The Structure Of DNA Flashcards
Outline, using a simple model, the process by which DNA controls the production of polypeptides
Polypeptide synthesis involves a type of nucleic acid, called RNA (ribonucleic acid). RNA is the intermediary between DNA and polypeptide synthesis. It is a single strand of nucleotide bases. It has ribose sugar and the nitrogen base, thymine, is replaced by uracil which bonds with adenine.
There are two types of RNA that are involved in polypeptide synthesis, messenger RNA (mRNA) and transfer RNA (tRNA).
In the nucleus, the double stranded DNA molecules unzip and the DNA code is transcribed into the single stranded mRNA molecule. The mRNA moves out of the nucleus into the cytoplasm and attaches to a ribosome. In the cytoplasm, the mRNA is translated into amino acids.
At the ribosome, the messenger RNA lines up forming a template. A group of three bases, called a codon, codes for a specific amino acid. There are codes that start and stop the chain formation. AUG is the starting point for translation.
tRNA has an anticodon (a non-amino acid forming codon) on one end and an amino acid on the other. A polypeptide is formed as each amino acid is added from tRNA to a chain following the sequence on the mRNA.
Explain the relationship between proteins and polypeptides
A protein is made up of one or more polypeptides. A polypeptide is made up of a chain of many amino acids.
Explain how mutations in DNA may lead to the generation of new alleles
Any change in the base sequence in DNA results in changes to the polypeptides that are produced and is a source of new alleles.
To produce changes in alleles, the mutation must occur in the sex cells of the organism which are then passed on to the next generation.
Discuss evidence for the mutagenic nature of radiation
There is much evidence for the mutagenic nature of radiation. Environmental factors that may increase the rate of mutation include X-rays, radiation from atomic bombs and ultraviolet light.
A mutagen is a natural or human-made agent (physical or chemical) which can alter the structure or sequence of DNA. Mutagens can be carcinogens (cancer causing) or teratogens (birth defects causing).
Radiation was the first mutagenic agent known. Its effects on genes were first noticed in the 1920’s.When X-rays were first discovered, they were thought to be harmless and were a great novelty. You could even buy an “X-ray machine” for your home for entertainment. Most of the first generation of scientists who worked with radiation died of cancer. Famous examples are Marie Curie and her daughter who both died of leukaemia.
Herman Muller received the Nobel Prize in 1946 for showing that genes had the ability to mutate when exposed to X-rays. Beadle and Tatum used X-rays to produce mutations in bread mould in the formulation of their “one – gene one – polypeptide” hypothesis.
The atomic bombs dropped on Hiroshima and Nagasaki also increased the evidence for mutations caused by radiation. There was a tenfold increase in cancer deaths directly after the bombs were dropped.
Mutagens may cause death in the individual but unless they affect the sex cells the effect is not passed on to the next generation.
Explain how an understanding of the source of variation in organisms has provided support for Darwin’s theory of evolution by natural selection
One of the foundation pillars for the theory of evolution is the variation that occurs among individual members of a species. The basis of this variation is the genetic makeup of the individuals in a species. It is this variation that selection acts upon. Mutation of DNA provides a source of new variations thus supporting Darwin’s theory of evolution.
Describe the concept of punctuated equilibrium in evolution and how it differs from the gradual process proposed by Darwin
Punctuated equilibrium differs from Darwin’s gradual evolution in that evolution is seen as long periods where there is little change in organisms, followed by a shorter period where there are rapid changes. Evolution is a sudden process rather than slow gradual change. The evidence for this comes from the fossil record where there are mass extinctions of organisms followed by the appearance of new species.
Describe the process of DNA replication and explain its significance
The process
Recall from Blueprint of Life, Subsection 3, that DNA is a double-stranded molecule twisted into a helix with each strand comprised of a sugar-phosphate backbone and attached bases - adenine (A), thymine (T), cytosine (C) and guanine (G) – connected to a complementary strand by pairing the bases, A-T and G-C.
A unit made up of a deoxyribose sugar, a phosphate molecule and one of four nitrogen bases (adenine, thymine, guanine and cytosine) is called a nucleotide. Thus, DNA consists of nucleotide units. The nitrogen bases that are part of each nucleotide are the rungs of the double helix of DNA. They only combine in one way, adenine binds to thymine and guanine binds to cytosine.
Along the DNA molecule, there are long sequences of genetic code made up of these bases. The replication of these long sequences accounts for the replication of the genetic code of an organism.
The process of DNA replication consists of the following steps.
Step 1: The DNA double helix is unwound by an enzyme.
Step 2: The DNA unzips forming two single strands
Step 3: Nucleotides are added to the single strands resulting in two identical strands of DNA
The two double stranded molecules are the chromatids.
The significance
The significance of this process is the genetic information is passed on from generation to generation. During sexual reproduction, the genetic code is copied and then half of the genetic information passes into each of the sex cells (ovum or sperm). When fertilisation occurs the new organism has half the genetic material from each parent.
The DNA in a cell contains the genetic information to make an entire organism. When a cell divides it takes with it an exact copy of the genetic code of that organism.
