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CELL BIO > WEEK 1 - nucleus > Flashcards

WEEK 1 - nucleus Flashcards

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Biology 101 flashcards
1
Q

What is the nucleus?

A
  • Membrane bound organelle in eukaryotic cells containing DNA (that is arranged in chromosomes)
  • surrounded by nuclear envelope (double nuclear membrane), seperates nucleus from cytoplasm
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2
Q

Nuclear lamins=

A

a meshwork of intermediate filaments supporting the nuclear membrane

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3
Q

Role of nuclear pores…

A

control movement of substances in and out of the nucleus

  • RNA is selectively transported into the cytoplasm
  • Proteins are selectively transported into the nucleus
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4
Q

Why is the nucleus important?

A
  • Separates fragile chromosomes from cell contents
  • DNA replication, transcription and RNA processing all in the nucleus
  • Separates RNA transcription in the nucleus from translation machinery in the cytoplasm
  • Nuclear envelope allows gene expression to be regulated
  • MRNA undergoes post transcriptional processing before moving from nucleus to the cytoplasm
  • Control of gene expression at the level of transcription
    e.g. expression of some eukaryotic genes controlled by regulated transport of transcription factors from cytoplasm to nucleus
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5
Q

What happens at the nucleolus and where is it found?

A

Sites at which ribosomes are assembled and ribosomal RNA is transcribed

One or more nucleoli are found in the nucleus

  • most prominent in cells that are synthesizing large amounts of proteins
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6
Q

Role of nuclear envelope

A

Encloses DNA

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7
Q

What does the inner membrane on the nuclear envelope contain?

A

proteins that act as an anchoring sites for chromatin and nuclear lamina

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8
Q

Characteristics of the outer membrane of the nuclear envelope…

A

continous with ER and studded with ribsomomes

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9
Q

Perinuclear space

A

Protein made are transported into perinuclear space

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10
Q

Characteristics of nuclear pores:

A

Each nuclear pore complex (NPC) - 30 proteins (nucleoporins)

Eightfold rotational symmetry

Large enough to accommodate ribosomal subunits

Pore filled with unstructured protein – contains numerous repeats of phenylalanine-glycine (FG) motifs – weak affinity for each other creates a gel like mesh inside NPC

^look back at slide 15.

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11
Q

Regulation of transport through NPCs…

A

Small proteins continually shuttle back and forth between the nucleus and the cytosol

Need import and export signal

Other proteins contain both nuclear localisation signals (NLS) and nuclear export signals (NES).

Relative rates of import and export determine the steady-state localisation of such shuttling proteins

changing rate of import, export, or both - change the location of a protein

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12
Q

Nuclear localisation signals:

A

Responsible for the selectivity of active nuclear import process

Most COMMON signal

Transport goes through mesh and can occur in either direction

Aslong as they are somewhere

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13
Q

Nuclear import receptors:

A

NLS must be recognized by nuclear transport receptors

Most receptors = karyopherins

Receptors use adaptor proteins that form an import receptor/NLS bridge

There is a variety of import receptors and adaptors – cells recognize a range of NLS

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14
Q

Nuclear export receptors:

A

Active – Ran-GTP

Inactive – Ran-GDP

Nuclear export of large molecules through NPCs depends on a selective transport system

Relies of nuclear export signals on macromolecules to be exported, export receptors bing to both the export signal, either DIRECTLY or VIA AN ADAPTOR, and to NPC proteins.

Import and export transport systems work in similar ways but in opposite directions: the import receptors bind their cargo molecules in the cytosol, release them in the nucleus, and are then exported to the cytosol for reuse export receptors function in the opposite fashion

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15
Q

Nuclear lamina:

A

Localised nuclear side of inner nuclear membrane

Meshwork of interconnected protein subunits (nuclear lamins)

Lamins – intermediate filament proteins – polymerise into 2D lattice

Gives shape and stability to nuclear envelope

Anchored by attachment to NPCs and integral membrane proteins Interacts directly with chromatin

STRUCTURE OF NUCLEAR LAMINA – pg 21 (rope)

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16
Q

What DNA stand for?

A

Deoxyribonucleic acid (DNA)

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17
Q

DNA consists of…

A

two long polynucleotide chains composed of four types of nucleotide subunits

Each chain is called a ‘DNA strand’

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18
Q

DNA structure:

A
  • The two strands run antiparallel to each other, and hydrogen bonds between the base portions of the nucleotides hold the two strands together
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19
Q

Nucleotide is composed of:

A

is composed of a five-carbon sugar to which a phosphate group and a nitrogen-containing base are attached

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20
Q

How are nucleotides linked?

A

covalently linked together in a chain through the sugars and phosphates, to form a “backbone” of alternating sugar–phosphate–sugar– phosphate

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21
Q

Bases in DNA:

A

base may be either adenine (A), cytosine (C), guanine (G), or thymine (T).

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22
Q

What makes the nucleotide negatively charged?

A

The phosphate

23
Q

Nucleosides=

A

base and the sugar linked by glycosidic bond

24
Q

How to form nucleic acid polymers:

A

nucleotides are joined together by phosphodiester bonds between the 5’ and 3’ carbon atoms of adjacent sugar rings.

25
Q

Creating phosphodiester bond:

A
26
Q

Structure of the double helix:

A

Nucleotides– covalently linked

Strands held together by hydrogen bonds between base pairs

All bases inside double helix, sugar-phospahte backbones on the outside

Strands run ANTI-PARALLEL

27
Q

Base pairing:

A

A two-ring base (purine) is paired with a single - ring base (a pyrimidine)
- this base pairing is the most energetically favourable arrangement
-each base pair is of similar width, constant distance apart along the DNA molecule

28
Q

How many hydrogen bonds form between A and T?

A

2

29
Q

How many hydrogen bonds form between G and C and why??

A

3
-bcos they are antiparallel

30
Q

How do the 4 base pairs lie?

A

Lie perpendicular to the axis of the helix

31
Q

Why does the double stranded DNA molecule wind into a double helix?

A

to maximise the efficiency of base pairing

-one complete turn per 10.4 bp
- creates two grooves, the major and minor groove

32
Q

(L2) Chromatin=

A

complex of DNA and tightly bound protein

33
Q

Chromosomes in eukaryotic DNA=

A

22 pairs of homologous chromosomes and 1 pair of non-homologous

EXCEPTIONS
- gametes (egg and sperm cells)
- & some highly specialized cell types – EACH human cell nucleus contains 2 copies of each chromosome

34
Q

Chromosome=

A

a single long DNA molecule along with the proteins that fold the fine DNA thread into a more compact structure.

35
Q

Karyotype=

A

= the general appearance of the complete set of chromosomes in the cells of a species or in an individual organism, mainly including their sizes, numbers, and shapes

  • Chromosomes 1-22 numbered in approximate order of size
36
Q

Centromeres=

A

attachment site for the two halves of each replicated chromosome (sister chromatids)

  • Not always in centre of chromosome
  • Keep chromosomes properly aligned during cell division
37
Q

Telomeres=

A

Repetitive stretches of DNA located at the ends of linear chromosomes

38
Q

Role of telomeres=

A

Protect the ends of chromosomes to keep them from unravelling – think shoelaces

39
Q

What happens to telomeres every time a cell divide in most types of cells?

A

In many types of cells, telomeres lose a bit of their DNA every time a cell divides. When all of the telomere DNA is gone, the cell cannot replicate and dies.

40
Q

Telomerase=

A

Enzyme in WBC and other frequently dividing cells that prevents their chromsomes from losing their telomeres -CELL LIVES LONGER

  • Telomerase adds TTAGGG repeats to the ends of chromosomes
41
Q

Telomeres role in cancer:

A

chromosomes of malignant cells usually do not lose their telomeres – fuels the uncontrolled growth

42
Q

Most important function=

A

form genes, incl. Info about when, in what type of cells, and in what quantity each RNA molecule and protein is to be made of.

43
Q

Diameter of the nucleus

A

nucleus is ~6 μm in diameter.

44
Q

How is DNA packaging accomplished?

A

by specialized proteins that bind to the DNA and fold it, generating a series of organized coils and loops that prevent the DNA from becoming an unmanageable tangle.

  • Although the DNA is very tightly compacted, it remains accessible to the enzymes in the cell that replicate it, repair it, and use its genes to produce RNA molecules and proteins.
45
Q

Heterochromatin=

A

chromatin regions that are condensed during interphase and transcriptionally inactive

46
Q

Euchromatin=

A

chromatin regions that are decondensed and DNA sequences are being transcribed into RNA

47
Q

Does euchromatin or heterochromatin stain more densely?

A

Heterochromatin

48
Q

interphase:

A

= period in eukaryotic cell cycle characterzed by:

G1 PHASE – cell undergoes growth

S PHASE – cell makes copy of its DNA

G2 PHASE – cell continues to grow, and preps for cell division

49
Q

What are histones responsible for?

A

responsible for most basic level of chromosome packing – a protein DNA complex called the nucleosome

50
Q

Nucleosomes continued….

A
  • Each individual nucleosome core particle consists of a complex of eight histone proteins—two molecules each of histones H2A, H2B, H3, and H4— and double-stranded DNA that is 147 nucleotide pairs long.
  • This histone octamer forms a protein core around which the double-stranded DNA is wound.
  • 147 nt pairs wraps 1.7 times around the histone core
  • Each nucleosome separated from next by linker DNA (few nucleotide pairs up to about 80)
  • Nucleosomes repeat at intervals of approx. 200 nucleotide pairs.
51
Q

Structure of nucleosome core particle

A
52
Q

Nucleosomes packed into a chromatin fibre

A

Chromatin in a living cell probably rarely adopts the beads on a string form

Nucleosomes packed on top of each other – DNA more condensed

Nucleosome to nucleosome linkages formed by histone tails, notably H4 tail.

53
Q

Looped domains

A

CELL BIO (8 decks)

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