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UNIT 7 DNA Structure, Replication, and Protein Synthesis
UNIT 7DNA Structure, Replication, and
Protein Synthesis
10.1 Objectives● Describe the structure of a DNA molecule. ● Describe the relationship between DNA and
genes. ● Describe the rules of base pairing.
We are now familiar with the chromosomes in the cells, how they behave during processes like mitosis and meiosis, where they are contained in cell (nucleus:) Let’s now look at how they relate to the DNA within them. https://www.dnalc.org/resources/3d/07-how-dna-is-packaged-basic.html (1:30)
! In 1953 Watson and Crick proposed that DNA is shaped like a double helix (allows more DNA to fit in cell).
! They used Rosalind Franklin’s X-ray diffraction photo. Rosalind's story
Discovery of DNA
Dr. Franklin’s famous “photo 51” which was ‘borrowed without permission” from her lab.
! It contains two chains connected in the middle like a ladder with rungs.
! This structure means massive amounts of information can fit into a small space, like a nucleus!
Discovery of DNA
Dr. Franklin’s famous “photo 51” which was ‘borrowed without permission” from her lab.
! DNA is a nucleic acid made up of two chains of nucleotides (monomer).
! Nucleotides contain: ! a five-sided sugar ! a phosphate group ! a nitrogenous base
● 4 types of nitrogen bases: 1. Adenine (A) 2. Guanine (G) 3. Cytosine (C) 4. Thymine (T)
● Adenine and Guanine are known as purines. ● They have 2 rings in their structure
● Cytosine and Thymine are known as pyrimidines. ● They have 1 ring in their structure
Purines
Pyrimidines
Adenine
Guanine
Cytosine
Thymine
Phosphate group
Deoxyribose (sugar)
Nitrogen base
● The backbone = sugar and phosphate
● Nitrogenous bases stick out from backbone
● Nucleotides join together in any order (any sequence of bases is possible)
● The 2 strands are held together by hydrogen bonding between the bases.
● Hydrogen bonds are considered weak bonds
Hydrogen bonds
Nucleotide
Sugar-phosphate backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
● Bonds can only form between certain nitrogenous bases….this is called base pairing
● Rules of base pairing: ● Adenine (A) pairs with Thymine (T) ● Guanine (G) pairs with Cytosine (C)
● Therefore….. ● The amount of Adenine in a DNA sequence equals
the amount of Thymine ● The amount of Guanine in a DNA sequence equals
the amount of Cytosine.
•A = T
•G = C
If you analyzed a piece of DNA that had 30% Guanine, can you figure out what percent Thymine it has? How?
What do you remember? 1. What are the 3 components that make up a
nucleotide?
2. How many strands is a DNA molecule composed of?
3. There are four types of bases. What are they?
4. How do the bases pair up in a DNA molecule?
5. What makes up the backbone of the DNA molecule?
6. How are the 2 strands of DNA held together?
Section 2 Objectives● Define DNA replication. ● Apply to rules of base pairing to replicate a
strand of DNA. ● Be able to explain the steps to replication and
the major molecules involved in each step.
● KEY ideas to remember: ● Eukaryotic DNA is located in the nucleus ● DNA molecules are long! ● A DNA molecule is double-stranded
● Each of those strands has the information needed to reconstruct the other half because base pairing is complementary.
DNA Replication
…This is how DNA replicates! Extension Learning: The 5' and 3' mean "five prime" and "three prime", which indicate the carbon numbers in the DNA's sugar backbone. The 5' carbon has a phosphate group attached to it and the 3' carbon a hydroxyl (-OH) group.
*If the top strand were separated, you could use the bottom strand to recreate the top! *because G = C and A =T
● This is called semi-conservative replication because we are conserving (keeping) the original strand and adding a new strand to form the two new strands
● Let's Watch!
● This unique DNA structure is a code for YOU! ● The arrangement of
your nucleotides represents and codes for your genes!
● The structure of DNA makes it easy to duplicate when creating new cells during mitosis and meiosis.
Steps to DNA Replication● Step 1-Replication fork formation: This is
performed by an enzyme known as DNA helicase the double stranded molecule must be “unzipped” into two single strands.
● Step 2-Primer Binding: Once the DNA strands have been separated, a short piece of RNA called a primer binds to the 3' end of the strand. The primer always binds as the starting point for replication. Primers are generated by the enzyme DNA primase.
Reminder: The 5' and 3' mean "five prime" and "three prime”; replication happens in the 5' to 3' direction on the leading strand, and 3’ to 5’ in the lagging strand
● Step 3-Elongation: DNA polymerase is responsible for creating the new strand by a process called elongation where new complementary base pairs are added to the strand.
● Step 4-Termination: Once both strands are formed, the new strand are “proofread” for any errors. Once completed, the parent strand and its complementary DNA strand coils into the familiar double helix shape. In the end, replication produces two DNA molecules, each with one strand from the parent molecule and one new strand.
