Lecture 11 (DNA structure)

Question 1

Chromosomes consist of both …

Answer 1

Proteins and nucleic acid

Question 2

The Xray diffraction pattern of DNA (photo 51) shows…

Answer 2

Helical structure

Bases perpendicular to the length of the DNA molecule

Double stranded

Question 3

The nucleotides point …

Answer 3

In towards the middle

Question 4

Chargaff discovery

Answer 4

That there is significant DNA variation between species, thus DNA could be the genetic material (not the proteins on the chromosomes)

Question 5

Chargaff’s rules

Answer 5

A=T and G=C

The composition of DNA varies between species

Question 6

What did the discovery of DNA as the genetic material involve?

Answer 6

Many experiments and scientists

Question 7

DNA structure

Answer 7

A nucleic acid molecule, composed of two chains which form a double stranded helix, each chain is made up of four chemical building blocks called nucleotides. DNA carries the genetic instructions for all cellular processes (growth, development, function and reproduction).

Question 8

Nucleotide

Answer 8

The building block of a nucleic acid, consisting of a five carbon sugar covalently bonded to a nitrogenous base and one to three phosphate groups.

Question 9

5 prime phosphate group

Answer 9

On a nucleotide… It is the fifth carbon which has a phosphate group attached to it

Question 10

3 prime hydroxyl group

Answer 10

On a nucleotide…It is the third carbon which has an -OH group attached to it

Question 11

What are the 3’ and 5’ ends in DNA?

Answer 11

A key feature of all nucleic acids is that they have two distinctive ends: the 5’ (5-prime) and 3’ (3-prime) ends. This terminology refers to the 5’ and 3’ carbons on the sugar. For both DNA (shown above) and RNA, the 5’ end bears a phosphate, and the 3’ end a hydroxyl group.

Question 12

Polynucleotide

Answer 12

Nucleotide monomers are joined together with phosphodiester bonds to form a polynucleotide = nucleic acid

Question 13

Formation of a phosphodiester bond

Answer 13

The hydroxyl group on the 3rd carbon of one nucleotide reacts with the phosphate group attached to the 5th carbon on another nucleotide

Question 14

DNA and RNA strands are synthesised in …

Answer 14

The 5’ to 3’ direction

Question 15

The two strands in DNA are….

Answer 15

antiparallel - one strand is 5’ to 3’ and the other strand in 3’ to 5’

Question 16

Two antiparallel DNA stands form a …

Answer 16

double stranded helix

Question 17

A-T pairs have

Answer 17

Two hydrogen bonds

Question 18

G-C pairs have

Answer 18

Three hydrogen bonds (therefore stronger interaction than A-T because G-C pairs have one more bond)

Question 19

How are the two strands of the DNA double helix held together?

Answer 19

Pair of complimentary bases form hydrogen bonds that hold the two strands together

Question 20

Hydrogen bond

Answer 20

The chemical bond that holds two bases together

Question 21

The Watson-Crick model of DNA structure

Answer 21

DNA has a double stranded helical structure

The sugar phosphate backbone is on the outside

The bases are on the inside

Stabilised by hydrogen bonds

The two polynucleotide strands are oriented in opposite direction

Question 22

What did the Watson-Crick model of DNA structure provide?

Answer 22

A stimulus for deciphering the genetic code

A possible mechanism for the replication of DNA

Question 23

What kind of replication does DNA undergo?

Answer 23

Semi-conservative model of replication - each DNA strand of the double helix is used as a template strand for the synthesis of two new strands. You end up with two identical DNA molecule

Question 24

Why does your cell replicate its DNA?

Answer 24

Because you body needs to make more cells constantly therefore DNA replication needs to occur at all times

You make more cells - in pregnancy, when repairing after injury, at earlier stages in life (baby-puberty)

Question 25

What are the building blocks of DNA? What are the three components of these building blocks?

Answer 25

Building blocks of DNA are nucleotides

Nucleotides are made up of a base, sugar and a phosphate.

Question 26

There are two types of bases - what are they called and how many ‘rings’ are in each?

Answer 26

Pyrimidines consist of one ring and include cytosine and thymine

Purines consist of two rings and includes adenine and guanine

Question 27

How does the sugar component in DNA and RNA differ?

Answer 27

DNA contains the sugar deoxyribose, while RNA contains the sugar ribose. The only difference between ribose and deoxyribose is that ribose has one more -OH group than deoxyribose, which has -H attached to the second (2’) carbon in the ring.

Question 28

How many polynucleotide chains make up a DNA molecule and how are these organised in relation to each other?

Answer 28

2 chains make up a DNA molecule and they run antiparallel to one another

Question 29

How is the DNA helix stabilised?

Answer 29

Hydrogen bonds

Question 30

Which nucleotide component(s) are located on the inside of the DNA helix?

Answer 30

Bases

Question 31

Which nucleotide component(s) are located on the outside of the DNA helix?

Answer 31

Sugar-phosphate back bone (sugar and phosphate part of the nucleotide)

Question 32

Does semi-conservative DNA replication allow genetic information to be passed onto newly synthesised cells and the next generation? Explain

Answer 32

The structure of the DNA double helix and selfpropagation of DNA through semiconservative replication allows for storage and maintenance of both normal and abnormal genetic information, and transmission to daughter cells somatically and to the next generation through the germline.