Protein Synthesis

Protein Synthesis• 3 major processes: –Replication → DNA copied to form 2 new DNA

molecules• Nucleus

– Transcription → DNA info copied to RNA• Nucleus

– Translation → building a protein according to RNA instructions• Cytoplasm

DNA Replication

DNA Replication• DNA Replication =

DNA DNA– Parent DNA makes 2

exact copies of DNA– Occurs in nucleus– Why??• Occurs in Cell

Cycle before MITOSIS so each new cell can have its own FULL copy of DNA

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Models of DNA Replication

Segments of single-stranded DNA are called template strands.

Copied strand is called the complement strand (think “c” for copy)

BEGINNING OF DNA REPLICATION(INITIATION)

• DNA helicase (think “helix”) – binds to the DNA at the replication fork (origin of

replication)– DNA strand separates into TWO – untwist (“unzips”) DNA using energy from ATP

by breaking hydrogen bonds between base pairs at several places along the segments of DNA called “origins of replication”

• Single-stranded DNA-binding proteins (SSBP) – stabilize the single-stranded template DNA during

the process so they don’t bond back together.

DNA Polymerase• DNA Polymerase- adds nucleotides to the DNA strands

makes POLYNUCLEOTIDES (1st function)

Helicase unzips the DNA molecule

DNA Polymerase adds nucleotides to create two NEW identical daughter molecules (A to T) and (G to C)

• Complementary bases match up – Two new strands are formed– A with T– C with G

ElongationAntiparallel nature: • Sugar (3’end)/phosphate (5’

end) backbone runs in opposite directions – one strand runs 5’ 3’,– other runs 3’ 5’

• DNA polymerase only adds nucleotides at the free 3’ end of NEW STRAND forming new DNA strands in the

5’ 3’ direction only!!!

DNA Replication (Elongation)After SSBP’s bind to each template…• Primase– primase is required for DNA synthesis– Like a “key” for a car ignition – makes a short RNA primers

• Short pieces of RNA needed for DNA synthesis• DNA polymerase – adds nucleotides to RNA primer makes

POLYNUCLEOTIDES (1st function)– After all nucleotides are added to compliment

strand…• RNA primer is removed and replaced with

DNA by DNA polymerase (2nd function)• DNA ligase– “seals” the gaps in DNA – Connects DNA pieces by making phosphodiester bonds

Elongation (con’t)• Leading (daughter) strand–NEW strand made toward the

replication fork (only in 5’ 3’ direction from the 3’ 5’ template strand–Needs ONE RNA primer made by

Primase–This new leading strand is made

CONTINOUSLY

Elongation (con’t)Lagging (daughter) strand• NEW strand synthesis away from

replication fork • Replicate DISCONTINUOUSLY – Creates Okazaki fragments• Short pieces of DNA

– Okazaki fragments joined by DNA ligase• “Stitches” fragments together

– Needs MANY RNA primer made by Primase

3

DNA Polymerase

5’ 3’

Leading strand

base pairs

5’

5’

3’

3’

Supercoiled DNA relaxed by gyrase & unwound by helicase + proteins:

Helicase

ATP

SSB Proteins

RNA Primer

primase

2DNA Polymerase

Lagging strand

Okazaki Fragments

1

RNA primer replaced by DNA Polymerase & gap is sealed by DNA ligase

Why Replication again?

DNA replication is necessary to create identical copies of DNA so it can be passed onto a new cell (cell division & reproduction)

Summary

Accuracy of Replication• Very low mistake rate (1/billion!) because

cells have enzymes (like DNA Polymerase) that proofread, recognize,

and fix mistakes! • HOWEVER, mistakes can happen MUTATIONS (cancer)

Mistakes Made during DNA Replication

• Mutation– Change in DNA (genetic material)• Frameshift(s) –extra or missing base(s).

• Substitutions –when the wrong nucleotide is

incorporated (mismatch mutation). • Deletions–Nucleotides are deleted shortening the

DNA

Review: What is DNA replication?

1. A new sugar-phosphate backbone is made for each new strand

2. Base pairs are added

3. Two strands are created in place of the original strand