DNA has not always been the accepted building block of genes and inherited material. Until the 1950′s, this role was believed to be filled by proteins.
The Search For Inheritable Material
In 1927, Griffith discovered bacterial transformation, which is the ability of bacteria to change their genetic makeup by absorbing foreign DNA molecules from other bacterial cells and incorporating the DNA into their own.
Then, in 1944, Avery, MacLeod, and McCarty published their findings that the molecule that Griffith’s bacteria was transferring was DNA.
In 1952, Hershey and Chase proved that it was DNA and not proteins that were the molecules of inheritance. They tagged bacteriophages (viruses that target bacteria) with radioactive isotopes, tagging the protein coat and DNA with different materials. They discovered that when the bacteria were infected with the virus, it was only the radioactive isotope they had tagged the DNA with that showed up.
Rosalind Franklin continued work started by Maurice Wilkins, and by carrying out X-ray crystallography analysis of DNA, found that DNA was a helix. Unfortunately, although her work was the essential backbone to Watson and Crick’s later discovery that DNA is a double helix, she didn’t get credit and was not named in the Nobel Prize.
Meselson and Stahl proved Watson and Crick’s hypothesis that DNA replicates in a semiconservative fashion. In order to prove this, they cultured bacteria in containing heavy nitrogen. They then moved them into a container with light nitrogen. The bacteria could replicate and divide once, and the new bacterial DNA had one heavy strand and one light strand, proving their hypothesis correct.
DNA is a double helix and looks like a twisted ladder
DNA has two complementary strands running in opposite sides from each other.
It’s a polymer with repeating units called nucleotides.
Each nucleotide has a 5 carbon sugar (deoxyribose), a phosphate molecule, and a nitrogenous base
There are four possible nitrogenous bases: The purines adenine, and guanine, and the pyrimidines thymine and cytosine. A goes with T and C goes with G.
The nucleotides of opposite chains are bound by hydrogen bonds.
DNA Replication in Eukaryotes
DNA replication is the process of making a perfect replica of the original DNA strand. Semi-conservative replication shows that the two new molecules of DNA have one old strand and one new strand.
Replication occurs during interphase
DNA polymerase catalyzes the replication of new DNA. It also proofreads each new DNA strand, fixing errors to minimise mutations.
DNA unzips at the hydrogen bonds connecting its two strands.
Each strand of DNA serves as a template for the new strand, based on the base-pairing rules.
Every time DNA replicates, some nucleotides on the end are lost. To prevent this from causing a problem, their DNA has nonsense repeating nucleotide sequences called telomeres.
RNA is a single-stranded helix.
It is a polymer, like DNA made of repeating units of nucleotides
It has ribose, a phosphate and a nitrogenous base
RNA does not have Thymine. Instead, it has Uracil. A pairs with U, C pairs with G.
There are 3 kinds: mRNA (messenger RNA) tRNA (transfer RNA) and rRNA (ribosomal RNA)
mRNA: Carries messages from DNA in the nucleus to the cytoplasm during protein synthesis. The nucleotides on mRNA are called codons.
tRNA: Carries amino acids to the mRNA to form a polypeptide. They have triplet nucleotides that are complementary to those of mRNA. These are called anticodons.
mRNA: Is structural. Makes up the ribosome, along with proteins
There are 3 main steps to protein synthesis: transcription, RNA processing, and translation.
Transcription is the process where DNA makes RNA. It is facilitated by RNA polymerase and takes place in the nucleus. The triplet codes on DNA are transcribed into codon sequences in the mRNA.
If the sequences in DNA triplets is: AAA TAA CCG GAC
The codons will look like this: UUU AUU GGC CUG (remember RNA does not have Thymine)
After transcription, the initial transcript is processed and edited by enzymes, who remove introns (noncoding sequences of RNA). The remaining exons are pieced back together to form the final transcript. The now shorter mRNA leaves the nucleus
Translation of mRNA Into Protein
Translation is the conversion of mRNA into an amino acid sequence.
It occurs in the ribosome.
Amino acids in the cytoplasm are carried by tRNA to the codons of the mRNA strand according to the base-pairing rules (think of it as trying to put a puzzle together.)
Some tRNA molecules can bind to two or more codons. For example, there are 4 separate sequences who code for the single amino acid: Serine.
Cells are not constantly synthesizing all the peptides it can make, as otherwise, the excess proteins would harm the bodies homeostasis. What this means is that the cells need to be able to turn their genes off sometimes. While this process is not well understood in humans, in bacteria it is a much more simple process, and much better understood.
The operon is the key to gene regulation. It is a cluster of functional genes, along with the “switches” that turn them on and off. There are two kinds. The Lac or inducible operon is normally turned off until it is actively triggered by something in the environment. The other is the repressible operon, which is always turned on unless it is actively turned off.
On the operon, there is the promoter. This is the binding site of RNA polymerase. RNA polymerase always needs to bind to DNA before transcription happens, so the promoter is the equivalent of an on the switch. There is also the operator, which is the binding site for the repressor, which turns of the Lac operon. The TATA box helps RNA polymerase bind to the promoter
Mutations are changes in genetic material. They are spontaneous and random. They can be caused by mutagenic agents, toxic chemicals, and radiation. They are often given a bad name, however, they are essential for natural selection.
A point mutation is the most simple form of a mutation. It is a base pair substitution, where one nucleotide becomes another. The effects of this can be seen when trying to read a sentence.
THE FAT CAT SAW THE DOG —— THE FAT CAT SAW THE HOG
The change isn’t too dramatic, and the sentence is still legible, albeit having a different meaning
Insertion and deletion cause much more dramatic changes. They occur when one nucleotide is lost, or an extra nucleotide is added to the sequence. These are also known as frameshift mutations.
THE FAT CAT SAW THE DOG — TTH EFA TCA TSA WTH EDO G
THE FAT CAT SAW THE DOG— HEF ATC ATS AWT HED OG
I went over chromosome mutations more in detail in my classical genetics post, so I’ll do a brief overview of some terms here.
Aneuploidy is a condition where someone has an abnormal number of chromosomes. Someone who is intersex is an aneuploid because of a chromosomal mutation that gave them an abnormal number of sex chromosomes.
The condition of having more chromosomes than average is called polyploidy. People with down syndrome are polyploids. More specifically, they have trisomy-21, meaning instead of 2 chromosome 21′s, they have 3.
These mutations are caused by nondisjunction when homologous pairs do not separate properly during meiosis.
It is important to know that chromosomal mutations do not always have disastrous effects. People with aneuploidy still live extremely fulfilled lives, and some don’t just learn to live, become happy with how they were born.
A genome is an organism’s genetic material. The human genome contains around 3 billion base pairs of DNA and 20,000 genes. 97% of that DNA does not code for protein production. Some of this DNA are regulatory sequences controlling gene expression, some are pseudogenes, which are former genes which accumulate over time. DNA is still very elusive, and scientists learn new things about it every day. Maybe one day, a scientist will read this blog, shaking his head at how wrong we were today.
Genetic Engineering and Recombinant DNA
Recombinant DNA is the act of taking DNA from two sources and combining them into one cell. This is the foundation of genetic engineering and biotechnology. Two pieces of this massive subject are gene therapy and environmental cleanup. The hope with gene therapy is that scientists may figure out how to insert functioning genes into humans to replace their nonfunctioning ones. Success could mean a cure for cystic fibrosis and sickle cell anaemia. Along with this, microbes could be engineered to decontaminate harmful chemicals at mining sites. GMO’s could be modified
However, the safety of genetic engineering. GMO’s, in particular, have become a major talking point. One major concern is that GMO’s will accidentally be introduced to the wild which could have major impacts on the ecosystems surrounding farmland.
Restriction enzymes are essential for scientists who work with DNA. They cut DNA at recognition sequences or sites. They are referred to as molecular scissors. The pieces of DNA that result from the cuts are called restriction fragments.
Gel electrophoresis is the act of separating large molecules of DNA based on their rate of movement through an agarose gel in an electric field. The smaller the molecule of DNA, the faster it travels. Before being placed in the gel, the DNA is prepared with restriction enzymes, providing small enough molecules for the scientists to work with.
Polymerase Chain Reaction
Discovered in 1985, a PCR is a cell free, an automated technique that rapidly copies or amplifies DNA. This is great for forensic sciences, where small pieces of DNA can be expanded, and then compared.
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