Chapter 12-2 Chromosomes

and DNA Replication Notes

I. The Review

A. Prokaryotic cells do not have a nucleus

1. DNA molecules are located in the cytoplasm

2. Usually a circular DNA molecule and it is

referred to as the cells chromosome

B. Eukaryotic cells have a nucleus

1. Can have 1000x more DNA than prokaryotic

cells

2. DNA is located in the form of a number of

chromosomes (in the nucleus)

a. Varies widely from species to species

Lots of DNA, but not all of it

codes for proteins!

C. DNA = Deoxyribonucleic acid (sugar =

deoxyribose)

1. Function: transmit and store hereditary, or

genetic information.

2. DNA molecules are LONG!

a. Ex: E. Coli (prokaryote that lives in human

colon) has 4,639,221 base pairs

b. This equals approximately 1.6mm

Chromatin (interphase)=

granular material

within the nucleus

Chromosome=

visible during

mitosis and can

be clearly seen

under the

microscope as an

“X” (two sister chromatids)

nucleosome

Let’sREVIEW...

histone

protein.

D. Nucleotide = Monomer of DNA

1. 3 parts: Phosphate group; deoxyribose

sugar; and one of the four Nitrogenous

bases –A, T, G, or C

Label Figure

E. Double Helix

1. Backbone = Deoxyribose & Phosphate Group

of each nucleotide (Held together by covalent

bonds)

2. “Steps of Ladder” = nitrogenous bases (Held to

complementary DNA strand by hydrogen bonds)

3. Any base sequence is possible following base

pairing rules:

a. A pairs with T

b. C pairs with G

F. How Does DNA Fit into your cells?

1. Forms a chromosome!

2. How? ….DNA coils

around proteins

called histones and

then this chromatin

supercoils around

itself to form

nucleosomes

a. Nucleosomes

bunch together to

form the visible

“X” chromosome

(prophase of

mitosis).

DNA is copied during a process

called1. replication.

2. translation.

3. transcription.

4. transformation.

replic

ation.

transla

tion.

transc

riptio

n.

transfo

rmatio

n.

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2437316/120076/micro04.swf::DNA%20Repli

cation%20Fork

II. DNA Replication – duplicates DNA

A. Background Information

1. We must pass exact copies of DNA from one

cell to the next

2. Before a cell divides, it must copy its DNA so

that all cells have a copy of the genetic

instructions (remember cell cycle – S phase

of Interphase)

3. This process involves LOTS of enzymes!

An Intro to the

Enzymes!

4. Each half of the DNA double helix (parental

strands) can serve as a pattern (“template")

for the replication of complementary

(daughter) strands forming two double helix

molecules.

Semi-Conservative

Replication

B. The Process

1. An enzyme (DNA Helicase) unwinds &

"unzips" the DNA (separates the 2 DNA

strands) by breaking the Hydrogen bonds

between base pairs

a.Replication bubble-small area where

replication begins

b.Two forks of replication move in opposite

directions along the chromosome.

2. DNA polymerase (enzyme) bonds free

nucleotides to elongate the new daughter

strands using base pairing rules(A-T; C-G).

a. The sugar-phosphate groups of the new

nucleotide are covalently bonded to the

growing DNA chain (forming the new

"backbone")

3. The DNA Polymerase

will also "proofread" the

order of bases & make

corrections in the new

strand.

a. It finds a mistake…backs up; removes the incorrect

base then adds the correct base.

b. This maximizes the odds that each molecule is a

perfect copy of the original strand.

4. Remember the 2 DNA strands are

antiparallel, one strand runs up and the other

runs down

5. Polymerase can only add N-bases in the

direction of replication

a. Leading strand-new continuous

complementary DNA strand synthesized along

the template strand in the mandatory 5’ 3’

direction

b. Lagging strand-a discontinuous synthesized

DNA strand (fragments) that elongates in a

direction away from the replication fork. As

more of the parental strand is exposed more

nucleotides are laid down.

6. DNA Ligase (an enzyme) joins the

fragments of the lagging strand

In addition to carrying out the replication of

DNA, the enzyme DNA polymerase also

functions to1. unzip the DNA molecule.

2. regulate the time copying

occurs in the cell cycle.

3. “proofread” the new copies to

minimize the number of

mistakes.

4. wrap the new strands onto

histone proteins. unzip th

e DNA m

olecu

le.

regu

late

the ti

me co

pyin...

“pro

ofread” t

he new

cop...

wra

p the n

ew st

rands o

n..

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C. Each new DNA molecule has one

nucleotide strand from the original DNA

molecule and one strand that is newly made

(semiconservative replication)

Lagging Strand

Label Figure

Template Strands

Replication Fork

DNA Polymerase

DNA Polymerase

DNA Ligase

Leading Strand

Okazaki

Fragments

The first step in DNA replication is

1. producing two new strands.

2. separating the strands.

3. producing DNA

polymerase.

4. correctly pairing bases.

producin

g two n

ew st

rands.

separa

ting t

he stra

nds.

producin

g DNA p

olymera

se.

corr

ectly

pairi

ng base

s.

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DNA replication results in two DNA

molecules,

1. each with two new strands.

2. one with two new strands and the other with two original strands.

3. each with one new strand and one original strand.

4. each with two original strands.

each w

ith tw

o new

stra

nds.

one with

two n

ew st

rand...

each w

ith o

ne new

stra

n...

each w

ith tw

o orig

inal

st...

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DNA Vocab List

Nucleotide

Base pairing

Chromatin

Histone

Nucleosome

Chromosome

Replication

DNA polymerase

Helicase

Ligase

Replication fork

DNA Base Pairing Practice-NO PARTNERS YOU ALL HAVE A DIFFERENT MESSAGE

First Message Assigned:

Number of DNA Message Assigned:_____ (carefully copy below the DNA message assigned):

DNA: _________________________________________________________

Complementary Strand: _________________________________________________________

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