Unit 1: Human Cells - Key Area 2 - Structure and replication of DNA

Question 1

What does DNA consist of?

Answer 1

Units called nucleotides

Question 2

What are nucleotides made of?

Answer 2

Phosphate, deoxyribose sugar, and a base

Question 3

What forms the genetic code?

Answer 3

The base sequence

Question 4

What holds each strand of DNA together?

Answer 4

Each individual strand of DNA is held together by a strong chemical bond between the phosphate of one nucleotide and the carbon 3 of the sugar, on another nucleotide.

Question 5

What holds the bases in adjacent strands together and what does this cause?

Answer 5

Weak hydrogen bonds hold the bases together and make them coil into a double helix structure.

Question 6

What does the 3’ end of a DNA strand have?

Answer 6

A deoxyribose sugar

Question 7

What does the 5’ end of a DNA strand have?

Answer 7

A phosphate

Question 8

What end can nucleotides only be added to?

Answer 8

The 3’ end.

Question 9

How many different nucleotides are there and what causes them to be different?

Answer 9

There are 4 different nucleotides, depending on the base they have.

Question 10

What are the 4 different bases?

Answer 10

Adenine, Cytosine, Guanine and Thymine

Question 11

What do the 2 DNA strands have running in different directions?

Answer 11

Their phosphate backbone.

Question 12

What is the double helix described as having?

Answer 12

The Double helix is described as having two anti-parallel chains of nucleotides because one side goes from 5’ to 3’ and the opposite side goes from 3’ to 5’.

Question 13

What is DNA arranged in?

Answer 13

Tightly coiled chromosomes.

Question 14

What is DNA polymerase and what is its function?

Answer 14

DNA polymerase is an enzyme that controls the formation of the sugar-phosphate bonding of the nucleotides into the DNA strand.

Question 15

What is a primer?

Answer 15

A short sequence of nucleotides at the 3’ end.

Question 16

What strand has to be replicated in fragments?

Answer 16

The lagging strand (5’ end).

Question 17

What is the 5’ end also called and why is this?

Answer 17

The lagging strand because it has to be replicated in fragments and is slower than the 3’ end.

Question 18

What is the 3’ end also called?

Answer 18

The leading strand.

Question 19

What is ligase and what does it do?

Answer 19

Ligase is an enzyme that joins all the DNA fragments together once they are all in place.

Question 20

For DNA replication to occur, what must the nucleus contain?

Answer 20

Primers, DNA (Template), Enzymes(Ligase and DNA Polyemrase), ATP and Nucleotides (the 4 types).

Question 21

Where does DNA replication occur?

Answer 21

In the nucleus.

Question 22

What is the first thing that happens to the DNA molecule in the DNA replication process?

Answer 22

A DNA molecule unwinding.

Question 23

What happens to the DNA molecule after it has unwound in DNA replication?

Answer 23

The hydrogen bonds between the adjacent bases are broken and these bases act as a template for a new DNA strand to be made.

Question 24

What happens after the nucleotides are exposed in DNA replication?

Answer 24

Free-floating nucleotides in the nucleus join on to their complementary bases. This happens simultaneously along the DNA strand.

Question 25

**Which end of the DNA** can **nucleotides be added to**?

Answer 25

The **3’ end.**

Question 26

How do you know which end of a DNA strand is the **5’ end**?

Answer 26

It is **joined** to a **phosphate**.

Question 27

Why is **DNA replication important**?

Answer 27

**DNA replication** is **important** so that **when cells divide** they have to have **the correct genetic information** they need to function properly **and that no information is lost**.

Question 28

**Why** is a **primer needed** in DNA **replication**?

Answer 28

**A primer is needed** in DNA replication because DNA polymerase can **only add nucleotides to a pre-existing chain**.

Question 29

What is the **function** of **the PCR**?

Answer 29

PCR **_amplifies_ DNA**. ( It creates many copies of a piece of DNA in vitro).

Question 30

What does the **amplification of DNA** involve the use of?

Answer 30

The **amplification of DNA involves the use of primers**. Each primer is a short strand of DNA complementary to a specific target sequence at the 3' end of the strand to be replicated.

Question 31

What are the **steps** involved in **PCR**?

Answer 31

* **DNA is heated (94–96 °) to separate**/denature the DNA stands. * I**t is then cooled (50–65 °C)** to allow the primers to bind to the target sequence of DNA. * **Heat tolerant DNA polymerase at 72 °C** then adds nucleotides to the primers (at the 3' end of the original DNA strands). * **Repeated cycles of heating and cooling** amplify the region of DNA. * **The first cycle of replication produces 2 identical; DNA molecules**, the second produces 4 molecules and so on and in an hour a short length can become a million.

Question 32

What is the **first process of PCR** called which is **carried out at 94 - 96 degrees** celsius?

Answer 32

**Denaturation**/separation.

Question 33

What happens in **the first stage of PCR**?

Answer 33

The **DNA is heated** so **it is denatured** and the **weak hydrogen bonds** between the bases **are broken** to leave two single strands.

Question 34

What is the **second process of PCR** called where **the temperature is cooler** at around **68 degrees** celsius?

Answer 34

**Annealing**

Question 35

What is the **last process of PCR** called which is carried out about **72 degrees** celsius?

Answer 35

**Elongation**

Question 36

What are the **practical uses of PCR**?

Answer 36

* **Solve crimes** - if not enough DNA is found to work on it can be amplified so the forensics people have more to work with (DNA profiling). * **Diagnosing genetic disorders** - testing for cystic fibrosis etc. * **Settiling paternity disputes** using (DNA profiling).

Question 37

What is **the role of ligase**?

Answer 37

**Joins fragments of DNA at the 5' end** strand/ lagging strand.

Question 38

What is the **role of DNA polymerase**?

Answer 38

**Adds nucleotides to the 3' end** of a strand when there is a pre-existing chain and primer.

Question 39

The 5' end is replicated in fragments, whilst the 3' end strand is replicated continuously. **Explain why the strands are replicated in different ways**?

Answer 39

DNA **polymerase can only add nucleotides at _the 3' end_ of a growing strand**.

Question 40

What happens to the **primers** in the **second stage of PCR**?

Answer 40

The **primers bind** or anell **to the target sequence of DNA**.

Question 41

Why is **the temperature raised again** in the **third process of PCR**?

Answer 41

So **the temperature** is **closer to the optimum temperature** for DNA polymerase **to add nucleotides** to the primers.