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Medical Genetics > Chromosomes & Genes > Flashcards

Chromosomes & Genes Flashcards

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Biology 101 flashcards USMLE® Step 1 flashcards
1
Q

What are the 2 types of nucleic acids in DNA, and which bases are derivative of them?

A
  1. Purines: adenine and guanine
  2. Pyrimidines: thymine and cytosine
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2
Q

In RNA, uracil replaces _______.

A

Thymine

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3
Q

What are the 2 general types of genes?

A
  1. Those whose product is a protein
  2. Those whose product is a functional RNA
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4
Q

What are the types of functional RNA?

A

t-RNA (transfer) the physical link between the mRNA and the amino acid sequence of proteins. Transfer RNA does this by carrying an amino acid to the protein synthesizing machinery of a cell called the ribosome
r-RNA(ribosomal) - help translate the information in messenger RNA (mRNA) into protein
nc-RNA (regulate gene expression) (non-coding)

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5
Q

What is the estimated number of protein coding genes? (Exome)

A

20,000 - 25,000
We recognize the function of about only 7,000 of these genes.
This represents less than 1.5% of the whole genome.

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6
Q

How many noncoding RNA genes are estimated to be in the human genome?

A

20,000 - 25,000
This is in addition to the exome.

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7
Q

Purine Adenine always pairs with ________

A

Pyrimidine Thymime

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8
Q

Pyrimidime cytosine always pairs with ________

A

Purine guanine

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9
Q

When is chromatin distributed throughout the nucleus evenly?

A

In interphase

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10
Q

When are chromosomes visible as discrete structures?

A

During cell division.
When a cell divides, chromatin condenses to appear as microscopically visible chromosomes.

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11
Q

Chromatin forms complexes with _______.

A

Histones (a family of chromosomal proteins)

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12
Q

Histones play a major role in the proper packaging of chromatin. What are the 5 major types of histones?

A

H1, H2A, H2B, H3, H4

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13
Q

A segment of DNA double helix winds around what?

A

An octamer, which is made up of two copies of each H2A, H2B, H3, and H4

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14
Q

How is the beads on a string appearance of DNA formed?

A

About 140 base pairs are associated with each histone core. After a short (20-60 bp) “spacer” segment of DNA, the next core DNA complex forms.

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15
Q

What is each complex of DNA with core histones called?

A

nucleosome

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16
Q

Where is the histone H1 found?

A

Bound to DNA at the edge of each nucleosome

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17
Q

What is a cylindrical solenoid fiber?

A

The long strings of nucleosomes that are themselves compacted into a secondary helical structure

Each complex of DNA with core histones is called a nucleosome

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18
Q

Chromatin Packing in Chromosomes Figure

A
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19
Q

How many chromosomes do we have?

A

22 pairs of autosomes and a pair of sex chromosomes

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20
Q

What are homologous chromosomes or homologues?

A

A pair of chromosomes that carry matching genetic information.

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21
Q

What is a karyotype?

A

The number and visual appearance (morphology) of the chromosomes in the cell.

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22
Q

Each band on a chromosome is designated by arm and a number, how is the numbering done?

A

From centromere to telomere

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23
Q

What is chromatin?

A

made up of protein, DNA, and RNA (makes up the chromosomes)

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24
Q

Name 4 features of the p arm of a chromosome?

A

very short, dense, non-coding, repetitive

25
Q

Which are the acrocentric chromosomes?

A

13, 14, 15, 21, 22

26
Q

What is the centromere?

A

Narrowing of the chromosome that divides between arms p & q

27
Q

G - banding (Giemsa banding)

A

type of karyotype, completed during the metaphase stage of mitosis. Cells are arrested in metaphase and stained to show G-bands. Can detect deletions/duplications of greater than 5 to 10 Mb
Estimate of the Total Number of Bands in One Haploid Set (=Resolution) is 400-500 bands

28
Q

prometaphase banding

A

higher resolution type of banding with an estimated total number of bands in one haploid set of 850 bands. Can detect deletions/duplications of 2-3Mb

29
Q

What is resolution as it relates to G-banding?

A

One of the variables in classical karyotyping by G-banding is the “resolution.” The resolution of the karyotype is a measure of the number of bands that are visible and therefore the smallest segment of the genome that can be detected using this method. Resolution increases with the overall length of the chromosome spread, thus it is desirable to obtain long chromosomes to yield more detailed information during karyotype analysis.

30
Q

What are heterochromatic bands?

A

Darkly staining bands
AT-rich, late-replicating, and correlate to generally inactive chromosomal regions that contain dense repetitive DNA sequences which do not contain genes and are not transcribed

31
Q

What are euchromatic bands?

A

Lightly-staining areas
GC rich, early replicating, and are associated with active regions. Also contains repetitive DNA sequences such as Alu elements which are the most abundant repetitive elements in the human genome

32
Q

Is the size of the chromosome associated with the amount of genes on that chromosome?

A

No

33
Q

What is euchromatin?

A

The major component of chromatin, relatively uncondensed and transcriptionally active. It stains lightly with G banding. Contrast with heterochromatin.

34
Q

What is constitutive heterochromatin?

A

highly repetitive DNA sequences (satellite) which do not contain genes and are not transcribed . Stains darkly with G-banding

35
Q

Which areas of chromosomes are constitutive heterochromatin?

A

proximal long arms (pericentric region) of chromosomes 1, 9, 16 and the distal long arm of Y

36
Q

What is facultative heterochromatin?

A

Areas of gene inactivation in the genome. An example is inactive X in females.

37
Q

What percentage of the genome is made up of single-copy DNA vs repetitive DNA sequences

A

50% of the genome is made up of single copy DNA and 40% of the genome is made up of repetitive DNA sequences

38
Q

What is satellite DNA?

A

synonymous with tandemly repeated DNA (tandem repeats). These are short repeats organized in tandem in a head-to-tail fashion. They make up 10-15% of the genome.

39
Q

Where are pentanucleotides found?

A

in large genetically inert regions on chromosomes 1, 9, and 16 and make up more than half of the Y chromosome

40
Q

Alu family of repetitive DNA

A

One group of a major class of repetitive DNA in the genome consists of related sequences that are dispersed throughout the genome rather than clustered in one or a few locations (unlike tandem repeats). The members of this family are approximately 300bp in length and are related to each other although not identical in DNA sequence. In total, there are more than 1 million Alu family members in the genome, making up at least 10% of human DNA.
They are also implicated in genetic conditions.
Also classified as short interspersed nuclear elements (SINEs)
Regions rich in GC content tend to enriched in Alu elements but depleted of LINE sequences

41
Q

LINE family (long interspersed nuclear element, L1)

A

another major class of repetitive DNA in the genome (like Alu family).
up to 6000 bp in length
Found in approximately 850,000 copies per genome.
Accounts for nearly 20% of the genome.
AT-rich regions of the genome tend to be enriched in LINE sequences but depleted in Alu elements

41
Q

Low copy repeats within chromosome arms are called ___________.

A

segmental duplications

41
Q

Segmental duplications: basic facts

A

Account for about 5% of the human genome
>1,000 base pairs (1kb) in length, can span hundreds of kilobase pairs
they are clustered within the genome
>90% sequence homology

42
Q

Which mechanism of structural variants are segmental duplications prone to?

A

Nonallelic homologous recombination (NAHR).
NAHR refers to aberrant (abnormal) recombination resulting from the misalignment of two highly similar paralogous copies (duplicated copies descended from the same origin) of segmental duplications, which further leads to structural variants, including deletions, duplications, translocations, and inversions.
Ex: Causes recurrent microdeletions/microduplications syndromes

43
Q

What percentage of the DNA sequence is a unique sequence (single copy)?

A

about 50%

44
Q

What percentage of the DNA sequence is made of repetitive non-coding DNA?

A

about 50%

45
Q

What are the types of repetitive DNA sequences, their prevalence, and features?

A
  1. Dispersed - 35% - interspersed - examples are SINEs and LINEs
  2. Satellite DNA - 10% - arrays of tandemly repeated DNA - examples are alpha- and beta-satellite DNA
  3. Segmental duplications - 5% - 1-500 kb with > 90% sequence identity
46
Q

What is the centromere?

A

Site of attachment to microtubules of the spindle apparatus during cell division
Point of connection of sister chromatids after DNA replication to form the kinetochore

47
Q

What type of DNA is predominantly present in the centromere?

A

alpha-satellite DNA with non-coding repetitive tandem repeats

48
Q

What DNA sequence makes up telomeres?

A

5’-TTAGGG-3’

49
Q

Why do telomeres shorten with age?

A

Because they shorten as cells divide.

50
Q

What do telomeres do and prevent?

A

Protect structural integrity and stability of chromosomes. Without telomeres, chromosome ends would fuse to each other producing unstable ring chromosomes or dicentric chromosomes.
A dicentric chromosome is a rare type of abnormal chromosome in which two chromosome segments, each with a centromere, fuse end to end. Dicentric chromosomes, despite their two centromeres, can be mitotically stable if one of the two centromeres is inactivated epigenetically or if the two centromeres always coordinate their movement to one or the other pole during anaphase. Such chromosomes are formally called pseudodicentric. The most common pseudodicentrics involve the sex chromosomes or the acrocentric chromosomes (so-called Robertsonian translocations; see later).

51
Q

What makes up the telomere structure?

A

Telomere structure is associated with six proteins that collectively create the Shelterin complex. This complex helps to create the final end cap structure of the chromosome.

52
Q

How are telomeres associated with disorders?

A

Mutations in telomere maintenance genes lead to disorders that are associated with short telomeres (Pulmonary fibrosis, Bone Marrow failure: dyskeratosis congenita (inherited bone marrow failure syndrome), aplastic anemia, cancers)

53
Q

Explain the composition of acrocentric chromosomes and name the related translocation mechanism.

A

Short p arms of acrocentric chromosomes (13, 14, 15, 21, and 22) are comprised of a stalk and satellite. Contain tandem repeats of ribosomal DNA genes. There is a large variation in length. This unstable area is prone to breakage and rearrangement (Robertsonian Translocation)
Balanced translocation doesn’t typically cause an issue but unbalanced translocation does. In Robertsonian Translocation, for example, the p arm of chromosome 13 may be lost which is important for that person’s offspring because they will get missing or extra copies of chromosomes.
https://www.youtube.com/watch?v=vbGw4VanNjk
Very good video on Robertsonian Translocation.

54
Q

Robertsonian Translocation Video

A

https://www.youtube.com/watch?v=vbGw4VanNjk

55
Q

How many genes make up the mitochondrial genome? And what are they involved in?

A

37 genes that encode 13 subunits of enzymes involved in oxidative phosphorylation, as well as ribosomal RNAs and transfer RNAs required for translating the transcripts of the mitochondria-encoded polypeptides

56
Q

What is the inheritance pattern of mitochondrial DNA?

A

Maternal

Medical Genetics (11 decks)

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