Bio week 8 chapter 9

What is DNA?

DNA (deoxyribonucleic acid) is the genetic material in all living things and makes up chromosomes in cells which contains many genes

Monomers are called nucleotides, a sequence of nucleotides form a gene, multiple genes form full chromosomes somatic animal cells have 46 chromosomes (23 pairs)

What is DNA composed of?

1. Deoxyribose sugar (5 carbon numbered 1’ to 5’ clockwise from oxygen)

2. Phosphate group

3. Nitrogenous base (4 types)

What are the 4 nitrogenous bases of DNA and what are the base pairs?

Cytosine and Thymine pair and are called pyrimidines (single ring)

Adenine and Guanine pair and are called purines (double ring)

A pairs with T (2 hydrogen bonds), C pairs with G (3 hydrogen bonds)

What is the structure of DNA?

The phosphate group of one nucleotide bonds covalently with deoxyribose sugar of the next nucleotide creating a sugar-phosphate backbone

The phosphate group is attached to the 5’ carbon of one nucleotide and 3’ carbon of the next nucleotide (5’ → 3’ strand)

DNA is made up of two strands that are twisted around each other to form a double helix. The strands are anti-parallel (one is flipped) so base pairings can occur. The base pairs are held together by hydrogen bonds

What is the structure of RNA and what are the different types of RNA?

Nucleobases are AUCG, Uracil instead of Thymine

RNA is a single stranded molecule

The types of RNA are Messenger RNA (mRNA), Transfer RNA (tRNA), and ribosomal RNA (rRNA)

How is DNA stored inside the nucleus?

DNA is wrapped around around histone proteins to form nucleosomes

Nucleosomes are connected to each other by a small strand of DNA that is free of histones (like beads on a string)

Nucleosomes go through further coiling to create a chromatin fiber

Chromatin fibers condensed and tightly packed results in chromosomes

What is the Central Dogma of Molecular Biology?

A theory that explains the flow of genetic information within a biological system (DNA → RNA → protein)

1. DNA replication (DNA copies itself to make more DNA)

2. Transcription (DNA is copied to make RNA (mRNA))

3. Translation (mRNA is read to make proteins)

What is semiconservative replication?

Semiconservative replication is the way DNA is copied, where each new DNA molecule contains one original (parental) strand and one newly made strand (using the parental strand as a template)

What happens during the initiation stage of DNA replication?

A set of proteins and enzymes are responsible for initiating the process of DNA replication

Specific nucleotide sequences called origins of replication are used as the starting points of replication. Multiple OOR are found along each double-stranded chromosome

Enzyme called helicase unwinds the DNA helix at the OOR, creating a Y-shaped structure called replication forks. Two replication forks form, extending in opposite directions

Replication occurs simultaneously at several places in the genome

What happens during the elongation stage of DNA replication?

Once the helix is unwound, an RNA primer sequence is added to start off the elongation process by an enzyme called primase

DNA polymerase adds DNA nucleosides to the 3’ end of the template, starting at the RNA primer. DNA polymerase can only synthesize DNA in the 5’ to 3’ direction

Primer sequence is removed later, and replaced with DNA nucleotides that complement the template, done by an enzyme called ligase

One DNA strand is created continuously towards the replication fork – this is referred to as the leading strand

The other strand must be created using Okazaki fragments given that DNA polymerase can only synthesize in 5’ to 3’ direction – this is called the lagging strand

What happens during the termination stage of DNA replication?

DNA replication carries on with the leading strand until we get to the end of the length of the chromosome in that direction

Exonuclease removes RNA primers, which are replaced by DNA nucleotides

Given that DNA polymerase only adds to the 3’ end, there is a segment of the lagging strand that is left without its base pairings

Ends of linear chromosomes are called telomeres, which have repetitive sequences that do not code for any genes

Due to the issue with the lagging strand, over time through multiple rounds of DNA replication, telomeres become shortened in length. Adult somatic cells continue to experience telomere shortening, which is associated with aging

What is telomerase?

An enzyme called telomerase assists with overcoming the telomere shortening problem in some cells (sex cells, adult stem cells and some cancer cells)

Telomerase consists of an RNA sequence, used to extend the 3’ overhang of the template strand; process is repeated several times

DNA polymerase is used to synthesize the rest of the complementary strand

What are the DNA Replication Machinery enzymes?

1. DNA Polymerase 3

2. Helicase

3. Primase

4. Ligase

5. Exonuclease

What is the function of DNA Polymerase 3?

To add nucleoside triphosphates to the 3’ end of the nucleotide on the new strand

What is the function of Helicase?

To break hydrogen bonds at the origin of replication, creating a replication fork

What is the function of Primase?

To add RNA primers allowing for replication

What is the function of Ligase?

To form phosphodiester bonds filling in any gaps between the Okazaki fragments

What is the function of Exonuclease?

To remove RNA primers

What is a nucleoside?

A nucleoside consists of a nitrogenous base attached to a sugar, without a phosphate group