DNA structure and replication molecular biology

1. DNA STRUCTURE
& FUNCTION

2. DNA
STRUCTURE
&
FUNCTION

4. DNA stands for...
DeoxyriboNucleic Acid

5. DNA is the genetic information in every cell
DNA holds the instructions for making proteins
The sequence of nitrogen bases in DNA determines the traits
of an organism

6. What are genes?
Genes are units of heredity
passed from parents to
offspring
A gene is a section of a DNA
molecule that codes for a
specific trait (protein)

8. -James Watson and Frances Crick (1953) published a paper
describing DNA as a double helix, or a twisted ladder. This
discovery won them a Nobel prize.
-Rosalind Franklin did much of the research, but was not given
credit because she was a woman. She later died due to the
radiation exposure from her research .

12. DNA
In cells, DNA (Deoxyribonucleic acid)
is the nucleic acid that functions as
the original blueprint for the synthesis
of proteins. DNA contains the sugar
deoxyribose, phosphates and a
unique sequence of the nitrogenous
bases adenine (A), guanine (G),
cytosine (C) and thymine (T).

14. Brief Insight into the Structure and Composition of DNA
The DNA molecules contain instructions a living entity
requires to grow, develop and reproduce. These
instructions are present inside each cell and are
inherited from the parents to their offspring.
It is made up of nucleotides which contain a
nitrogenous group, a phosphate group, and a sugar
group. The order of the nitrogenous bases – thymine(T),
guanine(G), cytosine(C), and adenine(A), is crucial in
determining the genetic code.
Genes are formed by the order of the nitrogenous
bases present in the DNA which is crucial for protein
synthesis. RNA is another nucleic acid that translates
genetic information into proteins from DNA.

15. The nucleotides are linked together for the formation of
two long strands which spiral to produce a structure
known as the double-helix which resembles that of a
ladder wherein the sugar and phosphate molecules
form the sides while the rungs are formed by the bases.
The bases located on one strand pair up with the bases
on the other strand, as in – guanine pairs
with cytosine and adenine pairs with thymine.
The DNA molecules are extremely long and hence
without the right packaging, they cannot fit into cells.
Thus, DNA is tightly coiled to produce formations
referred to as chromosomes. Every chromosome has a
single DNA molecule. In humans, there are 23 pairs of
chromosomes that are present within the nucleus of the
cells.

16. Types Of DNA
A-DNA: It is found at a relative humidity of 75%. In an environment where there is a
higher salt concentration or ionic concentrations, such as K+, Na+, Cs+ or in a state of
dehydration it endures in a form that contains 11 nucleotide pairs with a rise of
2.56Å vertically per base pair. It has the broadest helical diameter amongst all DNA
forms – 23Å DNA which is a typical helix that is right-handed with a rotation of 32.70 per
base pair.
B-DNA: The most common form, present in most DNA at neutral pH and
physiological salt concentrations, is B-form. It has 10 base pairs per turn from the helix
axis. There is a distance of 3.4Å with a helical diameter of 20Å. Watson-Crick’s double
helix model is defined as a B-form of DNA.
C-DNA: It is observed at a relative humidity of 66% and in the occupancy of a few
ions such as Lithium(Li+). It closely has 9.33 base pairs for every turn. The diameter of the
helix is about 19Å and the vertical rise for every base pair for the right-handed helix is
3.320.
D-DNA: It is observed rarely as an extreme variant. The 8 base pairs are titled
negatively from the helix axis with an axial rise of about 3.03Å
Z-DNA: It is found in an environment with a very high salt concentration. Unlike the
A, B, and C types of DNA, it is a left-handed helical structure. The backbone is arranged in
a zig-zag pattern formed by the sugar-phosphate linkage wherein the recurrent
monomer is the dinucleotide in contrast to the mononucleotide, which is observed in
alternate forms.

18. RNA
Ribonucleic acid (RNA) is a nucleic acid
which is directly involved in protein
synthesis. Ribonucleic acid is an
important nucleotide with long chains
of nucleic acid present in all living cells.
Its main role is to act as a messenger
conveying instructions from DNA for
controlling protein synthesis.
RNA contains the sugar ribose,
phosphates, and the nitrogenous bases
adenine (A), guanine (G), cytosine
(C), and uracil (U). DNA and RNA share
the nitrogenous bases A, G, and C.
Thymine is usually only present in DNA
and uracil is usually only present in
RNA.

20. Types Of RNA
Only some of the genes in cells are expressed into RNA. The following are the types
of RNA wherein each type is encoded by its own type of gene:
tRNA– The transfer RNA or the tRNA carries amino acids to ribosomes while
translation
mRNA – The messenger RNA or the mRNA encodes amino acid sequences of a
polypeptide
rRNA – The ribosomal RNA or the rRNA produces ribosomes with the ribosomal
proteins that are organelles responsible for the translation of the mRNA.
snRNA – The small nuclear RNA forms the complexes along with proteins which
are utilized in RNA processing in the eukaryotes.

22. Role Of Protein
To place these ideas in the proper context, remember that some proteins are
enzymes that aid cells by catalyzing chemical reactions. These chemical
reactions occur after the enzyme binds its substrate at the enzyme’s active site.
The enzyme’s active site matches the substrate molecule in size, shape and
chemical properties.
The size, shape, and chemical properties of an enzyme’s active site are due to
the combination of the enzyme’s amino acids, which are the individual
subunits of the enzyme. For the cell to reliably make an enzyme, the cell must
be able to control the placement of amino acids in a protein during the
synthesis of enzymes.
Proteins play a critical role in how cells successfully meet the challenges of
living. Cells use proteins to maintain their shape and to speed up important
chemical reactions such as photosynthesis and respiration.
A cell will not live long if it cannot reliably create the proteins that it needs for
survival.

23. What is the composition of DNA and RNA?
DNA and RNA are nearly identical polymers of nucleotides, except for the base
pairs. DNA contains thymine while the same is substituted with uracil in RNA.
Where are DNA and RNA found?
DNA is located in the nucleus of a cell and in the mitochondria. Meanwhile, RNA is
found in the cytoplasm, nucleus, and also in ribosomes.
How does propagation occur in DNA and RNA?
DNA is capable of self-replication but RNA cannot self-replicate instead, it is
synthesized from DNA (DNA transcription) when required.
What is the similarity between DNA and RNA?
Three out of the four nitrogenous bases in DNA and RNA are the same (cytosine,
adenine, guanine). They both possess a phosphate backbone to which the bases
attach.
Why is DNA a better genetic material than RNA?
The deoxyribose sugar of DNA contains one less oxygen-containing hydroxyl group.
DNA is a more stable nucleic acid. RNA, on the other hand, contains ribose sugar
and is more reactive than DNA. Therefore, DNA is a better genetic material than
RNA.

24. Nucleotide = monomer
Biomolecule: Nucleic Acid
Monomer: Nucleotide

25. Whatisanucleotide?
EachDNA nucleotideincludes:
1. deoxyribosesugar
2. phosphate
3. Oneof 4nitrogenousbases:
Adenine(A)
Thymine (T)
Guanine(G)
Cytosine(C)

26. DNA song
(tune of row, row, row your boat)
We love DNA
Made of nucleotides
Sugar, phosphate, and a base
Bonded on the side!

27. In the “ladder” of DNA:
1. alternating sugars and phosphates are the sides of
the ladder (sugar-phosphate backbone)
2. nitrogenous base pairs are the steps/rungs of the
ladder.

28. Antiparallel
Antiparallel: thestrandsaresideby
sidebutruninoppositedirections.
Always5’to3’direction

29. What holds together the two strands?
Hydrogen bonding between the base pairs holds the
two strands of a DNA molecule together

30. Complementary Base-Pairing
Adenine (A) = (T) Thymine (Apple Tree)
Cytosine (C) = (G) Guanine (Chewing Gum)

31. Practice
Fill in the complementary DNA strand
Notice: How many Adenines are present on both strands?
How many Thymines are present on both strands?
A A T A C T G C G G
G T

32. Chargaff’s Rule
Chargaff found that for any
DNA sample:
% adenine = % thymine
% cytosine = % guanine
Practice: If the DNA molecule is 20%
Thymine, what % of the molecule is
Cytosine?
Answer:
20% T = 20% A (40% total)
30% G = 30% C (60% total)
60 + 40 = 100% of molecule

34. RNA base
pairing:
A=U
G=C

35. Stop here!

36. DNA Replication

37. What is DNA replication?
During replication, DNA
is copied exactly, making
two identical molecules
of DNA

38. Where does replication happen?
eukaryotic cells: takes place
in the nucleus
-plants, animals, fungi,
protists
prokaryotic cells: takes place
in the cytoplasm
-no nucleus
-bacteria only

40. Process of DNA Replication
1. The parent DNA is “unzipped” by Helicase (-ase=enzyme),
breaking the hydrogen bonds between the two strands

41. Process of DNA Replication
2. The two parent strands of DNA are used as templates for
building two new strands of DNA .
Polymerase (-ase=enzyme), adds nucleotides according
to base-pairing rules.

42. Process of DNA Replication
3. The sugar-phosphate backbones of the new DNA
strands are connected. Finished!

43. Functions of DNA Polymerase
1. Adds nucleotides to the
new strands of DNA
according to base-pairing
rules
2. Proof reads the new
strands of DNA and
corrects any errors

44. Semi-conservative
DNA replication produces two DNA molecules that are
identical to the original DNA molecule
Each new DNA molecule has one parent strand and one
daughter strand