4.1 Biology - DNA, genes and chromosomes

Prokaryotic cells (bacteria) DNA

DNA in prokaryotic cells, mitochondria and chloroplasts is:

• short

• circular

• NOT associated with proteins

• NO introns (non coding DNA)

• found in the cytoplasm

eukaryotic (plants/living) DNA

DNA in eukaryotic cells is:

• very long

• linear

• coiled around histone proteins

• contains introns (non coding DNA)

• found in the nucleus

compared to each other - nucleotides are identical, nucleotides joined by phosphodiester bonds, a deoxyribose joined by phosphate DNA in mitochondria.

Describe the structure of DNA

Made up of deoxyribose sugar, a phosphate group, and one of four organic bases (A,C,T,G).

Double stranded and hydrogen bonds between the bases form the helix shape.

DNA

chromosome - supercoiled DNA + associated proteins

springy bit - Chromatin - semi coiled DNA + associated proteins

• EUKARYOTIC DNA

Chromosome

a compact structure formed when DNA is wound round histone proteins

homologous chromosomes

• a pair of chromosomes

• same size

• same shape

• same genes in the same positions (loci)(locations)

karyotype

• all the chromosomes in a cell arranged in their homologous pairs

gene

a level def

Sequence of DNA bases that codes for a sequence of amino acids in a polypeptide or for functional RNA e.g tRNA or rRNA

allele

A form of a gene

gene locus

the position of a gene on a chromosome

Non - coding DNA - INTRONS

DNA base sequences that code for amino acid sequences in between genes (intergenic DNA) - once called junk DNA

introns are found between exons within genes

Coding DNA - EXONS

region of DNA base sequences that code for protein

Structure of RNA

made up of ribose sugar, a phosphate group, and one of four organic bases (A, C, G, U)

single stranded

use of RNA - transfers genetic information from DNA to ribosomes for protein synthesis

The genetic code

Made up of base triplets - each sequence of 3 bases codes for a specific amino acid

summary about the genetic code

Genetic code is degenerate - most amino acids are coded for by more than one triplet

genetic code is universal - all organisms use the same genetic code (same triplets code for the same amino acid) - this is evidence for common ancestory

genetic code is overlapping - each base is only part of one triplet

The genetic code diagram

genome

the complete set of genes in a cell

proteome

the full range of proteins that a cell is able to produce

chromosome genes

introns and exons

describe how a gene is a code for the production of a polypeptide - no transcription or translation

base/ nucleotide sequence in triplets determines order/ sequence of amino acids sequence/ primary structure (in polypeptide)

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule

DNA polymerase catalyses the joining of the phosphate and deoxyribose together through condensation reaction to form a phosphodiester bond

give three ways in which the DNA in a chloroplast is different from DNA in the nucleus of a plant cell

in chloroplasts

• DNA is shorter

• fewer genes

• DNA circular not linear

• not associated with proteins unlike nuclear DNA

• introns absent but present in nuclear DNA

not all mutations in the nucleotide sequence of a gene cause a change in the structure of a poly peptide

why? 2 reasons

Triplets code for the same amino acid because they are degenerate

Occurs in introns which are a non coding sequence

compare and contrast DNA in eukaryotic cells with the DNA in prokaryotic cells

comparisons

• nucleotide structure is identical

• nucleotides joined by phosphodeister bond

• or deoxyribose joined to phosphate

• DNA in mitochondria/ chloroplasts same/similar (structure) to DNA in prokaryotes

contrasts

• eukaryotic DNA is longer

• Eukaryotic DNA contain introns, prokaryotic DNA does not

• Eukaryotic DNA is linear, prokaryotic is circular

• Eukaryotic DNA is associated with proteins/histones, prokaryotic DNA is not.