Bio 101 Unit 3

Gene

unit of hereditary transferred from parent to offspring — small region of chromosome that encodes a specific protein

• hershey + chase showed DNA contains genetic info

DNA

• made of nucleotide units: phosphate grp (outer circle), deoxyribose (hexagon), nitrogenous base

  • purine: A, G - double ring

  • pyrimidine: C, T, U - single ring

• double helix, 5’ to 3’ each strand

• hydrogen bonds hold bases together

RNA

single stranded copy of DNA that carries genetic info

• mRNA: (messenger) transcribed in nucleus and leaves to cytoplasm to bring info

• rRNA: (ribosomal) makes up ribosomes

• tRNA: (transfer) brings amino acids during translation

Transcription

making RNA copies in nucleus

1) initiation - RNA polymerase binds to promoter at start of gene and unzips DNA

2) elongation - RNA polymerase moves along copying RNA from DNA and joins complimentary bases together to form 3’ to 5’ RNA

3) Termination - at terminator sequence on DNA, all units separate and DNA winds into helix again

RNA processing

must happen after transcription before translation in order for mRNA to leave nucleus

• cap is added to 5’ end, poly-A tail added to 3’ end

• introns (not used to produce protein) are removed leaving only exons

Translation

making proteins in cytoplasm (multiple ribosome units attach to each exon to be efficient)

1) initiation - small ribosomal subunit binds to mRNA followed by large subunit. Initiator tRNA binds already containing base pairs and amino acid (anticodon matches up with START codon = MET amino acid)

2) elongation - second tRNA enters ribosome where its anticodon matches second mRNA codon and amino acids joined together by enzymes forming a peptide bond. tRNA leaves and amino acid stays in chain

3) Termination - at stop codon, release factor protein binds causing polypeptide to release where it will fold into functional protein

Gene regulation

highly regulated - requires lots of energy

• prokaryotes: genes organized as operons (grps of genes always transcribed together) which contain promotors (initiate), operators (affect activity of RNA polymerase), and repressors (end)

• Eukaryotes:

  • transcription factors: proteins that bind to gene to affect RNA polymerase activity (transcription only occurs if factors are correct)

  • alternate splicing: creates different proteins from same mRNA by rearranging exons

Mutations

changes in DNA sequences that are not always harmful

• point mutation: changes one or few base pairs

• frameshift mutation: insertion or deletion of nucleotide

DNA replication

1) helicase unwinds DNA (many origins) and single-strand binding proteins hold parent strands apart

2) primase adds short strand of RNA to each template (lagging strand needs more)

3) DNA polymerase adds nucleotides using RNA primer to create daughter strands (leading strand has one continuous)

4) ligases remove primer and form covalent bonds btwn DNA

Binary Fission

prokaryotes asexually replicate DNA to two identical daughter cells

• singular chromosome is replicated and each attach to opposite ends of cell and divides

DNA condensation

DNA condenses into chromosomes before division but is usually loose otherwise

• nucleosomes: loose DNA wraps around histones

• chromatin: nucleosomes coil tighter into thick string

• chromosomes: chromatin coils into X shapes

Cell cycle

Interphase: phase of cell that replicates DNA and carries out necessary functions

• G1: normal cell function + growth

• G0: cell most of the time

• S: cell replicates entire genome

• G2: additional growth for mitosis

Cell division

Mitosis: cells leave G2 and enter cell division phase

• 1) prophase: chromosomes + spindles form

• 2) prometaphase: chromosomes attach to spindle fibers

• 3) metaphase: chromosomes line up in center

• 4) anaphase: chromatids separate

• 5) telophase: two nuclei form around chromosomes

cell proceeds to cytokinesis:

• cleavage furrow: deepening ring of protein beneath cell membrane for cell to split

Checkpoints

ensure that DNA is replicated properly and is not damaged, and that chromosomes line up and separate properly

• if checkpoints fail, causes cancer cells: have distinct features - lose specialization, divide endlessly, can regenerate end of chromosome

• apoptosis: programmed cell death if cell does not meet requirements

Asexual vs sexual reproduction

• asexual reproduction: one parent replicates DNA and splits into two genetically identical offspring (bacteria, archaea, protists)

• sexual reproduction: DNA of offspring comes from two parents - egg fuses with sperm to create genetically diverse offspring to increase chance some will survive in changing environments

diploid cells

• diploid cell: two sets of chromosomes from each parent - human cells contain 23 sets of chromosomes (22 autosomes + 1 sex)

• homologous pair: chromosomes similar in size and structure and carry same sets of genes (even if different alleles)

Haploid cells

• haploid cell: one set of chromosomes (gametes)

• zygote: when two haploid sperm + egg cells fuse - first new cell of organism

• germ cells: in adults, these cells divide by meiosis to form haploid gametes

Meiosis

DNA replicates one, but nucleus divides twice - contains half as many chromosomes

• 1) prohase 1: homologous chromosomes pari up and attach to spindle

• 2) metaphase 1: homologous chromosomes line up in middle

• 3) anaphase 1: chromosomes seperate

• 4) telophase 1: nucleus forms

cytokinesis occurs

• 5) prophase 2: chromosomes attach to spindle

• 6) metaphase 2: chromosomes line up in middle

• 7) anaphase 2: chromaTIDS seperate

• 8) telophase 2: nucleus forms

cytokinesis occurs

4 total new cells created

increasing genetic variability

• crossing over: occurs in chiasma during prophase 1 - two homologous chromosomes pair up and exchange a piece each

• independent assortment: during metaphase 1, germ cell with 3 chromosome pairs has 4 possibel arrangements = 8 possible gametes

twins

• monozygotic twins: genetically identical - embryo splits

• dizygotic twins: two separate sperm with two separate egg cells

nondisjunction

chromosome fails to separate properly in anaphase 1 or 2 and can kill the whole cell

• down syndrome is caused by 3 copies of chromosome #21