Unit 4: DNA

DNA

Chromosomes

Chromosomes

are DNA wrapped around proteins called histones

Chromosomes are found

in the nucleus

Sections of DNA

Genes.

Identical copies of chromosomes

Chromatids.

Centromeres

hold sister chromatids together

Erwin Chargaff (1950)

A chemist who developed Chargaff’s Rules

Erwin Chargaff stated and proved

that the bases of DNA were paired together in a specific order

How does each base pair?

Adenine (A) pairs withThymine (T): A=T, T=A
ii. Cytosine (C) pairs with Guanine (G): C=G, G=C

If you know the amount of one of the bases, you can figure out all four using Chargaff’s Rules

Chargaff’s Rules

If you know that Adenine is 16% of the sample, then

there is 16% Thymine (A=T). Together, Adenine and Thymine make 32% of the total sample. According to Chargaff’s rules, knowing that 32% of the sample is Adenine and Thymine, we know how much Cytosine and Guanine is there by simply subtracting 100-32 and we are left with 68% of both remaining bases. By dividing (68/2) we figure out that there is 34% Cytosine and 34% Guanine.

Rosalind Franklin (1951)

A British female scientist who made a huge contribution to the study of DNA by establishing that it existed in a spiral three-dimensional shape with the use of X-ray diffraction.

Famous Photo-51

discovered/taken in 1952 most famous x-ray image of DNA that took 100 hours to get the image and calculations to analyze took 1 year

How did Photo-51 get leaked and what happened due to that.

Watson and Crick saw this photograph on her desk while attending one of her seminars and built structures of DNA based on this famous photo

The shape of DNA

a double helix, like a twisted ladder.

James Watson and Francis Crick (1953)

They created the first accurate DNA model with the help of Chargaff and Franklin’s research

What was the structure of the model.

The structure was a double helix with the sides consisting of subunits called nucleotides

A DNA nucleotide consists of

sugar- Deoxyribose, a phosphate, and a nitrogen base

The sides of the double helix

the alternating sugar and phosphate “backbone”.

The center of steps of the the “ladder” (DNA)

paired nitrogen bases.

What do each nitrogen base pair with?

Adenine always pairs with Thymine
Cytosine always pairs with guanine

What bonds hold the two sides together and why?

Hydrogen Bonds hold the two sides together and is twisted into the Double Helix Shape.

Hydrogen Bond are weak bonds. This is very helpful for DNA replication.

James Watson and Francis Crick were awarded with

the Nobel Prize in 1962 for their DNA model.

Frederick Griffith’s and his Experiment Transformation

Using two varieties of streptococcus, he originally searched for a vaccine for pneumonia. One variety of bacteria had a capsule (like a cell wall) the other did not have a capsule.

1. Injection with live encapsulated bacteria

resulted in the mice contracting pneumonia and dying

2. Injections with live bacteria (with no capsule)

resulted in the mice living, and their immune system destroyed the bacteria

3. Injection with heat

killed encapsulated bacteria, resulted in the mice remaining healthy.

4. Injection with dead encapsulated bacteria and live non capsulated bacteria.

resulted in the mice contracting pneumonia and dying.

The live non-capsulated bacteria gained some sort of dangerous property from the dead encapsulated bacteria.
(Dangerous property being a capsule)

This mysterious component was called “The transformation principle” or rather known today as DNA.

DNA (Deoxyribonucleic Acid)

a nucleic acid that is present in all living cells. This molecules contains all of the instructions that determine the traits a living organism needs to survive.

DNA is made of repeating subunits called

Nucleotides. Each nucleotide can have one of 4 bases Adenine Thymine Guanine Cytosine

5 Facts about DNA

1. Almost every cell in the human body has 2 meters of DNA.
2. DNA gives each organism its unique characteristics, but it functions in the same manner.
3. A DNA nucleotide consists of the sugar deoxyribose, a phosphate and nitrogenous base (Adenine, Thymine, Guanine, Cytosine)
4. DNA is double stranded.
5. DNA is often coiled around proteins called histones.

DNA is coiled into strands, called

chromosomes, which are found in the nucleus of Eukaryotes.

Genes

segments of nucleotides located on the chromosomes that give specific instructions for a single trait

DNA chromosomes in prokaryotes

form loose loops within the cell’s cytoplasm.
As prokaryotes do not have a nucleus, the DNA loop is in a part of the cell called the nucleoid region.

The suffix “oid” means.

“like” meaning it is like a nucleus because that is where the DNA is found.

RNA (Ribonucleic Acid)

It is a mirror copy of a segment of DNA (a gene), which then may be delivered to the ribosome to be converted into a protein.

The order of the bases on the RNA strand determines

the order of amino acids that will build certain protein

Proteins

molecules made up of amino acids and are needed to build and repair body structures. They also regulate body processes. They do work within the cell.

RNA can also make structures, such as

ribosomes and Transfer RNA molecules (tRNA).

The sugar in RNA

ribose

What is replaced in RNA and why?

Thymine is replaced by the nucleotide Uracil. (Uracil has a structure that allows it to leave the nucleus; whereas Thymine cannot leave the nucleus because of its structure.)

When pairing the bases of RNA, you still follow Chargaff’s Rules with an exception.

Cytosine still pairs with Guanine; C=G or G=C but Adenine will now pair with Uracil; A=U

RNA Nucleotide consists of

Sugar (Ribose)
Phosphate
Nitrogenous Base (Uracil, Adenine, Guanine, Cytosine)

RNA unlike DNA is

only a single strand

DNA replication

the process by which DNA makes a copy of itself.

DNA Replication (Step 1)

DNA helicase (enzyme) unwinds the DNA. The junction is called a replication fork.

DNA Replication (Step 2)

DNA polymerase adds the complementary nucleotides and binds the sugars and phosphates. DNA polymerase travels from 3’ to 5’ end. The original DNA is called the template strand.

DNA Replication (Step 3)

DNA polymerase adds complementary nucleotides on the other side of the ladder while traveling in the opposite direction

DNA Replication (Step 4)

One side is the leading strand-it follows the helicase as it unwinds

DNA Replication (Step 5)

The other side is the lagging strand-it’s moving away from the helicase (in the 5’ to 3’ direction).

Replication is called

semi-conservative, because one half of the original strand is always saved, or “conserved”

What is the problem with Replication

it reaches the replication fork, but the helicase is moving in the opposite direction. It stops, and another polymerase binds farther down the chain.

This process creates several fragments, called Okazaki Fragments, that are bound together by DNA ligase

During replication

there are many points along the DNA that are synthesized at the same time (multiple replication forks)
It would take forever to go from one end to the other, it is more efficient to open up several points at one time.

Transcription

the process of making a working copy of an original.

Transcription is the first step

in making a protein.

Transcription beings by

making a mirror copy of a gene on the DNA strand in the nucleus. This is called messenger RNA (mRNA).

It is a duplication of the original “blueprints of the cell”

The mRNA is synthesized

by an enzyme called RNA polymerase in the same manner that DNA is duplicated

Once mRNA is created

it moves through the nuclear membrane to a ribosome to synthesize a specific protein

Transcription (IANS)

1. One of the strands of DNA is used as a template to create a strand of mRNA
2. Requires the enzyme RNA polymerase
3. Transcription always starts at a region called the promoter.
4. Occurs in the nucleus

The mRNA has to be “translated” by

a molecule called transfer RNA (tRNA) (like a translator from English to Spanish or vice versa, the tRNA has to read the code on the mRNA and pair it with the appropriate amino acid it interprets for)

The mRNA is fed

like a ribbon through the ribosome three bases at a time (three bases are called a codon)

The code for one amino acid is

three bases on the RNA. The three bases represent a codon. This is where we have diversity in life

Diversity is in the

differences of sequence of amino acids that combine to make a protein. This pattern comes from the organism’s DNA (million dollar “blueprint”.)

How the tRNA brings amino acids from inside the cell to make the protein (Step 1)

These amino acids come from the foods (protein) that we eat and have been broken down by our digestive system and delivered to the cells through the circulatory and cardiovascular system

(An amino acid is the basic building block of a protein.)

How the tRNA brings amino acids from inside the cell to make the protein (Step 2)

The amino acids chemically combine together when the tRNA matches the three base codes on the mRNA

How the tRNA brings amino acids from inside the cell to make the protein (Step 3)

If the base code on the mRNA is ACG, the complementary codes on the tRNA need to be UGC to deliver the correct amino acid… in this codon’s case it is cysteine.

How the tRNA brings amino acids from inside the cell to make the protein (Step 4)

The growing chain of amino acids becomes the desired protein

The codes that are on the tRNA and complement the mRNA are called

anticodons

Mutations

A change in the nucleotide-base sequence of a gene. DNA molecule or mRNA code for a protein.

They are caused by mutagens.

Codon (IANS)

1. Each 3 bases on mRNA are called codons.
2. Each codon codes for 1 amino acid.

Translation (IANS)

1. RNA travels to the ribosome.
2. Ribosomes use the code on the mRNA to build a chain of amino acids.