DNA synthesis

Question 1

DNA is

Answer 1

𝐷𝑁𝐴 is the hereditary material that contains instruction for making proteins and 𝑅𝑁𝐴.

Question 2

DNA is made of

Answer 2

Nucleotides:

• Sugar.
• Nitrogenous base.
• Phosphate group.

Question 3

Nucleotides are linked together by

Answer 3

Phosphodiester bond.
- Phosphate group attached to the 5th carbon of one
sugar and the 3rd carbon of another

Question 4

𝐷𝑁𝐴 consists of

Answer 4

Double helix of nucleotides twisted into a double helix.
The two chains are antiparallel – run in opposite
directions.

Question 5

Strands of DNA held together by

Answer 5

hydrogen bonds between the bases:

• Adenine binds to thymine.
• Guanine binds to cytosine.

Question 6

𝐷𝑁𝐴 is always replicated

Answer 6

in the 5 to 3 direction.

• 5th end terminates in a phosphate group.
• 3rd end terminates in a sugar.

Question 7

𝐷𝑁𝐴 synthesis is

Answer 7

𝐷𝑁𝐴 synthesis , also known as replication, is the process of synthesizing a new 𝐷𝑁𝐴 molecule that occur during the cell’s 𝑆 phase of an eukaryotic cells (46 chromosomes replicate to form 92 chromosomes. 𝐷𝑁𝐴
synthesis occur in the nucleus of the eukaryotic cells, and in the cytoplasmic of a prokaryotic cells.

Question 8

Principle of 𝐷𝑁𝐴 Synthesis:

Answer 8

1. Universal process – 𝐷𝑁𝐴 replication occur in all living organisms.
2. Semiconservative process – the two original strands are separated during the process, and each
   will from a double stranded molecule with a newly synthesized strand.

Question 9

Process of 𝐷𝑁𝐴 Synthesis:

Answer 9

The 𝐷𝑁𝐴 molecule has several actin sites of synthesis working simultaneously. Each of those places are 
called replication fork, and in it, fallowing enzymes can be found: 
- Helicase 
- Single strand binding proteins (𝑆𝑆B)
- Topoisomerase
- 𝐷𝑁𝐴 polymerase 
- Primase
- Ligase

Question 10

The Helicase enzyme

Answer 10

An enzyme that separates the two 𝐷𝑁𝐴 strands by the removal of hydrogen bonds between the nitrogenous bases.

Question 11

Single strand binding proteins (𝑆𝑆B) enzyme

Answer 11

Proteins that bind the individual strands, preventing them from reforming the hydrogen bonds again.

Question 12

Topoisomerase enzyme

Answer 12

An enzyme that bind in advanced position to helicase in order to prevent knot formations by catting one strand to relieve the tension created helicase active.

Question 13

𝐷𝑁𝐴 polymerase enzymes

Answer 13

Family of enzymes that responsible of reading the template strand and bring complementary nucleotides. 𝐷𝑁𝐴 polymerase has two limiting factors:
- It can only read from 3 to 5 direction, and therefore write from 5 to 3.
This is creating two forms of strands:
* The leading strand – the strand synthesized in a
continues manner towards the
replication from.
* The lagging strand – the strand synthesized against
the replication from in short
pieces called Okazaki fragments.
- 𝐷𝑁𝐴 polymerase cannot initiate replication, and therefore a short 𝑅𝑁𝐴 primer is required.

***In the end of the process, 𝐷𝑁𝐴 polymerase 3 is doing a proof reading and correct any error that might have occurred.

Question 14

Primase enzmye

Answer 14

An enzyme responsible for the synthesis of a short primer (made of 𝑅𝑁𝐴).

Question 15

Ligase enzmye

Answer 15

Responsible to remove the 𝑅𝑁𝐴 primers and connect the fragments together into one
continuous strand.

Question 16

DNA Packing

Answer 16

𝐷𝑁𝐴 fiber is organized and packed as chromatin in three levels in eukaryotic cells. This organization is
made my proteins called histones. Histones are positively charged proteins that package and order the 𝐷𝑁𝐴 (which is negatively charged) into units called nucleosomes.

Question 17

The histones are

Answer 17

𝐻1 
 𝐻2𝐴. 
 𝐻2𝐵. 
 𝐻3. 
 𝐻4

Question 18

First organization (nucleosome formation)

Answer 18

Nucleosome is made of 8 histones proteins. Two of
each:
- 𝐻2𝐴.
- 𝐻2𝐵.
- 𝐻3.
- 𝐻4.
Histones are basic, therefore they attract the nucleic acid to be wrapped around them twice.

Question 19

Second organization (30𝑛𝑚 fiber)

Answer 19

The nucleosomes are arranged in triplets and connect by additional histone (𝐻1) to form a fiber with 30𝑛𝑚 thickness.

Question 20

Third organization (scaffolding)

Answer 20

Proteins called scaffolding proteins are forming fingerlike projections from the nuclear membrane. The 30𝑛𝑚 is warped around them.

Question 21

Forms of Chromatin

Answer 21

• Heterochromatin – tightly packed, not expressed or transcriptionally silent.
• Euchromatin – loosely packed, expressed or active transcription.

Question 22

Changing Chromatin Formation:

Answer 22

• Methylation – addition of methly1 group to nucleosides; resulting in 𝐷𝑁𝐴 more tightly packed.
• Acetylation and phosphorylation – histone modification; resulting in making the 𝐷𝑁𝐴 more
  loose.