Honors Biology - Unit 8: DNA Replication & the Cell Cycle

DNA replication

process in which DNA makes a copy of itself before cell division

“semi-conservative” process

the 2 daughter DNA molecules each contain half of the original molecule as well as new nucleotides

DNA helicase

an enzyme that unzips the double helix into 2 halves, exposing the nucleotide bases

DNA polymerase

an enzyme that assembles the new DNA molecules by adding the correct corresponding nucleotides in 5’ → 3’

leading strand

DNA polymerase builds the new DNA strand toward the replication fork 5’ → 3’

lagging strand

DNA polymerase constructs the new DNA strand in fragments (Okazaki fragments) away from the replication fork & fragments are joined together by DNA ligase to create a final strand

Okazaki fragments

fragments in which new DNA is built on the lagging strand

DNA ligase

an enzyme that joins together Okazaki fragments on the lagging strand to create a final DNA strand

G₁ stage

cell volume increases; cell differentiation/growth completed & cell begins to function

amount of genetic material = 2N

G₀ stage

stage outside the cell cycle in which cells do not copy their DNA or divide for differentiation or due to unfavorable conditions → may return to the cell cycle when triggered by a signal

S-phase

DNA replication occurs - all cell activity stops as replication is taking place

triggered by the buildup of hormone “S-Cyclin”

G₂ stage

cell function resumes; cell increases in volume as all cell contents double in preparation for cell division

amount of genetic material = 2 × 2N

M-phase

cell division occurs - mitosis then cytokinesis

triggered by the buildup of the hormone “M-Cyclin”

mitosis

formation & segregation of chromosomes, as well as the division of the nucleus

4 stages of mitosis

prophase, metaphase, anaphase, telophase

cytokinesis

the division of the cell membrane and cell contents into 2 cells

chromosome

tightly-coiled DNA in the nucleus that is only visible during M-phase, they are rod-shaped or X-shaped depending on when they are observed

chromatin

DNA in a non-dividing cell that is not tightly coiled & not visible, allows transcription/translation to occur between cell divisions

centromere

structure which holds together the sister chromatids in a doubled chromosome

histone

proteins that help organize DNA by winding it around “spools”

nucleosome

group of 8 histones; winds into a helix & condenses into a visible chromosome

homologous pair

chromosome pairs that contain similar genes (different alleles)

sister chromatids

result of DNA replication, genetically identical duplicate DNA molecules making up 2 halves of a doubled chromosome

sex chromosomes

determines gender as X and Y

female: XX

male: XY

autosomes

chromosome #1-#22

diploid cell

contains both halves of each homologous pair & contains a full set of genetic info (2N)

ex: human cheek cell (somatic/body cells) = 46 chromosomes (23 homologous pairs)

haploid cell

contains ½ the full set of genetic info & contains only one chromosome from each pair (1N)

ex: human sex cell (gametes) = 23 chromosomes (0 homologous pairs)

karyotype

image taken of a chromosome during mitosis; used to identify gender & possible chromosomal mutations

chromosomal mutations

occurs during DNA replication & meiosis, includes whole chromosomal mutations & partial chromosomal mutations

trisomy

whole chromosomal mutation

karyotype shows an entire extra chromosome (diploid # = 47)

monosomy

whole chromosomal mutation

karyotype shows a missing chromosome (diploid # = 45)

deletion

partial chromosomal mutation

a section of a chromosome is missing; chromosome appears small

duplication

partial chromosomal mutation

a section of a chromosome is repeated; chromosome appears oversized

translocation

partial chromosomal mutation

a section of a chromosome detached & reattached to a different chromosome

inversion

partial chromosomal mutation

a section of a chromosome detached & reattached in the wrong direction

cell differentiation

process by which immature cells develop specialized characteristics or functions

nondisjunction

failure of chromosomes to properly separate during meiosis