DNA structure and function Flashcards
The discovery of DNA
- Rosalind Franklin took the first clear X-ray diffraction image of DNA in 1952
- confirmed the spiral nature of DNA
Eukaryotic cells
DNA bound to proteins in chromosomes in nucleus enclosed in nuclear membrane
Also found in mitochondria and chloroplasts
Prokaryotic cells
Unbound circular DNA in nucleoid membrane - not bound by a nuclear membrane
DNA - deoxyribonucleic acid
smaller repeating subunits of nucleotides
double stranded
- bases held by weak hydrogen bonds
- nucleotides between sugar and phosphate units are held together by phosphodiester bonds
Nucleotide consists of
- phosphate group
- deoxyribose sugar / ribose sugar
- nitrogenous base
4 bases for DNA
complementary base pairs
- cytosine and guanine (3 H+ bonds)
- adenine and thymine (2 H+ bonds)
Nucleotides
building blocks of DNA
held together by:
strong chemical bonds - phosphodiester bonds
- between sugar and phosphate units
RNA - ribonucleic acid
uracil replaces thymine
one stranded
the purpose of DNA replication
- duplicate the code it carries - passed to daughter cells
preparation for cell division, mitosis and meiosis
DNA replication - eukaryotic cells
chromosomes gain a sister chromatid - x2 stranded
when does DNA replication occur?
S phase of interphase of the cell cycle
The process of DNA replication
1. - unwinding of DNA
- DNA helicase (enzyme) unwinds and separates the double stranded DNA by breaking the weak hydrogen bonds between the nucleotide bases, this exposes the nucleotide bases for replication
- the replication fork forms at the junction where the DNA strands separate
- As replication progresses, the replication fork moves along the parental DNA, continuously unwinding it
The process of DNA replication
2. - priming the template strand
- the enzyme primase attaches a short sequence of RNA primer (at short intervals on lagging strand) to the exposed DNA strand
- this primer signals DNA polymerase where to start adding new nucleotides
The process of DNA replication
3. - nucleotide addition
- DNA polymerase removes the RNA primers and then the free complementary nucleotide bases attach to the exposed bases on each template strand with the help of DNA polymerase
- since DNA strands are antiparallel, DNA polymerase moves in opposite directions on the two strands, leading and lagging
The process of DNA replication
4. - sealing and rewinding
- the enzyme DNA ligase seals the newly synthesised Okazaki fragments on the lagging strand, forming a continuous DNA strand, then rewind into a double helix
The process of DNA replication
5. - result
is the production of two identical DNA molecules
leading strand
- DNA polymerase synthesises continuously in the same direction as the replication fork (towards the replication fork)
- synthesis continuous
lagging strand
- DNA polymerase sythesises in short fragments (okazaki fragments) in the opposite direction (away from the replication fork)
- synthesis discontinuous
where is DNA found in eukaryotes
DNA bound to proteins in chromosomes in nucleus enclosed in nuclear membrane
Also found in mitochondria and chloroplasts
where is DNA found in prokaryotes
unbound circular DNA in nucleoid region of the cytosol - not bound by a nuclear membrane
Protein synthesis consists of
transcription
translation
Essential materials needed for protein synthesis
- enzymes
- codons
- amino acids
- nucleic acids
what is transcription
synthesis of mRNA using stored DNA
transcription process
- one section of DNA called a gene, is unwound and separated ready for copying
- a promoter attaches to help the DNA template strand to separate from the non-template strand, initiating transcription
- RNA polymerase (does not require primer) moves step by step along the DNA molecule, separating the two strands, only the template strand (also known as the non-coding strand) is copied
- the other strand - known as the coding strand - has the same code as the mRNA
- RNA polymerase then attaches the RNA nucleotide that’s complementary to each base (synthesises the mRNA in a 5’ to 3’ prime direction, anti-parallel to the template strand)
- After RNA polymerase enables elongation of the strand, mRNA molecule detaches as pre-mRNA
- pre-mRNA requires processing before it exits nucleus via nuclear pores
- stretches of non- coding DNA (introns) are removed and the remaining stretches of DNA (exons) join to form mature mRNA
