Unit 8: Replication/Chromosomes/Cell Cycle

DNA replication

• DNA copies itself prior to cell division

“semi-conservative” process

• 2 new “daughter” DNA molecules each contain ½ of the original “parent” DNA molecule as well as new nucleotides

Enzyme DNA helicase

unzips the double helix

DNA splits into

two halves, exposing nucleotide bases

Free DNA nucleotides found in the

nucleus combine with their complementary bases on the exposed DNA strands (both at the same time)

Leading strand

DNA polymerase enzyme continually builds the new DNA strand toward the replication fork (5’→3’ direction)

Lagging strand

constructed in fragments (Okazaki fragments) by the DNA polymerase (5’ → 3’ direction) away from the replication fork

• joined together by DNA ligase to create final strand

Why is the chromosome # in a diploid cell always an even #?

Because 2 parents donate the same number of chromosomes to the zygote during fertilization

• the diploid # must always be even

Why is it important that gamete cells have only 1 chromosome in each homologous pair?

So the resulting zygote has homologous pairs; an extra chromosome leads to genetic disorders…not super powers!

Interphase

• G₁

• S

• G₂

• Sometimes G₀ (thought of as an extension)

G₁ Stage (Gap 1/ “Growth 1”)

• cell volume increases (grows to normal size)

• cell differentiation/growth is completed

• cell begins to function

• DNA is “chromatin”

G₀ Stage

stage outside the cell cycle in which cells do not copy their DNA and do not prepare to divide

Many cells, such as mature nervous cells in the human body,

are in the G₀ Stage

• may last weeks or indefinitely

S Stage (Synthesis)

• DNA replication occurs

• all cell activity ceases as replication is completed

Onset of S Stage is trigged by

a buildup of the hormone “S-Cyclin”

G₂ Stage (Gap 2/”Growth 2”)

• cell function resumes

• cell increases in volume as all cell contents double in preparation for cell division (cell looks fat)

• DNA is “chromatin”

Genetic material in G₂ Stage

2 ⋅ 2N

• (2 identical DNA strands)

M-Stage

• cell division occurs

• mitosis followed by cytokinesis

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

Interphase ends

in parent cell

and begins in daughter cell

Mitosis

specifically the formation and segregation of chromosomes and division of the nucleus

Mitosis occurs in 4 stages:

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Chromosomes appear (become visible)

during prophase

• double-stranded

Cytokinesis

the physical division of the cell membrane and cell contents into 2 cells (2 equal piles)

Mitosis and cytokinesis together

represent the complete process of cell division

Chromosomes

• tightly coiled DNA found in the nucleus of a dividing cell during M-Phase

• each one represents a highly coiled DNA molecule; visible in cell

• 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 in the cell between cell divisions

Centromere

structure which holds together the sister chromatids in a doubled chromosome

Histone

• proteins that help organize DNA

• DNA is wound around histone “spools”

Nucleosome

• group of 8 histones

• nucleosome wind into helix

• helix condenses into visible chromosome

Homologous chromosome

• chromosome pairs that contain the same genes

• in all sexually reproducing organisms, chromosomes appears in these pairs (look like all other chromosomes but contain different genes than them)

• may/may not show “sister chromatids” depending on stage of cell cycle

Sister chromatids

• result of DNA replication

• genetically identical duplicate DNA molecule making up two halves of a doubled chromosome

Sex chromosomes

• determine gender

• known as X and Y

• female: XX

• male: XY

Autosomes

chromosome #1-#22

Diploid cell

• contains both halves of each homologous pair of chromosomes

• contains a full set of genetic information (2N)

Somatic cells

body cells (diploid)

Haploid cell

• contains ½ of the full set of genetic information

• contains only one chromosome from each homologous pair (1N)

All gametes

human sex cell (haploid)

In plants, gonads are the

flowers

Male gonads

testes

Female gonads

ovaries

Male gametes

sperm

Female gametes

egg cell (ova)

In plants, the male gamete is called

pollen

In plants, the female gamete is called a

ovule (contains egg cell)

• after fertilization, develops into a seed

Karyotype

• an image taken of chromosomes during mitosis

• used to identify both biological gender and possible chromosomal mutations

Human chromosomes are

numbered 1-22 and then there are sex chromosomes

Chromosomes are identified and paired according to

• centromere position

• banding pattern due to staining

• overall size (incl. arm length)

Chromosomal mutations

• occur during DNA replication and meiosis

• includes partial chromosomal mutations and whole chromosome mutations

Whole chromosomal mutations

• trisomy

• monosomy

*caused by “non-disjunction” (separation fails to occur)

Trisomy

karyotype shows an entire extra chromosome

• chromosome had an extra gamete when created, e.g. XXY

Monosomy

karyotype shows a missing chromosome

Partial chromosome mutations

• deletion

• duplication

• translocation

• inversion

Deletion

• a section of a chromosome is missing

• chromosome appears small

Duplication

• a section of a chromosome is repeated

• chromosome appears oversized

Translocation

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

Inversion

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