module 6-7 bio

DNA

Double-stranded, deoxyribose sugar, bases A, T, G, C; stores genetic info.

RNA

Single-stranded, ribose sugar, bases A, U, G, C; carries info from DNA.

Proteins

Chains of amino acids that perform most cell functions.

DNA location

Nucleus

RNA location

mRNA is made in the nucleus and moves to the cytoplasm.
tRNA and rRNA are in the cytoplasm (ribosomes).

Relationship Between DNA, Genes, Nucleotides, Amino Acids, and Proteins

Genes are DNA segments made of nucleotides.
The nucleotide order determines amino acid order, forming a protein.

Promoter

Where RNA polymerase binds to start transcription.

Coding region

Holds instructions for the protein.

Terminator

Signals transcription to stop.

Transcription process: Initiation

RNA polymerase binds to promoter.

Transcription process: elongation

Builds complementary mRNA strand.

Transcription process: Termination

mRNA and DNA separate.

mRNA Processing Before Leaving Nucleus

• Adds 5’ cap and poly-A tail.

• Removes introns (noncoding).

• Keeps exons (coding).

Alternative Splicing

A process that rearranges or skips exons, allowing one gene to make multiple proteins.

Organelle for Translation

Ribosomes (made of rRNA and proteins) perform translation.

Translation Process: Initiation

Ribosome binds mRNA at start codon (AUG).

Translation process: Elongation

tRNA adds amino acids.

Translation process: termination 

Stop codon ends translation; protein released.

Codon

A group of 3 mRNA bases that codes for one amino acid.

Differential Gene Expression

Cells turn specific genes on or off to make only the proteins they need, enabling cell specialization.

Role of Transcription Factors

• Activators increase transcription.

• Repressors block transcription.

Point Mutation

One nucleotide changes.

Frameshift Mutation

A base added or deleted, shifting the reading frame.

Silent mutaation

No change in amino acid

Missense mutation

Wrong amino acid added.

Nonsense mutation

Early stop codon.

Sources of Mutations

• Errors during DNA replication.

• Exposure to environmental mutagens (radiation, chemicals).

Cell Response to Mutations

• Repair enzymes fix DNA.

• If not fixable: cell stops dividing or undergoes apoptosis (programmed death).

Function of DNA

DNA is the body’s instruction manual; it holds the directions for making proteins like insulin and glucose transporters.

Parts of a Nucleotide

A phosphate group, a deoxyribose sugar, and a nitrogenous base (A, T, C, or G).

Phosphodiester Bonds

Bonds that connect the phosphate of one nucleotide to the sugar of the next; form the DNA backbone.

Hydrogen Bonds

Bonds that connect the nitrogen bases (A–T and C–G) between the two DNA strands.

Complementary Base Pairing

A pairs with T; C pairs with G. Example: 5’ CGAT 3’ → 3’ GCTA 5’.

5’ End of DNA

Has a phosphate group attached.

3’ End of DNA

Has a hydroxyl (–OH) group; new nucleotides are added here during replication.

Importance of the 3’ End

DNA grows only in the 5’ → 3’ direction because new nucleotides attach to the 3’ hydroxyl group.

Semiconservative Replication

Each new DNA molecule has one original (parental) strand and one new (daughter) strand.

Origin of Replication (ORI)

The site where DNA replication begins.

Replication Bubble

The open area formed when DNA strands separate at the origin of replication.

Replication Fork

The Y-shaped region at each end of a replication bubble where DNA is copied.

Helicase

Unzips the DNA by breaking hydrogen bonds between bases.

Topoisomerase

Relieves twisting strain ahead of the replication fork.

Primase

Builds short RNA primers to start DNA synthesis.

DNA Polymerase III

Builds new DNA strands and proofreads for mistakes.

DNA Polymerase I

Removes RNA primers and replaces them with DNA nucleotides.

DNA Ligase

Seals gaps between Okazaki fragments on the lagging strand.

Leading Strand

Made continuously in the same direction as the replication fork (5’ → 3’).

Lagging Strand

Made in short Okazaki fragments opposite the fork’s direction.

Okazaki Fragments

Short DNA pieces formed on the lagging strand that are later joined by ligase.

DNA Proofreading

DNA Polymerase III corrects errors as it builds the new strand.

Nucleotide Excision Enzymes

Remove incorrectly copied DNA sections so they can be rebuilt correctly.