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DNA structure Flashcards

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1
Q

DNA-

Deoxyribonucleic Acid

A

* relatively simple molecule

* carrier of genetic information

* Genes of every cell on Earth are made of DNA

* Cells maintain, replicate and express genetic information carried in DNA

* information in genes:

  • copied and transmitted to daughter cell multiple times
  • contains primarily instructions for making proteins
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2
Q

Dna Strands

A

consists of two long polynucleotide chains

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3
Q

DNA strand

Each strand made of 4 types of nucleotide subunits

A

( linked by phosphodiester bonds —> sugar-phosphate backbone with N-bases sticking out)

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4
Q

DNA Structure-

Two strands held togeher by hydrogen bonds between the bases -

A

purines ( adenine, guanine) and pyrimidines ( cytosine, thymine)

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5
Q

DNA

The opposing sugar- phosphate backbones have opposite polarity ( antiparallel)

A

complementary base pairing :

* enables the base pairs to be packed in the energetically most favorable arrangment ( same width 1- ring base pairs with 2 ring base –> same distance between sugar

*provides the basis for replication of nucleic acids

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6
Q

DNA structure

2 sugar-phosphate backbones twist around one another —-> form a double helix with 10 bases per helical turn

A

Base pairs fit together when the two strands are antiparallel ( opposite polarity)

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7
Q

Each strand of DNA molecule has a nucleotide sequence exactly complementary to its partner strand-

A

critical for copying DNA

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8
Q

DNA

A

carrier of genetic information

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9
Q

Genome

A

a complete set of genetic information in a cell

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10
Q

Genes-

A

fragments of DNA molecule coding for proteins + many non- coding sequences –> extremely long sequence of nucleotides ( message written in 4 letter code)

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11
Q

Human cell contain s- ____ ___ of DNA?

A

2 meters

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12
Q

Eucaryotic Cell Nucleus

-Provides a compartment in which the DNA and RNA-

A

-dependent functions are sequestered

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13
Q

Eucaryotic Cell Nucleus-

Enclosed by the ____ ____

A

nuclear envelope

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14
Q

Eucaryotic Cell Nucleus

May contain one or more ________? the site of ____

A

nucleoli

rRNA synthesis and ribosome assembly

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15
Q

Eucaryotic Cell Nucleus-

Nuclear Pores-

A

Nuclear envelope is penetrated by numerous nuclear pores

* allow passage of molecules and large particles:

from the nucleus to the cytosol and from the cytosol to the nucleus

* movement through the pores is regulated

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16
Q

Functions of the nucleus:

A

* DNA replication

* DNA packing -chromosomes

* DNA transcription —-> mRNA,rRNA and tRNA

* processing of mRNA

*mRNA transport

*ribosome assembly

* dissolution and reformation of the nuclear envelope during mitosis and meiosis

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17
Q

Eucaryotic Cell Nucleus

supported internally by the nuclear lamins ( intermediate filaments)

A

surrounded externally by a network of intermediate filaments

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18
Q

DNA packing - Chromosomes

name derived from Greek ( chroma + soma ) “ colored body”

A

composed of DNA + proteins = chromatin

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19
Q

DNA packing

Human genome- 3.2 x 10^9 nucleotides distributed over 24 chromosomes

A

Each cell contains 2 copies of each chromosome ( with a few exceptions) - one from the mother and one from the father

20
Q

ploidy

A

number of sets of chromosomes per cell

21
Q

haploid-

A

one set of chromosomes per cell

22
Q

diploid

A

two sets of chromosomes per cell

23
Q

tetraploid-

A

four sets of chromosome per cell

24
Q

Identification of Chromosomes

Chromosomes can be specifically identified in various ways

A

Karyotype-display of the full set of mitotic chromosomes

Banding patterns allows identification of human chromosomes

Giemsa dye-stains regions rich in A-T

25
Q

Chromosome functions-

* carry genes

A

Genes- segments of DNA containing instructions for making a specific protein or set of closely related proteins, or directing the RNA production

~30,000 genes in human cell + non coding sequences of DNA

Chromosomes undergo morphological changes during cell life.

Interphase chromosomes -tangled treats- cannot be distinguished in light microscope,

Metaphase chromosomes - highly condensed - easy to identify in light microscope

26
Q

Problem: extremely long DNA molecules in the cell must be packed into a very small volume and in such a way as to -

*prevent tangling

* be reversible

* allow rapid, localized, on demand access to DNA

A

solution:

DNA is packaged into a series of higher order structures by specialized proteins that coil and fold the DNA.

27
Q

Interphase-

A

DNA 1,000 times more compact than in the extended form

28
Q

Metaphase-

A

compaction ratio ~10,000 fold

29
Q

DNA Packing

DNA + proteins= chromatin

Proteins:

A

2 classes - Histone and nonhistone chromosomal proteins

30
Q

Histones

* large quanities ( ~ 60 million molecules/cell, total mass in chromatin about equal to DNA mass)

* responsible for the first level of chromatin packing

  • Nucleosomal histones= H2A, H2B, H3 & H4 small, highly conserved proteins responsible for coiling of DNA into Nucleosomes
  • H1 histomes - pack the DNA + nucleosomes into a coil
A

* positively charged

* the most highly conserved of all known eucaryotic proteins

31
Q

Nucleosomes

A

-fundamental packing units of DNA

32
Q

Nucleosomes

* made of-

A
  • protein core- complex of 8 histone proteins ( histone octamer-2 of each H2A, H2B,H3 & H4)
  • double stranded DNA- ~146 nucleotide pairs wrapped twice around octamer
  • linker DNA- up to 80 nucleotides.
33
Q

Formation of Nucleosomes

A
  • 1st level of packing
  • converts a DNA molecule into a chromatin tread- ~ 1/3 of its initial length

* beads on a string “ formation of chromatin”

34
Q

Formation of 30 nm fiber

A

* 2nd level of packing

* native form of DNA

*nucleosomes bundle together by H1 histones

* Unclear how nucleosomes are packed in a fiber- most probable zigzag model or selenoid structure

35
Q

Levels of DNA packing:

A
  1. Beads on a string
    - nucleosomes linked by linker DNA
  2. 30 nm fiber
    - Native form of DNA
    - Nucleosomes bundled together by H1 histones
    - Unclear how nucleosomes are packed in a fiber- most probable zigzag model
  3. Looped domain- Current model
    - Loops od 30 nm fiber attached to proteins that form the chromosomal axis

20,000-100,000 bp per loop

  1. Metaphase chromosome- final level of packing
36
Q

DNA packing - Chromosomes

Interphase cells

Chromatin in an interphase chromosome is not in the same packing state throughout the chromosome

A

* regions with genes that are being expressed are more extended, regions with quiescent genes- more compact —> chromosome structure can difffer from cell to cell during cell life.

37
Q

Forms of chromatin in interphase cell:

* Heterochromatin ( 10% of chromatin)

A
  • highly condenced

transcriptionally intactive

  • most of the heterochromatin does not contain genes

*concentrated around centromere and telomeres

38
Q

Forms of chromatin in Interphase cell:

* Active euchromatin (10% of chromatin)

A

* least condedsed

* histone h1 les tightly bound

* nucleosomal histones chemically modified

39
Q

Forms of chromatin in interphase cell:

Inactive euchromatin ( 80% of chromatin)

A

* more condensed than active euchromatin

* can become active euchromatin

  • it is not clear which of the levels of chromatin packing is present in euchromatin and in heterochromatin
40
Q

Miotic cell

Metaphase chromosomes

A

very condensed chromatin

41
Q

sperm cells

* sperm head-

A

most condensed form of chromatin

42
Q

DNA packing

Nucleosome Replication and Assembly:

A
  • nucleosomes must be moved out of the way to permit DNA to replicate or be translated
  • New nucleosomes must be assembled as DNA is replicated
  • New histones are synthesized at the same time as DNA replication
  • New nucleosomes assemble on the daughter DNA helices shortley after the DNA is replicated
43
Q

Dna PAcking

Nucleosome

A

-binding to DNA affected by:

_ base sequence ( AT- rich regions easier to bend)

-Binding of other proteins - may displace nucleosomes

44
Q

DNA packing

Eucaryotic cells have mechanisms to adjust the local structure of chromatin

* chromatin remodling complexes-

A
  • protein machines
  • use ATP to change nucleosome structure
  • make DNA more accessible to specialized proteins ( e.g. those involved in replication, gene expression, and DNA repair)
  • inactive during mitosis -helps maintain tightly packed chromosome structure
45
Q

DNA packing

Eucaryotic cells have mechanisms to adjust the local structure of chromatin

* reversible modification of histone tails

A

* N- terminal tails function in regulating chromatin structure

*undergo covalent modifications after nucleosomes assembly

* modified tails bind and attract specific proteins to diffrent chromatin regions ( some facilitate further chromatin condensation, some facilitate access to DNA)

*Histone modifying enzymes are strongly regulated

* diffrent combinations of tail modification and diffrent sets of histone binding proteins give diffrent signals ( e.g. for genes expression, replication)

46
Q

DNA packing

Eucaryotic cells have mechanisms to adjust the local structure of chromatin

A

Chromatin remodeling complexes and histone tails modifying enzymes may work in concert allowing rapid changes in chromatin structure according to cell needs.

cell Bio (14 decks)

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