DNA Replication

How DNA Makes Copies of Itself

Before a cell divides, its DNA is replicated (duplicated.) Because the two strands of a DNA molecule have complementary base pairs, the nucleotide sequence of each strand automatically supplies the information needed to produce its partner. If the two strands of a DNA molecule are separated, each can be used as a pattern or template to produce a complementary strand. Each template and its new complement together then form a new DNA double helix, identical to the original.

Before replication can occur, the length of the DNA double helix about to be copied must be unwound. In addition, the two strands must be separated, much like the two sides of a zipper, by breaking the weak hydrogen bonds that link the paired bases. Once the DNA strands have been unwound, they must be held apart to expose the bases so that new nucleotide partners can hydrogen-bond to them.

The enzyme DNA polymerase then moves along the exposed DNA strand, joining newly arrived nucleotides into a new DNA strand that is complementary to the template. Figure 1 shows the process part way through.

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Introduction (from Wikipedia):

Cell division is essential for an organism to grow, but when a cell divides it must replicate the DNA in its genome so that the two daughter cells have the same genetic information as their parent.DNA provides a simple mechanism for replication.

In transcription, the codons of a gene are copied into messenger RNA by RNA polymerase

Base Pairing: Since there are 4 bases in 3-letter combinations, there are 64 possible codons (43 combinations).

This enzyme makes thecomplementary strand by finding the correct base throughcomplementary base pairing, and bonding it onto the original strand.

Purpose: The purpose of replication is to conserve the entire genome for next generation.

The purpose of transcription is to make RNA copies of individual genes that the cell can use in the biochemistry.

Codons: These encode the twenty standard amino acids, giving most amino acids

DNA polymerases can only extend a DNA strand in a 5 to 3′ ′

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more than one possible codon. There are also three 'stop' or 'nonsense' codons signifying the end of the coding region; these are the UAA, UAG and UGA codons.

direction, different mechanisms are used to copy the antiparallel strands of the double helix. In this way, the base on the old strand dictates which base appears on the new strand.

Result: In replication, the end result is two daughter cells.

while in transcription, the end result is a protein molecule.

Product: Replication is the duplication of two-strands of DNA.

Transcription is the formation of single, identical RNA from the two-stranded DNA.

Enzymes: The two strands are separated and then each strand's complementaryDNA sequence is recreated by anenzyme called DNA polymerase.

In transcription, the codons of a gene are copied into messenger RNA by RNA polymerase.This RNA copy is then decoded by a ribosome that reads the RNA sequence by base-pairing the messenger RNA to transfer RNA, which carries amino acids.

Definition: DNA replication is the replication of a strand of DNA into two daughter strands, each daughter strand contains half of the original DNA double helix.

Transcription is the synthesis of RNA from a DNA template. The structure of DNA is not altered as a result of this process

EnzymesRequired: DNA Helicase, DNA Polymerase. Transcriptase (type of DNA Helicase), RNA polymerase.

Products: One strand of DNA becomes 2 daughter strands.

mRNA, tRNA, rRNA and non-coding RNA( like microRNA)

Product processing: In eukaryotes complementary base pair nucleotides bond with the sense or antisense strand. Thesre are then connected with phosphoester bonds

In eukaryotes, a 5’ cap is added, a 3’ poly A tail is added and introns are spliced out.

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by DNA helix to create a complete strand.

Methods to measure and detect:

Meselson and Stahl Experiment RT-PCR, DNA microarray, In-situ hybridization, Northern blot, RNA-Seq.

Translation process

The translation process is divided into three steps:

Initiation: When a small subunit of a ribosome charged with a tRNA+the amino acid methionine encounters an mRNA, it attaches and starts to scan for a start signal. When it finds the start sequence AUG, the codon (triplet) for the amino acid methionine, the large subunit joins the small one to form a complete ribosome and the protein synthesis is initiated.

Elongation: A new tRNA+amino acid enters the ribosome, at the next codon downstream of the AUG codon. If its anticodon matches the mRNA codon it basepairs and the ribosome can link the two aminoacids together.(If a tRNA with the wrong anticodon and therefore the wrong amino acid enters the ribosome, it can not basepair with the mRNA and is rejected.) The ribosome then moves one triplet forward and a new tRNA+amino acid can enter the ribosome and the procedure is repeated.

Termination: When the ribosome reaches one of three stop codons, for example UGA, there are no corresponding tRNAs to that sequence. Instead termination proteins bind to the ribosome and stimulate the release of the polypeptide chain (the protein), and the ribosome dissociates from the mRNA. When the ribosome is released from the mRNA, its large and small subunit dissociate. The small subunit

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can now be loaded with a new tRNA+methionine and start translation once again. Some cells need large quantities of a particular protein. To meet this requirement they make many mRNA copies of the corresponding gene and have many ribosomes working on each mRNA. After translation the protein will usually undergo some further modifications before it becomes fully active.

DNA Polymerase - Elongates a new DNA strand at a replication fork. Adds nucleotides one by one to the new and growing DNA strand.

DNA ligase - Joins sugar-phosphates of Okazaki fragments. Okazaki fragments are found on the lagging strand.

Primase - Starts an RNA chain from scratch that will eventually be replaced by DNA nucleotides (remember nucleotides come from DNA polymerase).

Helicase - Untwists the double helix at replication forks to make two parental strands available as template strands.

Topoisomerase - Relieves strain of ahead of the replication fork due to untwisting of the double helix.

These five enzymes are the most basic needed for DNA replication as has been described above. As I said earlier there are many more enzymes that can be utilized depending on species. Also, if you go more in depth and want to know about proofreading and repairing DNA, even MORE enzymes crop up. I would suggest reading your text book to get a better idea of what is going on. For instance, if we are talking about replication of the lagging strand of DNA, the order of enzymes being put to work would be as follows:

DNA Transcription

DNA consists of four nucleotide bases [adenine (A), guanine (G), cytosine (C), and thymine (T)] that are paired together (A-T and C-G) to give DNA its double helical shape. Nucleotide base sequences are the genetic code or instructions for protein synthesis.

There are three main steps to the process of DNA transcription.

RNA Polymerase Binds to DNA

DNA is transcribed by an enzyme called RNA polymerase. Specific nucleotide sequences tell RNA polymerase where to begin and where to end. RNA polymerase attaches to the DNA at a specific area called the promoter region.

Elongation

Certain proteins called transcription factors unwind the DNA strand and allow RNA polymerase to transcribe only a single strand of DNA into a single stranded RNA polymer called messenger RNA (mRNA). The strand that serves as the template is called the antisense strand. The strand that is not transcribed is called the sense strand.

Like DNA, RNA is composed of nucleotide bases. RNA however, contains the nucleotides adenine, guanine, cytosine, and uracil (U). When RNA polymerase transcribes the DNA, guanine pairs with cytosine and adenine pairs with uracil.

Termination

RNA polymerase moves along the DNA until it reaches a terminator sequence. At that point, RNA polymerase releases the mRNA polymer and detaches from the DNA.

Since proteins are constructed in the cytoplasm of the cell, mRNA must cross the nuclear membrane to reach the cytoplasm. Once in the cytoplasm, ribosomes and another RNA molecule called transfer RNA work together to translate mRNA into a protein. This process is called translation. Proteins can be manufactured in large quantities because a single DNA sequence can be transcribed by many RNA polymerase molecules at once.