genetics quiz 2

DNA packaging and cell reproduction

prokaryotes

bacteria, archaea

• always single-celled

• small cells without membrane bound organelles

• DNA is NOT packaged. It’s floating around in the cytoplasm

• just 1 circular chromosome + many small circular plasmids

how is DNA packaged in prokaryotes?

NOT packaged!

in the cytoplasm

1 circular chromosome

eukaryotes

protists, fungi, plants, animals

• can be single (protists) or multicelled

• larger cells with membrane bound organelles

• DNA packaged in the nucleus!!

  ^ also in the mitochondria and chloroplasts

where is DNA packaged in a eukaryotic cell and what is its structure like?

in the nucleus, mitochondria, and chloroplasts

nucleus: many linear chromosomes

mito. and chloro.: circular

how long would DNA from a human body be?

6.4 billion bases x 0.34nm between 2 bases

super long!!

we have about 50 trillion cells

how to prokaryotes fit all of their DNA into the cell?

supercoil their chromosomal DNA

• positive supercoiling (over-twisting)

• negative supercoiling (under-twisting)

• DNA coils on itself!

topoisomerases

enzymes that add or remove rotations by breaking the DNA, rotating the ends, and rejoining the pieces

• phosophodiester bonds are getting broken!

prokaryotic DNA is normally kept

negatively supercoiled

how do eukaryotes fit all of their DNA inside the cell?

DNA is packaged with proteins into chromatin and chromosomes

• proteins = histones

levels of DNA packaging in eukaryotes

10nm fiber

30nm fiber

300nm fiber

700nm fiber (now its called a chromosome!!)

histones

small, positively charged proteins

• why they’re positively charged: positive charge on R groups in the amino acid

• lysine rich!!!

significance of histones being pos. charged

DNA is neg. charged, so attracted to histones/wants to interact

• DNA wraps around histones!!

• beads on a string

which histone proteins make up the histone core?

8 total!

2 each of:

• H2A

• H2B

• H3

• H4

*H1 acts as a linker protein!!

nucleosome

histone core (8 histones) + DNA wrapped around it

~146 bp of DNA wrapped around each histone

chromatin

DNA complexed with histones

10nm

30nm

300nm

linker DNA

about 50 bp between nucleosomes

30 nm level of DNA packaging

10 nm fiber coils around itself

• 6 nucleosomes per turn

• H1 histone helps pull the nucleosomes together in a circle/ring

300 nm level of packaging

30nm chromatin fiber forms “looped domains” on a protein scaffold insidle the nuclear envelope

• protein scaffold is not a histone

700 nm level of packaging

highest level!!

• scaffold folds up on itself

• no longer chromatin, but chromosome

best level of packaging for gene expression

1st level (10nm fiber)

• as loosely packaged as possible!

2 types of chromatin

euchromatin: contains actively expressed genes (these are transcribed and translated)

• 10nm or 30nm fiber fall in this category

heterochromatin: few actively expressed genes

• 300nm fiber

• for most part, shuts down gene expression

DNA within cells will be at one of the first 3 levels of packaging as chromatin when in this phase

interphase

• before cell division occurs!!

mitotic chromosome

700nm fiber level

ready for cell division!

chromosome

a structure made of tightly packaged DNA and its associated proteins

• 700nm level, scaffold folding

gene

a discrete unit of heredity made of a specific sequence of DNA

• contains the genetic info for a particular trait

• multiple genes controlling different traits are contained on a chromosome

how many genes are on each of the 23 pairs of human chromosomes?

varies for each chromosome

in the hundred and thousands!

chromosomes are numbered based on their length

• #1 is the longest and contains the most genes!

karyotype

visual display of the chromosomes of an individual/cell

purpose: identify large scale issues

• missing or extra copies of chromosomes

• can also determine gender from it!!

autosomes vs. sex chromosomes

22 pairs of autosomes (44 total)

1 pair of sex chromosomes

trisomy 21

3rd copy of chromosome 21

aka down syndrome

klinefelter syndrome

XXY

Turner syndrome

XO

• only one X chromosome, that’s it!

1 “stick” in a karyotype would be called a

chromosome

homologous chromosomes / homologs

chromosomes that pair during meiosis and have an identical set of genes

ex: 2 copies of chromosome 2

are the DNA sequences of homologs identical?

no

have same genes located on them, but could have different alleles

heterologous chromosomes (non-homologs)

chromosomes containing different genes

• do NOT pair during meiosis

ex: a copy of chromosome 3 and a copy of chromosome 4

allele

one of 2 or more alternate forms of a single gene

encode diff. forms of a chara

homologs can either be homozygous or heterozygous. what does this mean?

homozygous: the homologs have the same version of a specific allele

EX: AA

heterozygous: the homologs have different alleles

EX: Dd

what happens to chromosomes before cell division

every chromosome is replicated

• look like an X once this happens

• one X = one replicated chromosome

the individual lines making up the X (aka 1 replicated chromosome) are called

chromatids

sister chromatids

the 2 subunits of a replicated chromosome

• they should be identical!

have same alleles!

non-sister chromatids

chromatids from different chromosomes

centromere

where chromatids remain attached

binary fission

cell division in prokaryotes

goal: exact copies of original cell, asexual reproduction

• 1 cycle every ~20 min

2 types of cell division in eukaryotes

mitosis and meiosis

purpose of mitosis

make identical copies of a cell

1 —→ 2

purpose of meiosis

reduce # of chromosomes

• pull homologous pair apart

• cells will have ½ the amount of DN

• cells go from diploid to haploid

G1 in cell cycle

cell grows, prepare for DNA replication

• check for any DNA damage before moving on to S phase to replicate

• If can’t fix damage: apoptosis (programmed cell death)

S phase

all DNA replicated exactly 1 time

• creates genetically identical sister chromatids

• same # of chromosomes, 2x the DNA

interphase

G1,S,and G2

• the preparation for cell division

• DNA is in chromatin form

G0

cells that cease division/ do not go thru mitosis

phases of mitosis

prophase

prometaphase

metaphase

anaphase

telophase/cytokinesis

prophase

• DNA condenses into chromosome form (700nm)

• the spindle apparatus (mitotic spindle) forms

spindle fibers are made up of

microtubules

chromosomes are where during prophase?

the nucleus

prometaphase

• nuclear envelope breaks down

• spindle fibers indirectly attach to chromosomes (attach to the kinetochores)

what is a kinetochore

collection of proteins on the centromere

• there’s 1 per chromatid

• spindle fibers attach here during prometaphase

metaphase

chromosomes are pulled to the metaphase plate

• homologs DO NOT interact

anaphase

the identical sister chromatids are pulled to opposite sides of the cell

• no longer sisters. they are 2 identical “daughter chromosomes”

why is the metaphase to anaphase transition so important?

spindle fibers need to properly connect to the kinetochores of the sister chromatids so they can be properly/evenly pulled apart

• don’t want an extra or missing chromosome in one of the daughter cells!

nondisjunction

when the sister chromatids don’t properly separate during anaphase

• can lead to a trisomy or monosomy

cohesin

the “glue” that keeps sister chromatids together

• breakdown of cohesin in the transition from meta to anaphase allows the sister chromatids to separate in anaphase

telophase and cytokinesis

• spindle apparatus dissolves (microtubules dissociate)

• nuclear envelopes form

• chromosomes decondense (700nm —→ 300, 30, or 10) back to chromatin form!!

• cell membrane divides the cell (cytokinesis)

result of mitosis

2 genetically identical daughter cells

• identical to original/parent cell!

why do we need mitosis?

• growth and development

• repair

in anaphase, there are no more ______

chromatids

double the amount of chromosomes!

daughter chromosomes

ex: meta —→ ana: 4 ——> 8 chromosomes and 8 ——> 0 chromatids