1. Chapter 12
Molecular Genetics

2. The structure of DNA and DNA replication

3. DNA - Deoxyribonucleic Acid
To understand genetics, scientists had to learn the chemical makeup of the gene.
Scientists discovered that genes are made of DNA
Scientists also found that DNA stores and transmits the genetic information from one generation of an organism to the next.
Scientists began studying DNA structure to find out how it carries info, decides traits, and replicates itself.

4. DNA – The structure
DNA is a long molecule made up of units called nucleotides.
Each nucleotide contains a 5-carbon sugar, a phosphate group, and a nitrogen-containing base.
There are four kinds of bases in DNA:
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)

5. The Nucleotides
Green = 5-C sugar
Peach = phosphate

6. Watson and Crick Watson and Crick made a 3-D model of DNA.
Their model was a double helix, in which two strands are wound around each other.
A double helix is like a twisted ladder
Sugar and phosphates make up the side of the ladder
Hydrogen bonds between the bases hold the strands together (the rungs)

7. DNA

8. A always pairs with T G always pairs with C Chargaff’s rule
Aka the “base pairing rule”
Chargaff’s rule states that bonds only form between certain base pairs
A always pairs with T
G always pairs with C
You must memorize this!!!!

9. Base-pairing

10. Where can you find dna?
Most prokaryotes have one large DNA molecule floating in the cytoplasm.
Eukaryotes have DNA in the nucleus.
Amazing fact:
The nucleus of a human cell contains more than 1 meter of DNA!

11. Chromosomes
Contain both DNA and protein, which are tightly packed together to form chromatin.
Chromatin consists of DNA that is tightly coiled around proteins called histones.
This forms a beadlike structure called a nucleosome.
Nucleosomes pack with one another to form a thick fiber, which is shortened by a system of loops and coils.
Nucleosomes allow enormous amounts of DNA to fit into such a small region.

14. DNA Replication
Before a cell divides, it copies its DNA in a process called replication.
During replication:
The DNA molecule separates into 2 strands – each strand of the DNA molecule serves as a model for the new strand.
Following the rules for base pairing, new bases are added to each strand.
The end result is two identical strands.

15. How does replication occur?
Its carried out by a series of enzymes
Some enzymes “unzip” the molecule of DNA – the hydrogen bonds between bases are broken
The two strands unwind and serve as templates to create two new strands
The key enzyme is called DNA polymerase – joins individual nucleotides to produce a DNA molecule
DNA polymerase is also involved in “proofreading” each of the new DNA strands to minimize errors.

16. DNA Replication

17. Let’s Replicate!
Original Strand = T A C G T T New Strand = Original Strand = ATCGGCAATCACGAT New Stand = A T G C A A
TAGCCGTTAGTGCTA

18. RNA and Protein Synthesis

19. Cookbook Story

20. RNA – The basics
For a gene to work, the genetic instructions in the DNA molecule must be decoded.
The first step is to copy the DNA sequence into RNA.
RNA is a molecule which contains instructions for making proteins.

21. RNA – The structure RNA is similar to DNA, except for:
RNA contains the sugar ribose
DNA has deoxyribose
RNA is single-stranded
DNA is double stranded
RNA has uracil in place of thymine – A, U, C, G
DNA has A, T, C, G

23. RNA Molecules – what do they do?
There are 3 kinds of RNA –
Messenger RNA (mRNA)
What is this in the story?
Has the instructions for joining amino acids to protein
Ribosomal RNA (rRNA)
Assembles the proteins
Transfer RNA (tRNA)
Carries each amino acid to the ribosome according to the coded message in mRNA
Contains an anticodon to pair up with mRNA
Cookbook
Cook
Translator

25. Transcription Process of making (or copying) RNA from DNA
Occurs in the nucleus
During transcription:
DNA strands are separated
RNA strand is built using one DNA strand as a template
DNA is transcribed (written) into RNA following base-pairing rules except that U binds to A
AU, TA
DNA Strand = TACGCTACGCCTAATACT
New mRNA Strand =
AUGCGAUGCGGAUUAUGA

26. Transcription to translation
The directions for making proteins are in the order of the four nitrogenous bases
This code is read 3 letters at a time
Each grouping of 3 letters is called a codon
AUG/CGA/UGC/GGA/UUA/UGA
Each codon stands for one amino acid
There is one “start” codon and 3 “stop” codons
Start = AUG
Stop = UGA, UAA, UAG

27. Translation
Process in which the cell uses info from mRNA to make proteins
Takes place in the ribosome
How does it work?
mRNA moves into the cytoplasm and attaches to ribosome
As each codon of the mRNA moves through the ribosome, the proper amino acid is brought into the ribosome by tRNA.
The ribosome joins together each amino acid and the protein chain grows.
When the ribosome reaches a stop codon, the polypeptide chain is released.

28. How do we figure out the amino acid?
Use a Codon Chart

30. Let’s do an example! DNA Strand = TACGCTACGCCTAATACT
mRNA Strand = AUGCGAUGCGGAUUAUGA
Split into codons = AUG/CGA/UGC/GGA/UUA/UGA
Create Protein =
Methionine -
Arginine -
Cysteine -
Gylcine -
Leucine

31. Let’s Review!

32. Mutations Mutations are mistakes made when cells copy their own DNA.
Mutations are changes in the genetic material of a cell.
There are two types of mutations:
Gene mutations – changes in a single gene
Chromosomal mutations – change in the number of structure of chromosomes and affect multiple genes

33. Gene Mutations Point mutations (substitution)–
Occur at a single point in the DNA sequence
Causes one base to replace another, which only affects one amino acid
Frameshift mutations (insertion, deletion) –
A base is added or removed
All the codons are affected/changed resulting in a completely different protein

34. Chromosome mutations Change in structure of chromosomes Four types:
Deletions = ABC-DEF  AC-DEF
Duplications = ABC-DEF  ABBC-DEF
Inversions = ABC-DEF  AED-CBF
Translocations = ABC-DEF  ABC-JKL GH-IJKL GH-IDEF