DNA Replication, Translation & Transcription Flashcards
What is a Nucleotide?
basic building block of nucleic acids (DNA or RNA)
— provide genetic instruction for organisms
Keynote: DNA is the genetic material found in all living organisms
What are the Components of a Nucleotide?
Each nucleotide has: - a base (example: nitrogenous base) - a phosphate - a pentose (5 sided) sugar (with 5 carbons) nucleotides in a strand
( DNA or RNA are linked together by covalent bonds between
the pentose sugar of one nucleotide and the phosphate of
the next one)
What are Nucleic acids?
Long chains of nucleotides which store and transmit genetic information
The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)
Differences between DNA & RNA
DNA:
1. Has deoxyribose sugar
2. Bases present are (A)(T)(G)(C)
3. DNA is doubled stranded (has 2 strands)
(Strands are antiparallel – run in opposite direction — in 3’ to 5’ direction)
(Strands are linked to each other by hydrogen bonding between Complementary base pairing)
==========================================================
RNA:
1. Has ribose sugar
2. Bases present are (A)(U)(G)(C)
3. RNA is single stranded (has 1 strand)
(Complementary base pairing — bases that can pair up)
In DNA:
(A) pairs with (G) & (C) with (G)
(Complementary base pairing linked by hydrogen bonds creates
specific sequences of bases)
How was DNA discovered?
Phoebus Levene, 1919, found:
DNA is composed of nucleotides made up of sugar, phosphate, and base
Erwin Chargaff, 1950, found
DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T)
Crick and Watson. 1953:
accurately modeling the structure of DNA
(Found DNA has double helix)
although Rosalin Franklin also contributed but gets overshadowed (she created an X-ray crystallography that showed double helix structure of DNA which Crick & Watson used to build the model)
What are the two big a Categories for Nitrogenous bases?
First Group:
purines — have a double ring structure
Include: (Adenine (A) and Guanine (G)
Second Group:
Pyrimidines —- have a single ring structure
Include: (Cytosine (C) Uracil (U) and Thymine (T)
Define Double Helix?
two complementary strands of nucleotides attached to each other. by bases and held together by hydrogen bonds
Types of Nitrogenous Bases
Nitrogenous base: A molecule that contains nitrogen and has the chemical properties of a base
nitrogenous bases in DNA are adenine (A), guanine (G), thymine (T), and cytosine (C)
nitrogenous bases in RNA are adenine (A), guanine (G), uracil (U), and cytosine (C)
How does genetic information change?
Mutations
Example: Mutation could cause a change in DNA that arises at regular intervals
Consequences
Could also lead to variety within a species —– leading to evolution
What is a nucleosome
DNA wrapped around 8 histone protein held together by a 9th histone
Explain Semi Conservative Replication
Semi Conservative replication (The accurate one)
two strands of DNA separate due to breaking of hydrogen bonds. New polymers of nucleotides are formed on each of the two strands.
Keynote:A strand of DNA on which a new strand is formed is called a template strand. This occurs because each nitrogenous base can only pair with its complementary partner:
Adenine (A) pairs with thymine (T)
Cytosine (C) pairs with guanine (G)
Due to complementary base pairing, each of the new strands will have the same base sequence as the old parent strand that was separated from the template strand. Thus, this type of replication results in two molecules being produced, one new strand and one old strand each of DNA molecules
Simple version
2 strands open up, each strand makes a new strand. The two molecules at the end are one new strand. and one old strand
Explain Conservative & Dispersive Replication
(Incorrect ones)
Conservative —- 2 strands of parent DNA remain together and form two new strands. The new strands went together and the old strands went together
Dispersive —- 2 strands opened up. Segments of the new and old strands were made together to form one strand
Explain Meselson and Stahl’s Experiment (1958)
cultured E. coli bacteria in a medium where the only nitrogen source
was Nitrogen 15. So the nitrogen in the bases of the bacterial DNA was Nitrogen 15. They then transferred the bacteria abruptly to a medium with the less dense Nitrogen 14 isotope. Meselson and Stahl spun samples of DNA collected from their bacterial culture at different times in an ultracentrifuge after transfer to Nitrogen 14 medium. After one generation the. DNA was intermediate in density between Nitrogen 14 and Nitrogen 15, disproving the conservative model. After two generations there were two equal bands, one still Nitrogen 14/15 and one at Nitrogen 14 density, disproving the dispersive model. Meselson and Stahl demonstrated that DNA replicated semi-conservatively, meaning that each strand in a DNA molecule serves as a template for the creation of a new strand.
Explain the process of DNA Replication
Detail
DNA replication is a semi-conservative process whereby pre-existing strands act as templates for newly synthesized strands
The process of DNA replication is coordinated by two key enzymes – helicase and DNA polymerase. DNA Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds. DNA polymerase links nucleotides together to form new strands in a. 3’ to 5’ direction, using the pre-existing strands as templates. One strand doesn’t form continuously but forms Okazaki fragments that connect to each other. These fragments later form one continuous strand through an enzyme catalyst. The daughter DNA molecules each rewind (coil) into a double helix in a semi-conservative fashion with each new double helix having an old and new strand. The two daughter DNA molecules are identical in base sequence to each
other and to the parent molecule, because of complementary
base pairing. Adenine will only pair with thymine and cytosine will only pair with guanine. Each of the new strands is complementary to the template strand on which it was made and identical to the other template strand
Explain the process of DNA Replication
In Brief & touch on its significance
DNA replication is the process by which the DNA molecule is copied to produce two identical DNA strands. Replication is tightly controlled by enzymes. The enzymes also proofread the DNA during replication to correct any mistakes. DNA replication is required before mitosis can occur. After replication, the chromosome is made up of two chromatids. Each chromatid contains half original and half new DNA. The chromatids separate during mitosis.
Define Transcription
The process by which DNA makes messenger RNA (mRNA)
(Happens in G1 of the cell cycle)
mRNA is made in nucleus —- mRNA is used to make protein
Explain the process of Transcription
in detail
RNA polymerase separates the DNA strands and creates a complementary RNA copy from one of the DNA strands. RNA nucleotides are assembled along one of the two strands of DNA in a 5’ to 3’ direction. The same rules of complementary base pairing are followed as in replication, except that uracil pairs with adenine, as RNA does not contain thymine. The RNA nucleotides are linked together by covalent bonds. Once the RNA sequence has been created, RNA polymerase detaches from the DNA molecule and the double helix reforms
Key note:
The strand that is transcribed is called the antisense strand and is complementary to the RNA sequence . The strand that is not transcribed is called the sense strand and is identical to the RNA sequence (with T instead of U)
Transcription of genes occur in the nucleus (where DNA is), before the RNA moves to the cytoplasm (for translation)
Define Translation
Production of protein by mRNA and tRNA.
(due to mRNA strand interacting with ribosome)
When mRNA arrives in the cytoplasm it can :
1) cause the formation of ribosomes around it
(free-floating ribosomes – which are made for use within the cell)
2) go to ribosomes fixed on the edoplasmic recticulum (RER) and synthesize proteins used for export/secretion)
Explain process of Translation
in detail
Messenger RNA containing a series of codons consisting of three bases binds to a site on the small subunit of the ribosome were each of which codes for one amino acid. Ribosomes bind to mRNA in the cytoplasm and move along the molecule in a 5’ – 3’ direction until it reaches a start codon (AUG). With transfer RNA molecules present around the ribosome in large numbers, each tRNA has a special triplet of bases called an anticodon and carries the amino acid corresponding to this anticodon. There are three binding sites for tRNA molecules on the large sub-unit of the ribosome but only two ever bind at once. A tRNA can only bind if it has the anticodon that is complementary to the codon on the mRNA. The bases on the codon and anticodon link together by forming hydrogen bonds, following the same rules of complementary base pairing as in replication and transcription.
The amino acids carried by the tRNA molecules are bonded together by a peptide linkage. The ribosome moves along the mRNA to the next codon. Another tRNA carrying an amino acid binds. A chain of three amino acids is formed. These stages are repeated until a polypeptide is formed.
Define a codon & a triplet code
1) A codon – A group of three bases
2) a triplet code — three bases code for one amino acid
Define an mRNA triplet codon, an anticodon, a start codon and a stop codon
1) mRNA triplet codon — three bases code for RNA
2) Anticodon — three bases code for tRNA
3) Start codon – three bases code that starts the process of translation (e.g AUG)
4) Stop codon — causes three bases code polypeptide chain to stop (e.g UAA/UAG & UGA)
Distinguish between a promoter and terminator region
Promoter region — gets transcription started
Terminator regions – ends the process of transcription
With respect to codon, illustrate degeneracy of the genetic code
DNA undergoes a mutation that changed one of the bases of the DNA strand and it was still coded. Thus, you will have multiple codons for one amino acid due to the mutation of DNA
Define gene expression
Process of rewriting a gene into a protein. It involves the transcription of DNA into mRNA and the translation of mRNA into protein
