STRUCTURE AND FUNCTION OF CELL / NUCLEUS / CHROMATIN

Question 1

WHERE DOES TRANSCRIPTION TAKE PLACE IN EUKARYOTES?

Answer 1

nucleus

Question 2

WHERE DOES TRANSLATION TAKE PLACE IN EUKARYOTES?

Answer 2

cytoplasm

Question 3

WHERE DOES DNA REPLICATION TAKE PLACE IN EUKARYOTES?

Answer 3

nucleus

Question 4

WHAT IS THE NUCLEAR ENVELOPE AND ITS FUNCTION?

Answer 4

separates the nucleus from the cytoplasm
-2 layers + perinuclear space (both membranes contain their own distinct complement proteins and outer membrane is connected to fibers of cytoskeleton by fibrous proteins (nesprins)
-nuclear pores which are protein forming channels that regulate the transport of substances from the cytoplasm to the nucleus

Question 5

WHAT IS THE NUCLEAR LAMINA AND WHAT ARE ITS FUNCTIONS?

Answer 5

network of lamins underneath the nuclear envelope
-provides mechanical support for nuclear envelope
-disintegration and reintegration of nuclear envelope during mitosis
1: phosphorylation – lamins disintegrate and carry bits of nuclear envelope with them
2: dephosphorization – lamins begin to reintegrate and reform the nuclear envelope

Question 6

WHAT KINDS OF CHROMATIN ARE FORMED BY CHROMOSOMES IN EUKARYOTIC CELLS?

Answer 6

linear chromosomes

Question 7

WHAT IS THE BARR BODY?

Answer 7

– one of two X chromosomes in female is inactive (due to heterochromatin) and is condensed and during interphase is visible under a microscope as a dark dot sticking to the side of the nucleus.

Question 8

BY HOW MUCH DOES DNA SHORTEN WHEN CONDENSED?

Answer 8

to 1/10 000 of its original length

Question 9

WHAT IS THE NUCLEOLUS AND ITS FUNCTION?

Answer 9

contains rRNA for the production of ribosomes
-not membrane bound

Question 10

WHEN DOES THE SYNTHESIS OF HISTONES TAKE PLACE?

Answer 10

during replication of DNA (S phase of cell cycle)

Question 11

WHAT IS THE FUNCTION OF NON HISTONE CHROMOSOMAL PROTEINS?

Answer 11

determine the spatial arrangement of chromosomes (they have solenoids anchored
to them)

Question 12

WHAT HISTONES ARE CONTAINED IN EUKARYOTIC CHROMOSOMES?

Answer 12

H1
H2A
H2B
H3
H4

Question 13

WHAT IS THE FUNCTION OF HISTONES?

Answer 13

they coil DNA → they build nucleosomes

Question 14

WHAT ARE THE DIFFERENCES BETWEEN PROKARYOTIC AND EUKARYOTIC CHROMOSOMES?

Answer 14

• euk. chromosome = linear
  prok. chromosome = circular

-euk. chromosome has more base pairs (10^8) instead of (10^6)

Question 15

WHAT IS THE STRUCTURE OF THE NUCLEOSOME?

Answer 15

-octamer of histones (2x H2A, 2x H2B, 2x H3, 2x H4)
-146/147 base pairs wrap around octamer
-around 60 bae pairs of free DNA in between nucleosomes
-H1 histone connects nucleosomes (condensation of chromosomes)
-1 nucleosomes is around 200 base pairs

Question 16

EXPLAIN THE STRUCTURAL LEVELS OF EUKARYOTIC CHROMOSOME.

Answer 16

• Double helix of DNA – diameter of strand is 2 nm
• Individual molecule of DNA is negatively charged à histones neutralize it and allow for the condensation of DNA - double helix wraps around the 8 histones – formation of nucleosome with the dimeter of the strand being 11 nm
• H1 histone causes further condensation into a solenoid -> chromatin strand with the dimeter of 30 nm
• solenoid is anchored onto the strand by non-histone proteins and cannot undergo further condensation -> loops of
  chromatin - 300 nm
• further condensation forms chromosome -> condensed part of chromosome = chromatid - 700 nm
• whole mitotich chromosme - 1400 nm = 1,4 x 10-3 m

Question 17

EXPLAIN THE STRUCTURE OF THE MITOTIC CHROMOSOME.

Answer 17

-2 chromatids
-centromere = central part, site of connection between chromatid and mitotic spindle
-telomere = ends of chromosome, protects against degradation of DNA/ random pairing of chromosomes

Question 18

WHAT IS THE REGULATION OF THE CHROMATIN STRUCTURE.

Answer 18

chromatin remodelling complexes – manipulate the level of condensation of DNA in nucleosomes – by modifying histones
A: acetylation - acetyl loosens chromatin; acetylate histones mainly found in euchromatin
B: methylation – methyl tightens chromatin; methylated histones mainly found in heterochromatin
C: phosphorylation – can loosen or tighten chromatin structure; depends on what is phosphorylated
- histones code = determines which genes are expressed or not

Question 19

WHAT ARE THE POST-TRANSCRIPTIONAL MODIFICATIONS?

Answer 19

-known as the histone code
-determine which genes are shown and not

METHYLATION = methyl tightens chromatin → methylated histones mainly found in heterochromatin

PHOSPHORYLATION = can loosen or tighten chromatin → depends on what is phosphorylated

Question 20

HOW MANY CHROMATIDS ARE IN THE HUMAN BODY?

Answer 20

92, double the number of chromosomes (46)

Question 21

WHAT IS THE DIFFERENCE BETWEEN TOPOISOMERASE I AND TOPOISOMERASE II?

Answer 21

topoisomerase I is ATP dependent while topoisomerase II is not ATP dependent

Question 22

WHAT IS A MONOCISTRONIC GENE?

Answer 22

transcript encodes for a single protein

Question 23

WHAT IS A POLYCISTRONIC GENE?

Answer 23

transcript encode for 2 or more proteins

Question 24

EXPLAIN HISTONE POSTTRANSLATONAL MODIFICATIONS

Answer 24

modifications via: methylation, acetylation, phosphorylation, ubiquitination

LYSINE ACETYLATION (HAT) -> abolishes positive charge = less compact nucleosome & transcription activated

LYSINE METHYLATION -> does not influence positive charge - serves as a mark - transcription can be activated or inactivated

LYSINE DEACTEYLATION (HDAC)

Question 25

WHAT DOES METHYLATION OF CYTOSINE IN CG DINUCLEOTIDES CAUSE?

Answer 25

-> interferes with binding of TFs
-> becomes marked and recognized by methyl binding proteins