M6 MOLECULAR BIOLOGY Elongation- an enzyme called DNA polymerase adds DNA
nucleotides to the 3’ end of the template. Since this enzyme can only
6.1 THE STRUCTURE OF DNA add new nucleotides at the end of a backbone, a primer sequence,
which provides this strating point is added with complementary RNA
DNA- genetic material passed from parent to offspring for all life on nucleotides. The primer is then removed later and the nucleotides
Earth. are replaced with DNA nucleotides. The strand complementary to
the parental DNA strand is synthesized continuously to the
1. DNA- nucleotides with three parts: deoxyribose, phosphate group replication fork so the polymerase can add nucleotides in this
and a nitrogenous base (Adenine, Guanine are double-ringed direction. The continuously synthesized strand is called the leading
purines; Cytosine and Thymine are single-ringed pyrimidines) strand. Because DNA polymerase can only synthesize DNA in a 5’-3’
The phosphate group of one nucleotide covalently bonds with the direction, the other new strand is out together in short pieces called
sugar molecule of the next nucleotide and so one forming a long Okazaki fragments which require a primer made of RNA to start
polymer of nucleotide monomers. The sugar-phosphate groups line synthesis. The strand with the fragments is known as the lagging
up in a backbone for each single strand of DNA and the bases stick strand. As synthesis proceeds, the DNA ligase removes the RNA
out from this backbone. The phosphate group is attached to the 5’ primer and is replaced with DNA nucleotides while sealing the gaps
carbon of one nucleotide and the 3’ carbon of the next. DNA is between fragments.
double-helixed held together by hydrogen bonds between the
bases. 1. DNA unwinds at the origin of replication.
2. New bases are added to the complementary parental strands.
Base pairing occurs between a purine and pyrimidine (A w T, C w G) Leading strand is made continuously while the lagging one is made
which are called complementary base pairs. AT connected by 2 H in pieces.
while CG is connected by 3H. The two strands are anti-parallel in 3. Primers are removed and replaced with DNA nucleotides and the
nature. 3’ carbon is in upward position while the other strand will backbone is sealed by DNA ligase.
have 5’ carbon in the upward.
TELOMERE REPLICATION
THE STRUCTURE OF RNA Telomeres- ends of the linear chromosome which needs replication
Has the same composition as DNA but is single stranded with because the ends of the lagging strand remain unpaired and may
nitrogenous base as ribose. Ribose has a hydroxyl group at the 2’ become shorter as cells continue to divide. They have repetitive
carbon, unlike deoxyribose which has only a hydrogen atom. sequences that do not code for a particular gene. It is the telomeres
Nitrogenous bases are CGA with Uracil replacing T. that get shortened during DNA replication and not the genes itself.
Telomerase- attaches to the end of the chromosome, and
HOW DNA IS ARRANGED IN THE CELL complementary bases to the RNA template are added on the end of
DNA molecule of a single human cell would come to a length of the DNA strand. Once the lagging strand template is sufficiently
about 2m. that is why DNA is packaged orderly to fit and function elongated, DNA polymerase can now add nucleotides that are
within the cell that is not visible to the naked eye. complementary to the ends of the chromosomes, replicating it.
Prokaryotes- DNA is twisted beyond the double helix in what is DNA REPLICATION IN PROKARYOTES
known as supercoiling. Process is much more rapid than eukaryotes that finishes at approx..
Eukaryotes- DNA is wrapped around proteins known as histones to 42 minutes.
form structures called nucleosomes. The nucleosome is linked to the
next one by a short strand of DNA free of histones. This is also Property Prokaryotes Eukaryotes
known as the bead on a string structure (beads are nucleosomes, Origin of replication Single Multiple
strings are short DNA Rate of replication 1000 nucleotides 50 to 100
Chromosome structure circular linear
Telomerase Not present Present
6.2 DNA REPLICATION
Occurs during the synthesis phase of the cell cycle before entering DNA REPAIR
mitosis or meiosis. The semiconservative model suggests that The DNA polymerase can make mistakes while adding nucleotides.
during replication, the two strands separate and each strand serves Most are corrected during replication but when this does not
as a template from which the new complementary strand is copied. happen, the mismatch repair mechanism is employed. Mismatch
repair enzymes recognize the wrongly incorporated base and excise
DNA REPLICATION IN EUKARYOTES it from the DNA, replacing it with the correct base.
Initiation- DNA is made accessible to the proteins and enzymes Nucleotide excision repair is when the DNA double strand is
involved in the replication process. Origins of replication are specific unwound and separated, the incorrect bases are removed along
nucleotide sequences which is multiple in eukaryotes at which with a few bases on the 5’ and 3’ end, and these are replaced by
replication begins and proteins bind here. The helicase unwinds and copying the template with the help of DNA polymerase. This repair is
opens up the DNA helix. As the DNA opens up, Y-shaped structures important in correcting thymine dimers caused by ultraviolet light
called replication forks are formed. where thymines are covalently bonded in one strand rather than
their complementary base. If not removed, it will lead to mutation.
Mutation- permanent change in the DNA sequence.
nucleotides to the 3’ end of the template. Since this enzyme can only
6.1 THE STRUCTURE OF DNA add new nucleotides at the end of a backbone, a primer sequence,
which provides this strating point is added with complementary RNA
DNA- genetic material passed from parent to offspring for all life on nucleotides. The primer is then removed later and the nucleotides
Earth. are replaced with DNA nucleotides. The strand complementary to
the parental DNA strand is synthesized continuously to the
1. DNA- nucleotides with three parts: deoxyribose, phosphate group replication fork so the polymerase can add nucleotides in this
and a nitrogenous base (Adenine, Guanine are double-ringed direction. The continuously synthesized strand is called the leading
purines; Cytosine and Thymine are single-ringed pyrimidines) strand. Because DNA polymerase can only synthesize DNA in a 5’-3’
The phosphate group of one nucleotide covalently bonds with the direction, the other new strand is out together in short pieces called
sugar molecule of the next nucleotide and so one forming a long Okazaki fragments which require a primer made of RNA to start
polymer of nucleotide monomers. The sugar-phosphate groups line synthesis. The strand with the fragments is known as the lagging
up in a backbone for each single strand of DNA and the bases stick strand. As synthesis proceeds, the DNA ligase removes the RNA
out from this backbone. The phosphate group is attached to the 5’ primer and is replaced with DNA nucleotides while sealing the gaps
carbon of one nucleotide and the 3’ carbon of the next. DNA is between fragments.
double-helixed held together by hydrogen bonds between the
bases. 1. DNA unwinds at the origin of replication.
2. New bases are added to the complementary parental strands.
Base pairing occurs between a purine and pyrimidine (A w T, C w G) Leading strand is made continuously while the lagging one is made
which are called complementary base pairs. AT connected by 2 H in pieces.
while CG is connected by 3H. The two strands are anti-parallel in 3. Primers are removed and replaced with DNA nucleotides and the
nature. 3’ carbon is in upward position while the other strand will backbone is sealed by DNA ligase.
have 5’ carbon in the upward.
TELOMERE REPLICATION
THE STRUCTURE OF RNA Telomeres- ends of the linear chromosome which needs replication
Has the same composition as DNA but is single stranded with because the ends of the lagging strand remain unpaired and may
nitrogenous base as ribose. Ribose has a hydroxyl group at the 2’ become shorter as cells continue to divide. They have repetitive
carbon, unlike deoxyribose which has only a hydrogen atom. sequences that do not code for a particular gene. It is the telomeres
Nitrogenous bases are CGA with Uracil replacing T. that get shortened during DNA replication and not the genes itself.
Telomerase- attaches to the end of the chromosome, and
HOW DNA IS ARRANGED IN THE CELL complementary bases to the RNA template are added on the end of
DNA molecule of a single human cell would come to a length of the DNA strand. Once the lagging strand template is sufficiently
about 2m. that is why DNA is packaged orderly to fit and function elongated, DNA polymerase can now add nucleotides that are
within the cell that is not visible to the naked eye. complementary to the ends of the chromosomes, replicating it.
Prokaryotes- DNA is twisted beyond the double helix in what is DNA REPLICATION IN PROKARYOTES
known as supercoiling. Process is much more rapid than eukaryotes that finishes at approx..
Eukaryotes- DNA is wrapped around proteins known as histones to 42 minutes.
form structures called nucleosomes. The nucleosome is linked to the
next one by a short strand of DNA free of histones. This is also Property Prokaryotes Eukaryotes
known as the bead on a string structure (beads are nucleosomes, Origin of replication Single Multiple
strings are short DNA Rate of replication 1000 nucleotides 50 to 100
Chromosome structure circular linear
Telomerase Not present Present
6.2 DNA REPLICATION
Occurs during the synthesis phase of the cell cycle before entering DNA REPAIR
mitosis or meiosis. The semiconservative model suggests that The DNA polymerase can make mistakes while adding nucleotides.
during replication, the two strands separate and each strand serves Most are corrected during replication but when this does not
as a template from which the new complementary strand is copied. happen, the mismatch repair mechanism is employed. Mismatch
repair enzymes recognize the wrongly incorporated base and excise
DNA REPLICATION IN EUKARYOTES it from the DNA, replacing it with the correct base.
Initiation- DNA is made accessible to the proteins and enzymes Nucleotide excision repair is when the DNA double strand is
involved in the replication process. Origins of replication are specific unwound and separated, the incorrect bases are removed along
nucleotide sequences which is multiple in eukaryotes at which with a few bases on the 5’ and 3’ end, and these are replaced by
replication begins and proteins bind here. The helicase unwinds and copying the template with the help of DNA polymerase. This repair is
opens up the DNA helix. As the DNA opens up, Y-shaped structures important in correcting thymine dimers caused by ultraviolet light
called replication forks are formed. where thymines are covalently bonded in one strand rather than
their complementary base. If not removed, it will lead to mutation.
Mutation- permanent change in the DNA sequence.
