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Medical Genetics > Sheet 1 > Flashcards

Sheet 1 Flashcards

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Biology 101 flashcards USMLE® Step 1 flashcards
1
Q

What is the structure of DNA?

A

A double stranded DNA molecule is wrapped around proteins called histones. This forms a DNA-protein complex known as a nucleosome.

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2
Q

Nucleosomes are packed together forming:

A

Coils of chromatin

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3
Q

Chromatin is anchored onto a scaffold forming ____

A

A chromatid.

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4
Q

An identical DNA molecule comprises:

A

The other sister chromatid

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5
Q

The two chromatids together form a ___.

A

Chromosome

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6
Q

In which phase of the cell cycle does DNA replication occur?

A

S phase (synthesis phase)

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7
Q

What happens in the S phase?

A

The DNA polymerase uses each strand of the DNA molecule as a template to synthesize daughter strands.

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8
Q

In which phase of the cell cycle does interphase occur?

A

G1-S-G2 phase (the stage before cell division)

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9
Q

The DNA in the interphase stage should be ____(loose and accessible/tight and inaccessible) and ____(inavailable for proteins to bind/available for proteins to bind)

A

Loose and accessible; available for proteins to bind.

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10
Q

Give a few examples of proteins at work during interphase.

A

1) The RNA polymerase in the G phase (growth phase) creating more mRNA molecules
2) Proteins that are necessary for the cell to grow and divide
3) Proteins that are used for replication (S phase)

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11
Q

When a chromosome consists of only ONE double-stranded molecule (1 chromatid) ____(after/before) the s phase, it is called ____.

A

Before; A chromosome {This chromosome has 1 chromatid}

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12
Q

When a chromosome conists of two sister chromatids (x-shaped) ____ (after/before) the S phase, it is called ___.

A

After; A chromosome {this chromosome has 2 chromatids}

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13
Q

What is a centromere?

A

The center of two joined sister chromatids (where the two lines of the X intersect).

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14
Q

Entering the S phase, the chromosome has how many chromatids?

A

1

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15
Q

Leaving the S phase, the chromosome has how many chromatids?

A

2

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16
Q

How many chromosomes are there before the S phase in humans?

A

46

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17
Q

How many chromosomes are there after the S phase in humans?

A

46

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18
Q

In which phase is the DNA tightly packed for easier separation of chromosomes?

A

M phase (mitotic)

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19
Q

Why are chromosomes only seen in M phase (metaphase)?

A

Because this is the phase in which chromosomes are most condensed (condensed chromatin).

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20
Q

Why can we say that there are technically no chromosomes in interphase?

A

Because chromosomes are diffused chromatin, whereas in the M phase, they’re condensed chromatin.

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21
Q

What are chromosomes classified based on?

A

1) Centromeric position

2) Arm length

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22
Q

How many arms does each chromosome have?

A

2

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23
Q

The upper chromosomal arm is ___(short/long) and called the ___(q/p) arm, while the lower arm is ____(short/long) and called the ___(q/p) arm.

A

Short, p; long, q

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24
Q

What are the four types of chromosomes?

A

1) Metacentric
2) Submetacentric
3) Acrocentric
4) Telocentric

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25
Q

What is a metacentric chromosome?

A

The centromere is in the middle and P and Q are indistinguishable.

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26
Q

What is a submetacentric chromosome?

A

Centromere is displaced from the center and P and Q are easily distinguishable.

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27
Q

What is an acrocentric chromosome?

A

The P arm is barely there.

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28
Q

What is a telocentric chromosome?

A

There is no P arm (abnormal)

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29
Q

How many chromosomes are found in humans?

A

46

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30
Q

How many pairs of chromosomes are found in humans?

A

23

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31
Q

How many chromosomes come from the mother (maternal)?

A

23

32
Q

How many chromosomes come from the father (paternal)?

A

23

33
Q

What does diploid mean?

A

Paired chromosomes, one from each parent

34
Q

How many pairs of chromosomes are autosomal?

A

22

35
Q

How many pairs of chromosomes are sex chromosomes?

A

1 pair (the 23rd)

36
Q

Which cells in our bodies are haploid (unpaired)?

A

Sex/germ cells

37
Q

The sex chromosome can either be:

A

X or Y

38
Q

What is mitosis?

A

The division of cells, producing identical daughter cells.

39
Q

In mitosis, what do we mean by identical daughter cells?

A

That they are identical in terms of the nucleus and DNA sequence, but not cytoplasm.

40
Q

Where does mitosis occur?

A

In the zygote, prenatal developmental process, and newborns and adults. Also, for growth and repair, wound healing, and proliferation of immune cells.

41
Q

During the M phase the chromosome consists of:

A

2 sister chromatids (2 DNA molecules)

42
Q

What are the phases of mitosis?

A

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

43
Q

What are the characteristics of prophase in mitosis?

A

1) Chromosomes begin to condense
2) Nuclear envelope disappears
3) Pair of centrioles appear at opposite ends
4) Spindle fibers appear

44
Q

Where do the centrioles attach to?

A

The centromeres

45
Q

What are the characteristics of metaphase in mitosis?

A

1) Each chromosome aligns individually along the midline (metaphase plate).
2) Spindle fiber attaches to the centromere on both poles

46
Q

What are the characteristics of anaphase in mitosis?

A

Separation of the sister chromatids (each X is pulled apart).

47
Q

If, for example, we had 6 chromosomes to start with, how many chromosomes would we have in anaphase during mitosis?

A

12 (each chromatid is comsidered a chromosome)

48
Q

What are the characteristics of telophase in mitosis?

A

1) Nuclear envelope reforms
2) Two daughter nuclei form
3) Chromosomes diffuse
4) Cleavage furrow (contractile ring) forms.
5) Cytokinesis (division of cytoplasm) happens

49
Q

What is meiosis?

A

The division of sex/germ cells, gives half the number of chromosomes and with varying DNA sequences (not identical daughter cells).

50
Q

What are the two stages of meiosis?

A

Meiosis 1: The two homologous chromosome separate (XX -> X X)
Meiosis 2: The two sister chromatids separate (X -> l l)

51
Q

What are the characteristics of prophase 1?

A

Same as mitosis:
1) Condensation of chromosomes
2) Nuclear envelope disappears
3) Pair of centrioles appear at opposite ends
4) Spindle fibers appear
BUT
Recombination (crossing over) happens here (swapping genetic material betwen non-sister chromatids of homologous chromosomes)

52
Q

Which phase in meiosis 1 is the longest?

A

Prophase 1

53
Q

What are the characteristics of metaphase in meiosis 1?

A

Same as mitosis, BUT:

1) Each chromosome aligns in pairs (homologs) along the midline (metaphase plate).
2) Spindle fiber attaches to the centromere on one pole only.

54
Q

What are the characteristics of anaphase in meiosis 1?

A

Same as mitosis, BUT:
Separation (disjunction) of the homologous chromosomes (each XX is pulled apart, reducing the chromosome number in half).

55
Q

What are the characteristics of telophase in meiosis 1?

A

The 2 haploid cells separate.

56
Q

What characterizes the end of meiosis 1?

A

When the haploid chromosome number is first seen.

57
Q

Reduction of the number of chromosomes occurs in:
A) Mitosis
B) Meiosis 1
C) Meiosis 2

A

B) Meiosis 1

58
Q

The steps of meiosis 2 are the same steps as ____.

A

Mitosis

59
Q

When does meiosis 2 happen?

A

Immediately after meiosis 1

60
Q

Which stage is not included in meiosis 2?

A

Interphase.

61
Q

What is the product of meiosis?

A

4 non-identical daughter cells (sperm/eggs)

62
Q

What are the five stages of prophase 1?

A

1) Leptotene
2) Zygotene
3) Pachytene
4) Diplotene
5) Diakinesis

63
Q

What are the characteristics of Leptotene?

A

1) Initiation of condensation

2) Homologous chromosomes align

64
Q

What are the characteristics of Zygotene?

A

1) Synapsis (pairing/matching up)

2) Tetrad formation

65
Q

What is the chiasma?

A

The point of synapsis

66
Q

What are the characteristics of Pachytene?

A

1) Synapsis is complete
2) Each pair of homologs is called a tetrad (bivalent)
3) Recombination (crossing over) happens

67
Q

What are the characteristics of Diplotene?

A

Chromosomes separate but remain bound at chiasmata

68
Q

Why do the separated chromosomes after diplotene remain at the chiasmata?

A

Because we need the chromosomes to stay aligned in pairs for metaphase 1.

69
Q

What are the characteristics of Diakinesis?

A

Further condensation until we reach metaphase 1 (most condensed at metaphase 1).

70
Q

What are Gregor Mendel’s laws?

A

1) Law of Segregation

2) Law of Independent Assortment

71
Q

What does the Law of Segregation state?

A

That each homologous chromosome will separate and randomly appear in different daughter cells, such that each gamete receives one allele for a given trait

72
Q

What does the Law of Independent Assortment state?

A

That chromosomes are aligned at the metaphase plate independently of each other, meaning that traits are inherited independently of each other. {The allele received for a gene does not influence another allele for another gene}.

73
Q

What is the formula for the number of different combinations of chromosomes in gametes?

A

2^n, where n= The number of pairs of homologous chromosomes.

(XX XX XX = 3 pairs = 2^3 = 8 combinations)

74
Q

What are the total results of meiosis?

A

1) Reduction of number of chromosomes
2) Diploid to haploid (essential for gametes)

3) Random assortment of maternal and paternal chromosomes
a) Genes on different chromosomes
b) Maternal/paternal chromosomes
c) Number of possible chromosome combinations = 2^23 = 8,388,608
d) Recombination between chromosome pairs increases the possible combinations

4) Segregation of alleles
5) Recombination/crossing over

75
Q

What are the advantages of crossing over/recombination?

A

1) Allows new combinations of genes to be produced
2) Important for normal chromosome disjunction
3) Ensures genetic diversity

Medical Genetics (7 decks)

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