Biology 1: Exam 3: With Victoria Mccurdy at Mississippi State University Fall 2017

DNA

stores genetic material by encoding it in a sequence of nucleotides and is packaged into chromosomes that are stored in the nucleus..

Humans

23 chromosomes = 22 homologous pairs + 1 sex pair..

5 Levels of Organization

1. Nucleotides

2. Single strand

3. Double Helix

4. Chromosome

5. Genome..

3 Component of DNA

1. Phosphate Group

2. Sugar

• Deoxyribose

3. Base

• Guanine

• Cytosine

• Adenine

• Thymine..

3 Components of RNA

1. Phosphate Group

2. Sugar

• Ribose

3. Base

• Guanine

• Cytosine

• Adenine

• Uracil..

Numbering

sugar carbons are 1' to 5' with the base attaching at 1' and phosphate at 5'..

Phosphodiester bonds

link nucleotide together and form sugar-phosphate backbone..

Directionality

nucleotide sequence is written 5' to 3'..

Strand

a linear linkage of nucleotides that has directionality..

Double stranded helix

has strands that run antiparallel with a sugar-phosphate backbone and bases projected towards the center (away from the backbone)..

Complementary Base Pairing rule

leads to complementary strands-

• Adenine (A) pairs with Thymine (T)

• Cytosine (C) pairs with Guanine (G)..

DNA has to replicated whenever a cell divides to produce new cells and follows the AT/GC rule

1. Parental strands separate and serves as a template for DNA synthesis

2. Replication is semi

conservative with each new helix composed of 1 original parent and 1 new daughter strand..

Steps

1. Replication begins at an origin of replication

2. The two strands unwind

3. Replication proceeds from 2 forks and is bidirectional..

Formation and Movement of the replication fork

1. Single

strand binding proteins: bind single strands to prevent them from reforming a double helix

2. DNA topoisomerase: prevents tangling of DNA ahead of the fork

3. DNA helicase: unwinds DNA at the fork..

Synthesis of DNA strands

1. DNA polymerase: covalently links nucleotides together to form DNA strands

• Can only add nucleotides to existing strands in the 5'

3' direction

• Extends DNA from the RNA primer

2. RNA primer (made by DNA primase) is needed to prime replication

• Removed and replaced by nucleotides

• DNA Ligase

seals the gaps between fragments..

Synthesis of Leading and Lagging strands

• Leading strand

is synthesized continuously 5'-3' towards the replication fork and requires only one primer

• Lagging strand

is synthesized discontinuously as short Okazaki fragments: away from the replication fork and requires a primer for each fragment..

DNA polymerase

is unlikely to catalyze bonds between adjacent mismatched nucleotides because they do not fit the enzyme active site optimally and has a 3'-5' proofreading mechanism that checks for mistakes afterwards..

Because DNA is unable to replicate the 3' end of the template DNA, there is a shortening of DNA with each round of replication

Telomeres prevent loss of genetic material and are added to the end of chromosomes by telomerase

DNA is highly compactable

• Has a double helix

• Nucleosomes: DNA wound around histone proteins + chromatin = DNA + histone proteins

• 30 nm diameter

• Radial loop domain

• Compacted radial loop domain

Euchromatin

regions of less compact DNA with active genes

Heterochromatin

regions of highly compact DNA with inactive/silent genes

Chapter 12:

Genes - transcription mRNA - translation polypeptides

Genes

segment of DNA that encodes a functional product

• Most genes encode a polypeptide (DNA

> mRNA -> polypeptide)

• Or a functional RNA molecule (DNA

> ribosomal RNA)

• Different genes are expressed in different cells

• Structure of a Gene

o Promoter: region where RNA polymerase binds to start transcription

o Regulatory sequence: regulatory proteins bind + control the rate of transcription

o Transcribed Region

contains the information that specifies amino acid sequence

o Terminator: signals the end of transcription

Transcription

produces an RNA copy of the gene

• mRNA sequence is complimentary to the DNA template strand (non

coding strand)

• mRNA = non

template strand = coding strand

• Initiation: a sigma factor recognizes a promoter and recruits an RNA polymerase

• Elongation: makes a complimentary RNA strand

o DNA RNA

o G pairs with C

o A pairs with U

o T pairs with A

• Termination: DNA polymerase dissociates after the terminator sequence (bacteria) or polyA signal sequence (eukaryotes)

mRNA

a copy of a genes that contains information to make a polypeptide

• mRNA vs tRNA

o mRNA = codons (sets of 3 RNA nucleotides that specify an amino acid)

o tRNA = anticodons (sequences of 3 nucleotides that are compilatory to a codon

• Organization

o Ribosomal

binding site (prokaryotes only)

o Start codon: specifies the first amino acid

o Coding sequence: codons that determine the sequence of amino acids

o Stop codon: specifies stop (UAA, UAG, UGA)

Translation

produces polypeptides using information in mRNA (one language to another)

• Translation: has initiation, elongation, and termination