Section 3: DNA Replication

Transcription

1 Section 3: DNA Replication Main Idea: Replication- process by which DNA is copied during the cell cycle DNA Polymerase- a group of enzymes that bond the new nucleotides together 1

2 DNA Replication Replication Process: 1. Enzymes begins to unzip the double helix along the chromosome (called origins of replication). -Hydrogen bonds are broken -Original molecule separates and bases are exposed 2

3 DNA Replication Replication Process: 2. Free-floating nucleotides pair (one by one) with the exposed bases on the template strand -DNA Replication is continuous on one strand -DNA Replication is non-continuous on the other strand (more complex: involves formation of many DNA segments that are joined together) 3

4 DNA Replication Replication Process: 3. Two identical molecules of DNA result. -DNA is semiconservative -one strand original and one strand new (copied) 4

5 DNA Replication The replication process has a built-in proofreading function to correct errors If the wrong nucleotide is added, DNA polymerase can detect the error, remove the incorrect nucleotide, and replace it with the correct one 5

6 Transcription -You want to play skeeball, but the skeeball lane only takes tokens -You have quarters, but no tokens -Do you go home in defeat? No, you exchange the quarters for tokens! 6

7 Transcription -In a similar way, your cells cannot make proteins directly from DNA -DNA must first be converted into RNA in a process called transcription 7

8 RNA carries DNA s instructions -Central dogma: information flows in one direction, from DNA to RNA to proteins 8

9 RNA carries DNA s instructions -The central dogma involves three processes: 1. Replication copies DNA 2. Transcription converts DNA into RNA 3. Translation interprets RNA into a string of amino acids, called a polypeptide. -Either a single polypeptide or many polypeptides working together make up a protein 9

10 RNA carries DNA s instructions -Prokaryotic cells: replication, transcription, and translation occur in the cytoplasm at approximately the same time -Eukaryotic cells: replication and transcription occur in the nucleus; translation occurs in the cytoplasm 10

11 RNA carries DNA s instructions -RNA acts as an intermediate link between DNA in the nucleus and protein synthesis in the cytoplasm -You can think of RNA as a temporary copy of DNA that is used and then destroyed 11

12 RNA carries DNA s instructions -RNA, ribonucleic acid, is a chain of nucleotides made of three parts 1. Sugar 2. Phosphate group 3. Nitrogen-containing base 12

13 RNA carries DNA s instructions RNA differs from DNA in three ways: 1. Sugar 2. Base Pairs 3. Number of strands 13

14 RNA carries DNA s instructions RNA verses DNA ribose deoxyribose A, U, C, G A, T, C, G Single-stranded Double-stranded U=Uracil takes the place of T in RNA 14

15 Transcription makes 3 types of RNA -Transcription is the process of copying a sequence of DNA to produce a complementary strand of RNA -During transcription a gene, not an entire chromosome, is transferred into an RNA message 15

16 Transcription makes 3 types of RNA -RNA polymerase: enzymes that bond nucleotides together in a chain to make a new RNA molecule 16

17 The Transcription Process 1. RNA polymerase recognizes the transcription start site of a gene, with the help of other proteins and DNA sequences -A large transcription complex consisting of RNA polymerase and other proteins assembles on the DNA strand and begins to unwind a segment of the DNA molecule, until the two strands separate from each other 17

18 The Transcription Process 2. RNA polymerase, using only one strand of DNA as a template, strings together a complementary strand of RNA nucleotides -RNA base pairing rules: A-U C-G -The growing RNA strand hands freely as it is transcribed and the DNA helix zips back together 18

19 The Transcription Process 3. Once the entire gene has been transcribed, the RNA strand detaches completely from the DNA. -Exactly how RNA polymerase recognizes the end of a transcription unit is complicated. It varies with the type of RNA. 19

20 Transcription makes 3 types of RNA -Transcription produces three major types of RNA molecules -Not all RNA molecules code for proteins, but most play a role in the translation process -Each type of RNA molecule has a unique function 20

21 Transcription makes 3 types of RNA 1. Messenger RNA (mrna): an intermediate message that is translated to form a protein 2. Ribosomal RNA (rrna): forms part of ribosomes, a cell s protein factories 3. Transfer RNA (trna): brings amino acids from the cytoplasm to a ribosome to help make the growing protein 21

22 Transcription makes 3 types of RNA -The RNA strand must be processed before it can exit the nucleus of a eukaryotic cell -This step occurs during or just after transcription *We ll talk more about processing after translation* 22

23 Transcription is similar to replication -The process of transcription and replication share many similarities: *Both are catalyzed by large, complex enzymes (DNA or RNA polymerase) *Both involve unwinding of the DNA double helix *Both involve complementary base pairing to the DNA strand *Both are highly regulated by the cell 23

24 The end results of transcription & replication are different -Replication ensures that each new cell will have one complete set of genetic instructions *It does this by making identical sets of doublestranded chromosomes *This double-strand structure makes DNA especially well suited for long-term storage because it helps protect DNA from being broken down and from potentially harmful interactions with other molecules *Replication only occurs once during each round of the cell cycle because each cell needs to make only one copy of its DNA 24

25 The end results of transcription & replication are different (Transcription) -In contrast, a cell may need hundreds or thousands of copies of certain proteins, or the rrna and trna molecules needed to make proteins -Transcription enables a cell to adjust to changing demands *It does so by making a single-stranded complement of only a segment of DNA and only when that particular segment is needed 25

26 The end results of transcription & replication are different (Transcription) *In addition, many RNA molecules can be transcribed from a single gene at the same time to help produce more protein *Once RNA polymerase has transcribed one portion of a gene and has moved on, another RNA polymerase can attach itself to the beginning of the gene and start the transcription process again *This process can occur over and over again 26