contain?

A

In all cases the female egg will contain an X chromosome (as the mother is XX)

47
Q

What are the chromosomes apart from sex chromosomes called?

A

The remaining chromosomes in the organism are called autosomes (they do not determine sex)

48
Q

What are karyotypes?

A

Karyotypes are the number and types of chromosomes in a eukaryotic cell – they are determined via a process that involves:

49
Q

In what 3 ways a karyotype be obtained?

A

Harvesting cells (usually from a foetus or white blood cells of adults)
Chemically inducing cell division, then arresting mitosis while the chromosomes are condensed
The stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids or not

50
Q

What is a karyogram?

A

The chromosomes are stained and photographed to generate a visual profile that is known as a karyogram

51
Q

How are chromosomes arranged in a karyogram?

A

The chromosomes of an organism are arranged into homologous pairs according to size (with sex chromosomes shown last)

52
Q

When is karyotyping used and what can it be used for?

A

Karyotyping will typically occur prenatally and is used to:

Determine the gender of the unborn child (via identification of the sex chromosomes)
Test for chromosomal abnormalities (e.g. aneuploidies or translocations)

53
Q

What is down syndrome caused by?

A

Down syndrome is a condition whereby the individual has three copies of chromosome 21 (i.e. trisomy 21)

54
Q

What causes a tripling of chromosome 21?

A

It is caused by a non-disjunction event in one of the parental gametes

55
Q

What does 3 copies of 21 lead to?

A

The extra genetic material causes mental and physical delays in the way the child develops

56
Q

What is the first step of autoradiography?

A

Cells are grown in a solution containing radioactive thymidine (tritiated thymidine – 3H-T)

57
Q

What is done with the tritiated thymidine?

A

The tritiated thymidine is incorporated into the chromosomal DNA of the cell (3H-T is used as thymidine is not present in RNA)

58
Q

How are the chromosomes isolated? autoradiography

A

The chromosomes are isolated by gently lysing the cells and fixing the chromosomes to a photographic surface

59
Q

What is the surface emerged in? autoradiography

A

The surface is then immersed in a radioactively-sensitive emulsion containing silver bromide (AgBr)

60
Q

What is done with the radiation released? autoradiography

A

The radiation released from the tritiated thymidine converts the Ag+ ions in silver bromide into insoluble metal grains

61
Q

what is left on the surface? autoradiography

A

Following a period of exposure, excess silver bromide is washed away, leaving the silver grains to appear as small black dots

62
Q

what happens once the film is developed? autoradiography?

A

When the photographic film is developed, the chromosomal DNA can be visualised with an electron microscope

63
Q

What technique is used to measure chromosome length?

A

John Cairns pioneered a technique for measuring the length of DNA molecules by autoradiography

64
Q

Previously, why was the measuring of chromosome length so inaccurate?

A

Previously, chromosome length could only be measured while condensed during mitosis (very inaccurate due to supercoiling)

65
Q

How did cairns help with aiding the accuracy of measuring chromosomes?

A

Cairns used autoradiography to visualise the chromosomes whilst uncoiled, allowing for more accurate indications of length

66
Q

What can tritiated uracil help with visualising?

A

By using tritiated uracil (3H-U), regions of active transcription can be identified within the uncoiled chromosome

67
Q

What two other findings did john cairns encounter?

A

DNA replication involves formation of a replication bubble (and prokaryotic replication involves a single origin of replication)

DNA replication is bi-directional (it occurs independently at both ends of the replication bubble)

68
Q

Is chromosome number the same for every species?

A

Chromosome number is a characteristic feature of members of a particular species

69
Q

can organisms with different diploid numbers interbreed?

A

NO
Organisms with different diploid numbers are unlikely to be able to interbreed (cannot form homologous pairs in zygotes)

70
Q

What happens when species with different chromosome numbers interbreed?

A

In cases where different species do interbreed, offspring are usually infertile (cannot form functional gametes)

71
Q

Does chromosome number indicate complexity?

A

Chromosome number does not provide a valid indication of genetic complexity

72
Q

Does genome size indicate complexity?

A

Genome size can vary greatly between organisms and is not a valid indicator of genetic complexity

73
Q

WHat size genome do viruses have?

A

Viruses and bacteria tend to have very small genomes

74
Q

What size genome do prokaryotes have?

A

Prokaryotes typically have smaller genomes than eukaryotes

75
Q

What size genome do plants have?

A

Sizes of plant genomes can vary dramatically due to the capacity for plant species to self-fertilise and become polyploid

Biology IB HL (56 decks)

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