Unit 1 KA1-KA3 Flashcards
DNA structure
- DNA consists of two strands of nucleotides
- The nucleotides making up each strand are held together by strong chemical bonds between the deoxyribose sugar of one nucleotide and the phosphate molecule of another nucleotide.
- These links form the sugar-phosphate backbone of the DNA molecule.
- The two strands of nucleotides are held together by weak hydrogen bonds between these bases.
- two strands are antiparallel-deoxyribose sugar is at the 3’ end and the phosphate is at the 5’end
- The two strands of nucleotides are twisted into a double helix.
Prokaryote Cells
- Have a single circular chromosome and smaller circular plasmids.
- e.g. bacteria
Eukaryote Cells
- Have linear chromosomes in the nucleus
- These linear chromosomes are tightly coiled and packaged with associated proteins called histones.
- There are also small circular chromosomes inside mitochondria and inside the chloroplasts of a green plant cell.
- e.g. yeast
Replication of DNA
-Prior to cell division, DNA is replicated by the enzyme DNA polymerase.This ensures that each new cell produced as a result of cell division receives an exact copy of each gene it requires to function.
-DNA replication results in the formation of 2 new molecules. Each new molecule contains:-
• 1 strand of the original parent molecule and
• 1 new strand
Requirements for replication of DNA
- DNA
- Primers
- ATP (for energy)
- Enzymes : DNA polymerase & ligase
- A supply of the 4 types of DNA nucleotide
-A primer is a short strand of nucleotides which binds to the 3’ end of the template DNA strand allowing polymerase to add DNA nucleotides.
DNA polymerase adds DNA nucleotides, using complementary base pairing, to the deoxyribose (3’) end of the new DNA strand which is forming.
Ligase joins fragments of DNA together.
Replication process
- Double helix unwinds.
- Hydrogen bonds between bases break & strands separate into 2 template strands.
If the leading strand (with the 3’ at the end) :-
- A primer binds to the 3’ end of the strand and DNA nucleotides in the cytoplasm move in and find their complementary base.
- DNA polymerase adds these nucleotides continuously to the deoxyribose 3’ end of the new DNA strand which is forming.
- Weak hydrogen bonds form between base pairs.
- Strong chemical bonds form between adjacent nucleotides.
If the lagging strand (with the 5’ at the end):-
Synthesis lags behind until enough of the strand is exposed to add a primer and then nucleotides to the 3’ end. This addition of primers and nucleotides is repeated in fragments as more of the strand becomes exposed. The strand only becomes complete when these fragments are joined together by the enzyme ligase.
- When the new complementary strands are complete, each strand coils into a double helix.
The polymerase chain reaction (PCR)
- PCR is used to target specific sequences of DNA and amplify them (produce multiple copies of them).
- PCR involves the use of primers. Each primer is a short strand of nucleotides which is complementary to a specific target sequence at the two ends of the region of DNA to be amplified.
Requirements for PCR
- DNA (as a template)
- Primers
- Heat tolerant DNA polymerase
- A supply of the 4 types of DNA nucleotide
Process of PCR
- DNA is heated (to 92-98°C) to separate the two DNA strands
- Cooling to (50-65°C) allows primers to bind to their complementary target sequence
- DNA is heated again (to 70- 80°C) so that heat tolerant DNA polymerase can replicate the region of DNA
Practical applications of PCR
-In Forensics (to help solve crimes):
A tiny quantity of DNA from blood, semen or tissue found at a crime scene can be amplified to produce enough material for DNA finger printing
-To Diagnose Genetic Disorders:
DNA from embryonic cells can be amplified to diagnose genetic disorders before birth
-To Settle Paternity suits:
A sample of DNA from the child, mother and possible father can be amplified and used to determine paternity.
Nucleotide
-Each nucleotide consists of a deoxyribose sugar, a phosphate and a base.
The base is attached to carbon 1 (C1) and the phosphate to C5 of the deoxyribose sugar.
-A nucleotide contains one of 4 different bases:- adenine (A), thymine (T), guanine (G) and cytosine (C)
The shape of each base is complementary to one other base:-
• adenine pairs with thymine
• guanine pairs with cytosine
Gene expression
-Gene expression involves the transcription and translation of DNA sequences.
Only a fraction of the genes in a cell are expressed – the genes expressed depend on the proteins required by that cell.
Transcription and translation involves three types of RNA:-
• messenger RNA (mRNA)
mRNA carries a complementary copy of the DNA code from the nucleus
to the ribosome
• transfer RNA (tRNA)
tRNA picks up and carries its specific amino acid to the ribosome
• ribosomal RNA (rRNA)
rRNA and proteins form a ribosome
Structure of RNA
Ribonucleic acid (RNA) is single-stranded and is composed of nucleotides containing:- • a ribose sugar • a phosphate • one of 4 bases – cytosine guanine adenine uracil (instead of thymine)
Transcription of mRNA
- RNA polymerase moves along the DNA unwinding the double helix and breaking the hydrogen bonds between the bases.
- RNA polymerase synthesises a primary transcript of mRNA using RNA nucleotides and complementary base pairing. Uracil in RNA is complementary to adenine.
- When the mRNA primary transcript is complete, the weak hydrogen bonds holding the DNA and mRNA bases together break.
- The open DNA strands reunite and the molecule recoils into a double helix.
- Each group of 3 bases on mRNA is known as a codon- each codon codes for a specific amino acid.
RNA splicing
- RNA splicing occurs within the nucleus and results in the formation of a mature mRNA transcript.
- The primary mRNA transcript consists of introns and exons.
- After splicing, the mature mRNA transcript leaves the nucleus and enters the cytoplasm, where it attaches to a ribosome.
- The next process, translation, now occurs - this involves the formation of protein from mRNA.
