DNA Structure and Replication: A Look into the Double Helix and its Reproduction, Study notes of Crystallography

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DNA Structure and Replica2on

Structure of DNA

| James Watson and Francis

Crick (with Maurice

Wilkins) —awarded the

Nobel Prize in 1962 for the

construc2on of the double

helix model of DNA

| Rosalind Franklin —used X-­‐

ray crystallography to

produce an image of the

double helix model of DNA

DNA—Deoxyribonucleic Acid

|Contains your genes

{Genes are sec2ons of DNA that code for a certain trait (hair color, height, nose shape…)

|Double stranded in the form

of a double helix (twisted

ladder)

|Contains four nitrogen

bases, phosphate and

deoxyribose

DNA Nitrogen Bases

4 Nitrogen Bases

| Purines (2 rings)

{Adenine {Guanine (PUGA2)

| Pyrimidines

(1 ring)

{Thymine {Cytosine

Write the Complimentary DNA Strand

| ATGCGGATACATCCGATCGATGCAATCATGACATA

| TGACGGACACTTACAGATTCTGTGAGTCTCAACGG

| GAGCATTAGCAGGACTGCACGATATTGTCCTAGTA

| GTAGTAGTAGTAGTAGTAGTAGTAGTAGTAGTA

DNA Replication

Definition: process that

copies the entire genome

(all of the DNA) of a cell

In humans, your cells will

replicate 3 billion

nucleotides in 6 hours!

8

Synthesis Phase (S phase)

|S phase during interphase of the

cell cycle

|Nucleus of eukaryotes

10 Mitosis -prophase -metaphase -anaphase -telophase

G

1

G

2

S

phase interphase DNA replication takes place in the S phase.

| Barbara McClintock is credited for the discovery of the replica2on process. | She was awarded the Nobel Prize for her work in 1983. 11

| The two DNA strands open forming Replication Forks (Y-shaped region) 13 Replication Fork Parental DNA Molecule 3 5 3 5

| As the DNA strand opens at the replication forks, Replication Bubbles form | Eukaryotic DNA will have MANY replication bubbles | Prokaryotes (bacteria) have a single bubble 14 Bubble Bubble Bubble

Proofreading New DNA

|DNA polymerase initially makes

about 1 in 10,000 base pairing

errors

|DNA Polymerase proofreads and

corrects these mistakes

|The new error rate for DNA that

has been proofread is 1 in 1 billion

base pairing errors

16

REPLICATION STEP 3

17

• The enzyme Ligase attaches all the replication bubble fragments together.
• The new double helixes are identical- each contains one original template strand and one copied strand
• Replication is semi- conservative since each new DNA molecule is made of one old strand and one new strand.

SUMMARY OF REPLICATION STEPS ONE MORE TIME:

| DNA is unwound with the help of the enzyme Helicase. It separates the hydrogen bonds between the nitrogen bases. | Each original strand of nucleo2des acts as a template (paaern) to make a new chain | The new strand is assembled as DNA polymerase matches free-­‐floa2ng nucleo2des w/complementary nitrogen bases in the replica2on bubble | DNA ligase links together the individual replica2on bubbles. 19

ENZYME FUNCTION

Helicase Unwinds helix by breaking hydrogen bonds between nitrogen bases DNA Polymerase *Builds new strands inside replication bubble by attaching nucleotides to their complementary base *Proofreads for mistakes Ligase Forms bonds between replication bubbles so DNA can return to the helical form. 20