3.1 & 7.13.4 & 7.2

Genetic information is stored in molecules called nucleic acids.

There are 2 types of nucleic acids

DNA: deoxyribonucleic acid◦ Double stranded

RNA: ribonucleic acid◦ Single stranded

Nucleotides are the building blocks of nucleic acids

A single nucleotide consists of:◦ A pentose sugar◦ A phosphate group◦ A nitrogenous base

Components are held together with covalent bonds

Also known as an organic base, or a nitrogen base

5 different bases:◦ Adenine (A)◦ Cytosine (C)◦ Guanine (G)◦ Thymine (T) – only found in DNA◦ Uracil (U) – only found in RNA

Nitrogen Bases with a double ring structure Adenine and Guanine

Nitrogen bases with a single ring structure Cytosine, Thymine, and Uracil

In RNA, the sugar is “ribose” In DNA, the sugar is “deoxyribose”

(The difference is the presence or lack of oxygen on the 2nd carbon)

In a single nucleotide, the 1st carbon of the pentose sugar is covalently bonded to the nitrogenous base.

The 5th carbon of the pentose sugar is covalently bonded to the phosphate group

The 3rd carbon of the pentose sugar is covalently bonded to the phosphate group of the next nucleotide in the chain. This bond is called a phosphodiester bond.

Remember DNA, is double stranded

The 2 strands of DNA are complimentary

The nitrogen bases of 2 complimentary nucleotides hydrogen bond to each other to create the double strand

Adenine always bonds to Thymine and Cytosine always bonds to Guanine

Covalent Bond

Covalent Bond

Phosphodiester bond (Covalent Bond)

Hydrogen Bond

There are 2 H bonds between Adenine and Thymine

There are 3 H bonds between Guanine and Cytosine

How many would you expect uracil to make with a potential nucleotide?

DNA forms a double helix

The helix is created by H-bonds between non-consecutive nucleotides

The 2 DNA strands will each have a phosphate at the end of one strand, and a sugar at the opposite end.

The end that has a phosphate is referred to as the “5 prime end” (5’)

The end that has a sugar is referred to as the “3 prime end” (3’)

The 2 strands are ANTIPARALLEL (because their 3’ and 5’ terminals are at opposite ends)

DNA is extremely long

If you took the DNA of a single cell and stretched it out into one long double helix, it would measure 1.8 in length

If fits into a cell because it is very tightly packed – which also keeps it organized!

Just like thread is spun around a spool to keep it organized, DNA is coiled around a group of eight proteins called histones.

The complex of histones and DNA is called a NUCLEOSOME

It takes 200 nuleotides to form a nucleosome

The histone are positive, the DNA is negative – so they are strongly attracted!

Histone proteins:

◦ 8 histone proteins (4 types, 2 of each type) inside each nucleosome

◦ 1 histone protein outside each nucleosome, which functions to organize and hold the nucleosome together

A series of nucleosomes coil into chromatin fibres

The chromatin fibres then coil to form a supercoil

The supercoiled chromatin is what makes up a chromosome

A chromosome is one unbroken double-stranded DNA helix

Not only do nucleosomes keep DNA organized, they also prevent trancription

Transcription is when DNA is used as a template to produce an RNA strand. For this to occur, the enzyme RNA polymerase must attach to the 3’ end of a DNA strand.

When DNA is organized in a nucleosome, the promoter region is inaccessble so transcription cannot take place

When the cell requires transcription, enzymes will alter the shape of the nucleosome to allow RNA polymerase to attach.

DNA contains genetic information

But in actualilty only a small portion of DNA constitutes genes

“Unique genes” / “Single-Copy genes” / Codable Genes” make up 1.5% of human genetic material.

These are the genes that carry out genetic information.

The remainder (and majority) of DNA are “repetitive sequences” that have no known function (non-coding regions)

Since repetitive sequence vary from person to person, they are useful in DNA profiling, which allows for DNA fingerprinting to identify sample from individuals.

In eukaryotic cells, many genes are discontinuous. A single gene is interrupted with a long non-coding sequence.

EXON – the coding sequence INTRON – intervening, non-coding sequence

For life to perpetuate, cell must replicate (undergo cell division – mitosis and cytokinesis)

Before mitosis can occur, the DNA in the nucleus must duplicate

DNA replication is semiconservative

The parent double helix produces 2 daughter double helices.

Each daughter molecule will have a parental strand and a daughter strand (an old strand and a new strand)

The new strand is made up of “free floating nucleotides” or deoxyribonucleoside triphosphates that are found in the nucleus.

1. The enzyme DNA helicase unwinds the double helix and separates the complimentary strands by breaking the hydrogen bonds between them.

Single-stranded binding protein (SSBs) prevent the complimentary nitrogen bases from reforming their hydrogen bonds.

DNA gyrase relieve tension produced by the unwinding of DNA

DNA will replicate small segments of the larger strand at a time.

So only small segments will be unwound and separated by helicase at any give time.

These segments are called replication bubbles.

The junction where the 2 strands are still attached is called the replication fork

2. The enzyme primase creates an RNA primer – which is 10 - 60 RNA nucleotides.

The RNA primer temporarily attaches to the 3’ end of a DNA strand.

The purpose of the primer is to create a starting point for the DNA nucleotides to attach

3. Starting from the RNA primer, the enzyme DNA polymerase III adds free floating deoxyribonucloside triphophates to the DNA strand using complimentary base pairing rules.

The original parent strand acts as a template

DNA is always synthesized in the 5’ -3’ direction .

This means: a nucleoside that is being added will bond its phosphate group (at the 5’ end) to a nucleoside that is already apart of the strand.

The next nucleoside will bond to the 3’ end of the previous nucleotide with its 5’ end.

The strand that is built continuously and in the same direction of helicase is the leading strand

The other strand is called the lagging strand.

It is synthesized discontinuously in the direction away from the replication fork and in the opposite direction of helicase.

As a result, short fragments (1000-2000 nucleotides in length) are produced called Okazaki fragments

(at the beginning of each Okazaki fragment there will be a RNA primer)

4. The enzyme DNA polymerase I removes the RNA primers from both the leading and lagging strands and replaces them with the appropriate DNA nucleotides.

5. Enzyme DNA ligase will attach the Okazaki fragments of the lagging strand together.

6. As the 2 new double strands of DNA are made, they will automatically twist into a helix.

DNA polymerase I and III “proofread” the newly created strands checking for mistakes.

If there is a mistake, the enzymes act as an exonuclease

It removes the incorrect nucleotide and replaces it with the correct one.

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(* show Replication Fork first)